Thursday, October 25, 2007

Semiconductor blue-green laser diodes

according to US Patent Number: 5371756 direct blue-green laser diodes can be manufactured using CdZnSe, ZnSSe, and ZnSe, for each of the p-type clad layer and the n-type clad layer contains at least one of ZnSSe, ZnSe and ZnMgSSe. This direct blue-green laser diodes would be cheaper to manufacture compared to diode pumped solid state lasers using high-power infrared diodes and then doubling or tripling the frequency to produce visible light.

Here are the patent claims of the said patent:

1. A semiconductor blue-green light emitting device in a double heterostructure configuration, comprising:

a substrate;

a light emitting layer which contains at least one of CdZnSe, ZnSSe, and ZnSe;

a p-type clad layer and an n-type clad layer each of which contains at least one of ZnSSe, ZnSe and ZnMgSSe and which are provided at two sides of said light emitting layer, respectively;

a cap layer of (Al.sub.X Ga.sub.1-X).sub.0.5 In.sub.0.5 P (0.ltoreq.X.ltoreq.1) which is deposited on said clad layer located at an opposite side of said light emitting layer from said substrate; and

a contact layer of Al.sub.X Ga.sub.1-X As (0.ltoreq.X.ltoreq.1) which is deposited on said cap layer.

2. A semiconductor blue-green light emitting device according to claim 1, in which said cap layer and said contact layer are made of organic group V materials.

3. A semiconductor blue-green light emitting device according to claim 1, which further comprises a dielectric insulating layer which is formed on said contact layer and which has a stripe opening, and a metal electrode which is provided in said stripe opening.

4. A semiconductor blue-green light emitting device according to claim 1, in which said clad layer disposed at the opposite side of said light emitting layer from the substrate is configured in a mesa stripe with its two side surfaces and two laterally positioned surfaces being covered with dielectric insulating films, respectively.

The story of cheap Green Laser Pointer

When I was new to the exciting world of high-power green lasers, like many folks I didn't really know much about what I was getting in to. I had learned that there were pocket-sized 'laser pointers' that could pop balloons and light matches, and I wanted in on that kind of fun.

I had found the Wicked Lasers web site and saw what they had but it seemed kind of pricey. So like many of us, I went to ebay to see what was there, cheaper.

I should have known better because a) cheaper means both 'less expensive' and 'poor quality', and b) if something seems too good to be true then it probably is.

Nonetheless, I soon found an auction listing for an '80mW Green Laser Pointer'. The price was almost $100 less than Wicked Lasers wanted for their 75mW laser. And the vendor was in Canada, which meant faster shipping and no duties / brokerage, and no hassles at the border. So I plunked out the money, and in a few days, it arrived.
green ebay laser

The Good, Bad, and Uncertain

Sure enough, the first match I put it to ignited almost instantly. I was wowwed, I was happy. Strangely though, the second match I tried, smoked a bit then did nothing at all.

Soon I realized that it was only super-powerful in the first few seconds. Then its power tapered off considerably. You could even see it, shine the bright green dot on a white wall and it went from blinding, to just bright, really fast.

During all this time, I was still reading and learning and gaining more experience. I quickly learned the complicated way that green lasers work - much more complicated and finicky than a red laser pointer. Over at Green Lasers UK they have a great little tutorial on how green lasers work and how they differ from red lasers. Suffice to say, I soon understood that there are two ways to state laser power: peak, and average. Wicked Lasers might cost more but the stated power is their average. Cheap lasers usually seem inexpensive because they quote you the peak power - a rating which it might only attain once or twice, if ever.

By this time, I had decided that I wanted to get involved with lasers quite a bit, so I decided to invest in a laser power meter. The LaserCheck by Coherent was the most inexpensive, small meter that would work fine for the ranges I was looking for.
Coherent LaserCheck

So I got my meter, checked all my lasers, and lo and behold, it worked for all the red and infrared ones, but not the cheap green one! The reading was crazy, like over 400mW! Was my new LaserCheck not working? No, it works fine. I soon learned that the LaserCheck, which measures optical brightness, is easily confused by multiple-wavelength lasers. My 'green' laser was putting out green at 532nm, but also infrared, at 808nm and/or 1064nm. (For information as to why there is IR in a green laser, see the link above.)

Aha! Here is another way for the unscrupulous to get away with selling what seems like a high-powered laser. If you take a cheap green 5mW pointer and remove the IR filters, you will still have 5mW of green light, but now you also have a lot (as much as over 100mW) of IR light as well! So you can sell your cheap green pointer, as a 50mW, 80mW, or even 100mW laser, make lots of money, and claim that you are not lying since it is in fact generating that much IR!

Safety Matters

So what's wrong with a little IR in the mix? First, it's not a little it's a lot. Second, it's invisible, you can't see it. Third, if you're relying on laser goggles rated at 532nm to protect your eyes, you may as well wear no goggles at all because they won't do anything to block the IR, and worse than nothing, you might take some extra risks in the belief that your eyes are protected since the green dot is dull and doesn't hurt to look at! Additionally, the IR is not as well-collimated, so it spreads out more, which means you might not even know you're getting dangerous specular reflections of IR in your eyes, because the green is not reflecting your way.

Wicked Lasers sells great laser goggles at a great price - but they are not rated for IR! If you buy cheap lasers, shell out for expensive goggles. If you buy quality lasers, you can afford the lower-priced goggles. Where would you rather spend your lasers-money? Good expensive lasers, or goggles?

Wicked's goggles work well for the wavelengths they're designed for. These pics compare what the goggles look like to your eyes, and what they look like to infrared laser light:
Goggles for different lasers Guess which ones are for IR Lasers
If you were relying on goggles for 532nm (the orange/red lenses) and your laser had lots of IR in it... well you get the point right?

Ok sure, the weak green laser might burn pretty good because it's got all that IR in it. But it will only burn stuff good up close, because the IR isn't well-collimated. And you might be slowly and unknowingly losing your eyesight, every time you use the laser!

See, there are no pain receptors at the back of the eye. You can't see the IR so there is no blink response or aversion response. And the brain is very adept at compensating for partial vision loss, by filling in the blank spots with information from the opposite eye. Most people don't even know they're born with a 10 degree blind spot in both eyes (where the optic nerve enters) because the blind spot in the right eye is compensated with data from the left, and vice versa. So little bits and pieces of your vision might be slowly going away, permanently, and by the time you realize it, it is far too late to do anything! This is why having IR mixed in with your green laser is unsafe.

Knowledge Is Power

Ok. So I have established that my cheap green laser has a high initial peak but then quickly tapers off, and I am pretty sure it has no IR filter. A few more tests pretty much confirmed the IR filter situation: I took photos of the dot, using a DV Camera in normal and in 'nightshot' mode. In 'nightshot' mode it can see near IR light, and the results were pretty conculsive, and obvious.
IR halo around green dot
Here you can see the IR 'halo' clearly visible around the green dot. The dot is distorted and stretched because it is being refracted through a cheap plastic prism.

Still learning more, I finaly found out what brand-name the laser is appearantly made by. It's a NewWish, which are a new manufacturer in China. The manufacturer probably aren't trying to be malicious; appearantly they're relatively new and they just produce what is ordered. If the distributor wants to save a dollar or whatever by omitting the IR filter, that's them, not the maker's fault.

What is bad of course are all the guys on ebay and elsewhere, hawking these things, and they don't even know about the potential hazards. Or if they know, they don't care.

Fixing It

Ok, so I've got a laser which is potentially dangerous to me and others around me. I smartened up and ordered a WL Fusion 125mW but in the meantime I'm not just going to throw out the cheap green laser. What to do?

Obviously the answer is to add an IR filter. The caveats: If it is just a cheap green pointer, then adding the IR filter will make it safe, and also reduce it to being just a cheap pointer. No burning power, no balloon popping. Just boring old safety. But, these are the risks you run when you buy on the cheap.

So, where to get an IR filter, and how to know if it's working? Edmunds Optics sells IR filters that cut-off just over 700nm and also pass over 90% of the green wavelengths, so they're a good choice. A bit pricey though, at $40-ish for a 12.5mm size. Talk about adding insult to injury, eh? Buy an overpriced unsafe laser, then spend another $40 to make it safe and take all the fun out of it.
IR filter

I'm still looking for cheaper sources...crap there's that word again, 'cheap'. Ok, here's what I did. I have a lot of electronics junk. I dug up an old USB web cam from my junk box. Web cams, in fact all digital camera type devices, incorporate an IR filter of some kind. Why? Their image CCDs are very sensitive to IR light. If there were no IR filter all their pictures would look funny, i.e. would look different than we see the world, because they see the IR and we don't. This webcam, new, cost me about $40. But it was worth nothing today, so I disassembled it, got the lens carrier out, and removed the lens. The IR filter was square, about 12.5 x 12.5 mm, and was glued into the plastic lens holder. Before trying to get it out, I figured to test it first.

Note: The IR filter is blocking IR from outside, getting into the camera. So the side of the filter that faces out of the camera is the side you want to face into the laser. Incase it makes a difference, and I think it does.

So I slid the lens holder over the laser and got out my trusty LaserCheck. If it works, I will get a realistic reading. If it doesn't, I'll get a nutsy reading like I did before.

Results worked! I got a sensible reading! I proceeded to take a series of test readings, at the three wavelengths, with the filter and without:

WavelengthNo FilterWith Filter

532 nm401.0 mW52.1 mW

808 nm35.4 mW3.68 mW

1064 nm64.1 mW8.16 mW

Well, now this is interesting! At 808nm, the IR is reduced by about 90%. And at 1064nm the IR is reduced by about 87%. It's impossible to know how much the green is reduced, since we can't get a good reading of just the green without a filter. But 50mW is still acceptable - a heck of a lot better than a 5mW pointer anyways...?

So, I pried the IR filter out of the webcam's lens holder and (keeping track of which side was which) I used a little medium-strength lock-tite to affix it inside the aperture cover of the green laser. Now my green laser has an IR filter! Yippee! Now it's reasonably safe!

Here's a closeup of the IR filter, glued into the laser's aperture cap:
IR filter in place

Obviously the IR filter I am using, was never designed to block such intense sources of IR, so there is bound to be some IR leakage. Who knows, the filter might even break down at some point, so I will test it periodically and find out.

But, what about the other problem? The power spike then rapid loss? Well now I can get accurate measurements, so it must be time for another chart:

Initial Peak125.0 mW

After 5 Seconds44.8 mW

After 10 Seconds28.8 mW

After 15 Seconds16.7 mW

Oh... Uh... Ok. So my 80mW laser has an initial peak of 125mW. Yeah that's good. That's why it will burn a match or pop a balloon almost instantly, when first turned on.

The rest of the numbers aren't so great though, are they? I didn't bother to test the reading past 15 seconds, I don't really want to know. I guess the good news is, if I want to let my cat chase the green dot around, I just have to leave the laser on for half a minute first, then it's probably safe for her to chase after that.

Footnote Regarding the Measurements:
The LaserCheck wavelength setting simply adjusts a 'formula' that the LaserCheck uses to calculate power - it does not filter anything. So setting it to 808 doesn't just measure the 808 light and ignore the other lines, it simply measures everything and assumes it is all 808nm. So the readings are not assumed to be absolute or completely accurate - I have neither the experience or equipment to measure the relative strength of the separate lines. This is merely the best I can do, with the tools and experience I have at home in my livingroom. Your mileage, as always, may vary.


With the IR filter infront of the laser, the IR 'halo' is completely gone now, as visible in these Before and After shots:
IR halo around green dot IR image: No IR Halo
Both images taken with the DV camera in Nightshot mode, the dot at about 3 feet, after reflecting through a cheap plastic prism (the cause of all the deformation and 'star' effects is the prism).

So, not really the great deal I initially thought. The cost of the laser, the cost of the IR filter I had to add, and the final results showing the under-achieving performance, all add up to... yep, that's right: If it seems too good to be true, it probably is!

I should add: I'm not bitter or angry. I'm not upset at being cheated or ripped off. I took the chance, I bought a cheap laser from an unknown dealer on ebay. I'm dissapointed, but mostly in myself for letting myself get sucked in by the too-good-to-be-true vibes. I also have to acknowledge the educational aspect: it was my first green laser, my first 'high power' laser, and it has inspired me to learn quite a bit about DPSS frequency doubled lasers, IR filters, etc.

Save your money, and buy your lasers only from reputable dealers - where there is a warranty, a returns policy, and some level of responsibility.

I've shared my experiences with you all, so you can hopefully learn from them, save your money, save your eyeballs, and play safe!

The Fine Print
Information provided on this page is based on my own personal experiences. Although I purchased my Cheap Green Laser at Ebay, I have not identified the individual seller. Outside of the general recommendations given on this page I won't warn against any specific laser or brand. I suggest laser buyers accept the responsibility of doing their homework before making a purchase, as much as I suggest laser sellers do the same and learn about the products they are selling.

source: Stephanie Maksylewich

Retinopathy from a Green Laser Pointer

Dennis M. Robertson, MD; Jay W. McLaren, PhD; Diva R. Salomao, MD; Thomas P. Link, CRA

Arch Ophthalmol. 2005;123:629-633.

Objective To report retinopathy following exposure to light from a commercially available class 3A green laser pointer.

Methods A 55-year-old woman with a ring melanoma was scheduled for enucleation. The eye (visual acuity 20/20) had a healthy-appearing macular retina. The retina was exposed to light from a commercially available class 3A green laser: 60 seconds to the fovea, 5 minutes to a site 5° below the fovea, and 15 minutes to a site 5° superior to the fovea. Color photographs were obtained before and after exposure. The eye was enucleated 20 days after exposure.

Results Laser power measurements averaged less than 5 mW. Retinopathy was observed 24 hours after laser exposure. This was characterized by a yellowish discoloration at the level of the retinal pigment epithelium (RPE) in the subfoveal region and at the site superior to the macula where the retina received 15 minutes of laser exposure. Each site developed granular changes at the level of the RPE within 5 days of exposure. Histologic study showed RPE damage in the exposed subfoveal and parafoveal regions.

Conclusion A class 3A green laser pointer caused visible retinopathy in the human eye with exposures as short as 60 seconds.

Author Affiliations: Department of Ophthalmology, Mayo Clinic, Mayo Foundation, and Mayo Medical School, Rochester, Minn.

Experiments for High Power Green laser Pointer

Conducting experiments with laser pointers is both educational and loads of fun. Listed below are some basic experiments you can perform with green laser pointers. Please when experimenting, always keep safety foremost in mind.

Thermal experiments

Burning plastic

This is the easiest of the thermal experiments to perform and only requires a 55mW laser pointer.


-Use thin black plastic such as shopping bags.

-Hold the laser pointer very close to the bag and rest it on something solid to keep the beam steady.

-The first sign of success will be tendrils of smoke from the point you are aiming at.

-When you’ve finished, holding the plastic up to the light should reveal small pin pricks burnt into the plastic.

Bursting or popping balloons

Laser pointers of roughly 75mW or higher in power can pop dark or black colored balloons.

Instructions/tips: Dark or black balloons are best because they are better at absorbing green light. White or green balloons will have too much reflection and the absorbed energy will not be enough to pop the balloon. If you don’t have any dark/black balloons, you can use a sharpie (black texta) to put a black spot on the balloon that you can aim the laser at. Make sure the balloon is fully blown up because if it is not blown up properly and lacks tension, popping it will be difficult.

Lighting matches

This is harder to do than popping balloons and requires a minimum of 95mW or higher laser pointer.


-As with balloons, the color of the match head is also important. If the color is too light such as white, too much energy from the laser beam will be reflected and there will not be enough energy remaining to light the match. Black or dark red matches are ideal. You can also use a sharpie to darken the match head.

-Make sure both the laser pointer and the match are fixed and not moving

-don’t hold the laser pointer too close to the match or you may get ash and debris on the laser pointer lens.

Cutting black electrical tape

Another one of the more difficult burning experiments that requires a laser pointer of at least 95mW in power.


-Attach the loose end of the tape to a fixed point such as the edge of a table then leave roughly 10 cm of tape with the tape roll suspended at the end. The tape roll will create tension that makes cutting the tape easier.

-keep the laser steady and aimed at the edge of the tape.


Experiments with mirrors can range from the very basic to the very advanced and are only limited by your imagination. The advantage of mirror experiments is that there are no specific power requirements and low powered laser pointers will be just as effective (but less visible) as the higher powered laser pointers.

Laser accordion

Very easy to do and can be visually very impressive.


-Carefully line up two rectangular mirrors of similar size parallel to each other.

-change the angle of the laser pointer to the mirror to create more beams between the mirrors

Measure your pulse

Sounds difficult but is actually very easy to do.


-attach a very small mirror (no more than 4cm in diameter) directly over the inner part of your wrist where pulse is normally measured. Use a small round object like blue tack or chewing gum (messy but effective) to attach the mirror to your wrist.

-reflect the laser pointer beam of the mirror onto a smooth surface such as a wall at least 4 meters away.

-if the laser pointer and your wrist are held steady, the rhythmic movements in the reflected image on the wall will be your pulse.

Security alarm

This experiment is an extension of the laser accordion experiment.


-at a security point such as an exit/doorway or a hall way, line up two rectangular mirrors and create a grid of laser beams. The beam should be aimed to fall on a photoelectric detector that is connected to an alarm.

-once the beam is broken, the alarm will be activated.

-IR lasers could be used to create an invisible security alarm.

Cool beam effects

There are a number of experiments ranging from very simple to quite advanced that can create beautiful beam effects such as the ones listed below.


The crystalline structure of ice is full of hollows, gaps and imperfections that will scatter and diffract the laser beam.

Laser drawing and writing


-make sure the exposure time on your camera is at least 2 seconds

-aim the laser pointer beam at a smooth non reflective surface with a good contrast

-then use the beam point to write/draw pictures

-use smoke or fog to provide additional effect

Bend light

Using wave propagation theory, you can actually bend light from your laser pointer and see it clearly.


-Aim the laser at a smooth surface 4 meters away such as a white wall

-Use an object with a very precise smooth edge such a new razor blade.

-partially block the laser beam with the edge and note the interference patterns produced.

-the patterns will not be flat or smooth. Instead there will be a diffraction pattern

Moving objects

No kidding, high power laser pointers have a beam powerful enough to move objects. At least they can move the vanes in a Crookes radiometer.


-you need a Crookes radiometer that has a complete vacuum in the globe and the vanes have virtually frictionless support.

-aim the laser pointer beam at the silver side of the vanes and they should start to move very rapidly.

Beam splitting

There are various methods and numerous pieces of equipment that can split the beam of a laser. This is one of the simplest methods.


-using microscope slides, split the laser pointer beam by angling the beam on the slide.

-for really spectacular results, you can use a diffraction grating.

Home made laser microscope

The humble green laser pointer is versatile enough to be used as a very basic microscope.


-put a drop or two of water on a transparent piece of glass such as a microscope slide.

-shine the laser through the glass onto a smooth surface such as your roof. You’ll see a magnified image of the water and its contents on your roof.

Time tunnel

Laser pointers can be used to create a light tunnel reminiscence of Doctor Who and several other famous science fiction programs


-you attach a mirror to an electric motor at a slight angle.

-aim the laser beam on the mirror when the motor is turned on and the laser beam will spin around itself to create a laser cone or tunnel


808nm Diode from Green Laser Pointer

While fitting a heatsink to the outside of a green laser pointer I broke the cyanoacrylate glue holding the driver board to the brass housing. I did not think this was a problem. I was quite wrong. A laser diode is very fragile inside. I thought I only had to be careful with heat and static shock. Here are some photos of the inside of a dead 808nm laser diode.

This is the diode at 10x.

This is the laser diode chip. It is about .020 inch square! It sits on the front edge of the end of the can. I did not find it with my naked eye. I thought the laser diode was lost until I looked at 60x.

This is the photo diode at 60x.

This is the laser diode at 60x from the front. The two lines are for alignment and are about .025 inch apart.

Here is a picture of the Laser Diode and the pin it used to conect to. The wire hanging on the pin was soldered to the laser diode. If you look closely the blob of solder is still on the end of the wire.

Here is the photodiode. Notice the wire soldered to the corner. The ring is the strain relief for the pin. The pin was pulled out of the diode.

I could not see the wire on the pin with my naked eye but with a little help it is still there!

This is a strand of hair to show the scale involved.

Can this be repaired?

Here is Chris Luebner's method. I don't think I will try. Without a 60x stereo microscope the method he describes would be very difficult.

Looks like the bonding wires got yanked off the diode, set it up so the diode is facing a cm or so from some white paper and tie the can to the positive side of 2 d cells hooked up in series.

Then add a 2 ohm resistor in serieswith the negative side of the battery and gently touch the top of the diode (with the gentleness and steadyness of a neruosurgeon) with the free reistor lead to see if it still lases.

You should see a dim red elipse if it still is alive, be sure you are more than a foot away when you view it and do not do so for very long. Technically it is defused to an eye safe value after a mere couple inches when shone on paper, but it's still VERY very bright and you will ge a nasty headache because your eyes still _FEEL_ it as if you are looking at a 1/2 watt argon laser.

If you have an IR sensitive camera or night vision scope handy you can use that to view it, be sure that you have the pinhole attenuator for the NV scope or it will die.

If the diode is still good you may try a soldering pin (Yes, really smd tips are much too big! Wire wrap a brass pin to a 15W temp controlled soldering pen tip.) with indium tin solder. To re-attach a wire try to move the bonding wires using a cactus needle to the diode, or if they are too far gone a tiny piece of wire like a single strand from a cheap bad headphone wire works well. Never use any other solder but indium/tin on the pin, and never let it get very hot and allways pre-heat your part on a hotplate before soldering it or it will fail to bond if you don't.

If all else fails I do have some fiber coupled 1-2W laser pump diodes that i'll sell ya for 125.00 each. The c-mount diode inside has a circulaser lens that makes the beam round and would be ideal for pumping a dpss crystal.

hope this helps, Chris


DIY Green laser pointer power meter

Here is a simple cheap and accurate way to measure laser output power.

Required material :
- A green LED like this :

- An ammeter with a 1000mA range
Connect the ammeter to the LED as shown in diagram.

Place the LED into the hole of the laser head, fire the laser and read the value on the ampmeter. Use this formula to obtain the optical power : P = uA / 2.8

Note : it must be a green coloured LED for correct values (not a transparent green LED). Radio Shack sells them.

Readings : 14 uA for 5mW, and my pointer measured 310uA for 110 mW


eBay Laser Pointer Scam Exposed

The scam itself is based on the type of light given out by the laser pointer, the way power is measured, changes made to the laser pointer and ignorant and/or dishonest sellers.

Type of light

These lasers are advertised as green lasers and should give out green light and only green light. That how ever is not the case. Virtually all supposedly high power green laser pointers on ebay emit a combination of green light with a wave length 532nm light and infrared light with wavelengths of 808nm and 1064nm light.

All genuine high power green laser pointers have a built in filter that prevents them from emitting infrared light. The laser pointers on eBay are missing these filters which have either been removed or deliberately manufactured with out. Removing the infrared filter from a laser pointer increases its power and makes it a lot more dangerous.

Take a 240mW laser pointer commonly found on eBay as an example. This laser pointer is likely to have only 40mW of green light with the remaining 200mW of light being infrared light. Customers pay for laser pointers with 240nm of green light and only get laser pointers with 40mW of light. Effectively cheating them of 84% of their money.

The other problem with missing infrared filters is the lasers are very dangerous. Infrared light spreads out like a torch beam compared to green light which is in a narrow beam. Green light being visible can be avoided but the Infrared light because it is invisible cannot be avoided. Most safety glasses that protect against green light are completely transparent to infrared light. This means a person wearing safety glasses and using one of these lasers is less likely to avoid the beam and much more likely to suffer damage to their eyes.

Measurement of power

The power of most laser pointers will peak in the first few seconds then stabilize at a level much lower than the peak. Reputable sellers will give the average power as the advertised power. Ebay sellers on the other hand use the peak power as the advertised power. This is very deceptive because it misleads people into thinking the laser is much more powerful than it really is.


These laser pointers are only designed to handle powers from around 5mW to 10mW. The sellers modify the laser pointers by increasing the power from the batteries to the laser diode so the diode emits a more powerful laser beam. The problem with this that the diode is not built to handle the higher power and will have a much shorter life cycle and is likely to burn out. Many ebay lasers lose power or burn out completely in a matter of hours or even minutes. The lens in the laser pointer the acrylic coatings are also not designed for the increased power and are likely to damaged with reduced performance or fail completely.

These eBay sellers are not laser experts and generally have little or no understanding of the laser pointers they sell. This means they are not able to give qualified advice or recommendations and the level of support and service they can provide is minimal. Their general response to customers when there are problems with laser pointers is to ship it back for a replacement. Given that the cost of the laser is not much more then the cost of shipping it, most customers cut their losses at that point.

Possible remedies

Laser pointers with powers over 5mW are classified as hazardous goods and are not allowed on eBay. If you have been scammed by one of these sellers, you can threaten to report them to eBay and they may have their account suspended or even closed.

If you paid with PayPal, you can also open a dispute for goods not as described. The dispute may not be decided in your favor but most sellers are willing at that stage to issue you a refund. If you paid with a credit card, you can start a charge back with your credit card company in a similar manner with a greater chance of success than PayPal.

The last option is to bluff. Claim that you will contact eBay, the police and the government department in charge of consumer affairs in their country if they do not issue you with a refund. Considering they are selling an item prohibited by eBay and illegal to sell (legal to possess) in a number of countries, this bluff is very effective.

This article is provided courtesy of Dragonlasers at

5mW Green Laser Pointer for Astronomy

I bought the BTG-6-plus laser pointer from (Beam of Light Technologies) to use as a sky pointer for showing my kids, my friends, and my friends' kids the sky. I've seen all sorts of claims from various different people about how well or poorly these work, so I thought I'd write up a review here.

Green laser background
There is a class of lasers, IIIa, which by law must be less than 5mW (of measured optical output, not electrical input). This class is legal to sell in the United States, and legal to operate outside in the United States (local or state exceptions may exist) provided you don't do anything stupid. Shining the laser at aircraft in flight, or moving cars, or other equally retarded acts can easily land you in prison for an extended time (and rightly so). Apparently a man who wanted to see if he could hit airplanes as they were landing was in fact successful. Thankfully, none of the pilots crashed, but the man was reported to have received a seven year prison sentence.

The next higher class, IIIb ranges from 5 to 500 mW. You can also legally purchase this class of laser in the United States. But there are restrictions on it's use, because these lasers are capable of permanently damaging vision. You can't use it in an environment where the beam could escape to the outside. To be explicit here, this means you can't legally use them outside. Now you may want to adopt a "no blood, no foul" attitude, and that's fine for you. But just know that if you ever make a mistake, or run into a narrow-minded individual, you don't have a legal leg to stand on - prepare for a good screwing. Furthermore, based on my own <5mW product, there is no reason outside of inferiority complex to get a higher power product for astronomical use.

So how safe are these things (the sub-5mW class IIIa version)? They won't burn you. They won't cause permanent eye damage. Tests were performed on individuals who were scheduled to have an eye removed for medical reasons. For the purposes of the test, the eye was normally functioning. Test subjects stared directly at 5 mW lasers with there to-be-removed eye for five to fifteen minutes from various angles. No permanent eye damage occured. Some changes in tissue were noticed. Of course, in a real-world incident, laser light entering the eye would likely last for less than one second, as people naturally look away from bright things and close their eyes, so there is no real danger of direct damage.

Having said that, these things are damn bright. If you hit a car driver at night with this, he'd be effectively blinded for at least a few seconds afterwards -- long enough to crash and die and have you go to prison for manslaughter. These are not toys for children. They should never be shined at people ever, and most especially not cars or planes.

Finally, why green? Our eyes are most sensitive to green light. The same measured power output of a red laser would not produce a visible beam, because our eyes aren't as sensitive to red. By the time we could see a red laser, it would probably be reaching a dangerous level of intensity.
Z-bolt offers several green laster pointers in the IIIa class. Note that these are often referred to as 5mW lasers, but they always have to be less. From everything I've read, there is a lot of variation in how much under 5mW the lasers are. I'm not sure how much to believe, but some claim that you can end up with as little as 1.5 mW from some of these products. For this reason, I chose the BTG-6-plus, because this particular product is guaranteed to be tested by Beam-of-Light to be between 4.5 and 5mW limit. Mine actually came with a hand-written sticker on it that said 4.92 mW.

It also came in a very nice wooden box, and a pair of batteries. For my order, they were also giving away a free red laser pointer with it. I don't much care about this, but the free red laser pointer was packed into a second plastic foam case, which was much too big for the red laser pointer, but perfect for the green one. I don't know if they always give out this second case for the green pointer, but if you buy a pointer from them I suggest you ask them about it. The wooden box is very nice, but not very practical. The plastic foam case on the other hand is much more practical for slipping into your pocket or some luggage, and it provides nice protection. It closes with a flap that has two snaps in it, and it has slots for a spare pair of batteries. This is the case I'll be using whenever I'm carrying this pointer.

So, how does my laser work? It works GREAT! Exactly as described - a green beam of light protrudes up and more or less stops right on the object you are pointing to. The end of the beam is a bit more blurry, and fades slightly, but it really seems to have an end where the beam essentially stops. It's extremly apparent what you are pointing to. I haven't yet tested to see how far away from me it remains visible, although people standing six feet away from me have been able to see it without a problem. What about light pollution? Many web sites say that in light-polluted conditions you won't see the beam, and you'll need more power. I suppose it depends on what they mean. The first time I used it, I was in a rural area, although not very far from the city, and there was a setting gibbous moon. Limiting magnitude was around 5.0, maybe 5.5. The laser was bright and easy to see.

I've also used it in Cambridge Massachusetts, easily one of the most light-polluted cities on this planet. On the best moonless nights, limiting magnitude is 4.0. Again, the laser is easily visible, not quite bright, but not dim either. However, this only accounts for the light pollution - I was on a dark rooftop on a slight hill above other lights. So the light pollution was in place, but I had no lights in my eyes. If, by light pollution, you mean standing on a brightly lit street with a street light above you, then no, you won't see the beam. But if you mean, can you see it from a dark spot in the worst light-polluted sky imaginable? Yes, you can see it.

Just for perspective, I used it about 45 minutes after sunset. The sky was still quite bright, with 20 minutes of nautical twilight left, and an hour of astronomical twilight. Limiting magnitude was perhaps 3.5. The beam was visible in these conditions. Dim, but unmistakably visible.

These lasers are also supposed to work poorly in cold weather. I've used it in below-freezing temperatures. I was carefully to keep it in an inside pocket, or up my sleeve, when I wasn't using it. It worked fine. It tends to come on at less than full brightness, and then brighten up after a fraction of a second.
So why DOES that beam of light simply stop at the target, instead of fading out in the distance, or seeming to go on "forever". Well, the answer's obvious if you do the math. If the laser is one foot away from my eyes, to the side, and I'm looking towards the "end" of the beam, then we can start to think in triangles, where the base is 1 foot long. If I look at a point 100 feet along the beam, then we have a tall skinny triangle with sides of 1 foot, 100 feet, The small angle for this triangle is 0.57 degrees. That's the angle between my sight line and the laser beam. But that means that the other angle is 89.42 degrees. The first 100 feet of beam covers 89.42 degrees of view to my eye. Let's look a thousand feet down the beam. We now cover 89.94 degrees of our field of view. Going ten times farther filled an additional 0.37 degrees of our field of view with a beam. At 10,000 feet, we get to 89.99 degrees - and we gained 0.05 degrees or three arcminutes. Beam-of-light technologies claims their beam from this product reaches 25,000 feet. If that's the case, then the additional 15,000 feet past what we just calculated will add 0.003 degrees to our view of the beam, or 10 arcSECONDS.

The first 10,000 feet gives us a laser beam across almost 90 degrees of our view. And the next 15,000 feet of beam visually lengthens the visible beam by a size smaller than the disk of Saturn, Jupiter, or Venus. In other words, while the beam is fading out gradually, the part of it that we can actually see, the close part goes almost all the way to where we're pointing, while the long long section that fades out, adds almost no visible length to the beam. Even the section of the beam starting after one thousand feet away only lengthens the visible beam by the size of a crater on the moon that's too small to see with the naked eye.
Where to buy?
There are tons of people out there selling green lasers, and lots of horror stories. I chose Beam of Light Technologies because they've been in business for more than five years - I know this because I found a couple of negative reviews of them online from that long ago. But I found no recent bad reviews, and they were still in business. I'm perfectly satisfied with the product and with their service, although one could argue that when everything goes well, you haven't really tested their service.

I apologize for writing in this space that Howie Glatter never answered my email. Apparently, spamassassin ate the email, and I found it later. By that time I'd already purchased my product. He has a good reputation, seems a bit pricy, but otherwise I can't comment on the quality of his products or services.


Green Laser Pointer SAFETY

Hazards of Laser Light

Laser beams can be rather hazardous, particularly for the eye (sometimes also for the skin), because they can have high optical intensities even after propagation over relatively long distances. Even when the intensity at the entrance of the eye is moderate, laser radiation can be focused by the eye's lens to a small spot on the retina, where it can cause serious permanent damage within fractions of a second – even when the power level is only of the order of a few milliwatts. Possible damage mechanisms are based on thermal as well as photochemical effects. Laser damage of the eye is not always immediately noticed: it is possible e.g. to burn peripheral regions of the retina, causing blind spots which may be noticed only years later.

laser safety issues

Eyes are particularly sensitive, but laser radiation can also cause skin injury. For infrared light, this occurs mainly via thermal effects (thermal skin burns), similar to burning the skin with other means. The penetration depth depends on the wavelength, and for such reasons a laser beam at 1.5 μm wavelength causes more pain on the skin than a 1-μm beam. While such burning should in most cases not have serious consequences, ultraviolet light can in addition induce photochemical reactions. These can lead to changes in the pigmentation, to erythema (sunburn), photokeratitis (corneal flash burns, a painful condition of the cornea) and (most importantly) to skin cancer. In the eye, UV radiation can cause photokeratitis and cataracts in the eye's lens. (For these reason, the XeCl excimer laser has acquired the nickname "cataract machine".)

How much light an eye can tolerate depends on many circumstances: not only the intensity, but also particularly the wavelength and the duration of irradiation (e.g. the pulse duration). There are detailed sets of rules for calculating safe exposure limits (maximum permissible exposure, MPE) for a given situation. Such rules are occasionally revised according to new scientific findings.

Not Only Light is Dangerous

Note that laser safety issues do not only arise from the direct effects of laser radiation, but also e.g. from

  • high electric voltages in laser power supplies (e.g. for discharge lamps)
  • the use of hazardous chemicals
  • potentially exploding or imploding glass tubes (e.g. arc lamps)
  • the risk of causing a fire
  • fumes, dust, hot droplets of molten material (e.g. in laser material processing)
  • secondary radiation (e.g. ultraviolet light or even X rays) when high intensity beams heat certain targets to high temperatures.

In fact, probably most victims of accidents with lasers have been hurt by such hazards (particularly by electric shocks) rather than by laser radiation.

Particularly Hazardous Situations

The following list of important safety issues can never be complete, but is meant to improve the awareness for the multitude of possible hazards:

  • High-voltage power supplies can be dangerous, if workers can get into contact with the inner parts or with a defective high-voltage cable.
  • Some lasers require the handling of hazardous chemicals, e.g. carcinogenic dye solutions in dye lasers. One should know the risk e.g. of such solutions penetrating the skin, and be trained to safely handle such things.
  • Infrared laser beams are much more hazardous than visible light with the same power level, because their radiation is focused to the retina just in the same way as visible light, while the blinking reflex of the human eye (normally closing the eye's lid rather quickly when the intensity is too high) is not active. Also, no warning is possible e.g. through weak stray light: nothing can be seen until serious damage occurs.
  • Ultraviolet lasers (UV lasers) endanger not only eyes, but also the skin (see above).
  • Pulsed laser sources, e.g. Q-switched lasers or regenerative amplifiers, generated pulses with a peak power many orders of magnitude higher than the average output power even of a high power laser. A single pulse from a hand-held miniature laser can totally destroy an eye.
  • High power lasers (e.g. with powers in the kilowatt region) can not only damage the eye but also the skin within short exposure times, and can easily start a fire e.g. when the beam hits materials like wood or plastics; toxic fumes may also be generated.
  • In open laser setups, parasitic specular reflections (caused either by parts of the setup or by movable metallic tools, watchbands, rings, etc., but also by the residual reflectivity of anti-reflection coatings) may allow hazardous beams leaving the setup, which might hit someone's eye.
  • Optical fibers, e.g. transporting high optical powers between different rooms, may release dangerous radiation when being damaged. They therefore need to be specially protected and marked.

Often less dangerous are e.g. the following cases:

  • setups with low-power visible beams, where the blinking reflex of the human eye may provide sufficient protection against occasional exposure of an eye with moderate power levels
  • sources operating in certain eye-safe spectral regions (e.g. the mid-infrared region with wavelength longer than ≈1.4 μm) where the light is absorbed in the eye's lens and can thus not reach the (more sensitive) retina
  • fully closed laser setups with an interlock, which automatically switches off the radiation source as soon as the case is opened

Safety Classes

To give some guidance on adequate handling and required precautions, laser devices are assigned to different safety classes, with class 1 being the least dangerous one (containing e.g. lasers with microwatt power levels) and class 4 the most hazardous one. Note that the assignment to a laser safety class does not only depend on the laser power, beam quality and laser wavelength, but also on the accessibility of hazardous areas: even a high power laser may be in a low safety class when there is no risk that dangerous radiation can leave a fully encapsulated housing.

Details like the large diameter or the divergence of involved laser beams are largely ignored in such simplified classification schemes. The concept is solely to classify the laser product itself according to some emission limits, rather than evaluating a particular setup containing a laser. The classification is indirectly based on some exposure limits for the eye (see above), but also takes into account a number of worst case assumptions concerning e.g. the distance of persons to the laser aperture, the exposure duration and the possible use of optical instruments. Therefore, the classification tends to overestimate certain risks very strongly, and a complete safety assessment has to consider the details of the whole setup and the way it is used.

Safety ClassSimplified Description
1The accessible laser radiation is not dangerous under reasonable conditions of use.
Examples: 0.2-mW laser diode, full enclosed 10-W Nd:YAG laser.
1MThe accessible laser radiation is not hazardous, as long as no optical instruments are used, which may e.g. focus the radiation.
2The accessible laser radiation is limited to the visible spectral range (400-700 nm) and to 1 mW accessible power. It is not dangerous for the eye in case of limited exposure (up to 0.25 s → blink reflex).
Example: some (but not all) laser pointers.
2MSame as class 2, but with the additional restriction that no optical instruments may be used. The power may be higher than 1 mW, but the beam diameter in accessible areas is large enough to limit the intensity to levels which are safe for short-time exposure.
3RThe accessible radiation may be dangerous for the eye, but is limited to 5 times the permissible optical power of class 2 (for visible radiation) or class 1 (for other wavelengths).
3BThe accessible radiation may be dangerous for the eye, and under special conditions also for the skin. Diffuse radiation (as e.g. scattered from the some diffuse target) should normally be harmless. Up to 500 mW are permitted in the visible spectral region.
Example: 100-mW continuous-wave frequency-doubled Nd:YAG laser.
4The accessible radiation is very dangerous for the eye and for the skin. Even light from diffuse reflections may be hazardous for the eye. The radiation may cause fire or explosions.
Examples: 10-W argon ion laser, 4-kW thin disk laser.

Fig. 1: International laser safety classes, with somewhat simplified and approximate descriptions. For details, consult the applicable laser safety standard documents.

Note that there are different classification schemes (e.g. international and American ones), using classes like 1 to 4 but with somewhat different definitions. (The American system uses classes I, IA, II, IIIA, IIIB and IV similar to the classes 1 to 4 of the international system, but with significant deviations.) Particularly important standards are

  • the IEC 60825-1 international laser safety standard of the International Electrotechnical Commission (IEC),
  • and those based on the US user standard ANSI Z-136 (with various variations Z-136.X, in particular the Z-136.1, which has been revised in 2007)

The IEC standard has been fully adopted by the European standardization organization as EN 60825-1 and is published in national versions such as e.g. DIN EN 60825-1 in Germany. Note that these standards cover much more than only defining safety classes; they also determine the measures to be taken in order to safely work with laser products in such classes. There are also government regulations such as e.g. the relatively outdated 21 CFR 1040.10, which is still relevant for the US, although the IEC / EN standard is now also accepted there with some additions.

Generally, it is the duty of the manufacturer of a laser product to classify the product and to accordingly equip it with warning labels. However, the classification may change when a laser product is modified by a user, and the user is then responsible for reclassification.

The Nominal Hazard Zone

Originally, the required safety measures for a given laser setup where basically determined only by the safety class of the laser. As mentioned above, this classification does not reflect details such as beam divergence, which can be very relevant for safety issues: a strongly focused laser beam can be so divergent that within a moderate distance after the focus the intensities fall below the allowable exposure level for the eye. In such situations, one sometimes defines a "Nominal Hazard Zone" (NHZ) within which safe exposure levels may be exceed, in order to apply certain restricting measures to this zone instead of the whole room.

Technical Precautions

Examples for frequently used technical laser safety precautions are:

  • the use of protective goggles (→ eye protection), strongly absorbing radiation with wavelengths near the laser wavelength
  • full or partial encapsulation of laser setups, ideally with absorbing housing materials, avoiding specular reflections
  • interlocks that automatically switch off lasers or block laser beams e.g. when a protective box or a door is opened
  • key-operated switches for power supplies, preventing unauthorized use
  • written warnings (indicating e.g. the types of lasers behind a door), warning lights (indicating that hazardous laser sources are operated) and automatic door locks, preventing people from entering dangerous areas
  • beam stoppers (not only for main beams, but also for parasitic reflections), preventing dangerous beams from leaving the optical setup
  • low-power visible pilot beams and alike, marking the paths of dangerous invisible laser beams

However, the first measure to be implemented is proper laser safety education for all staff. Personal instruction by a knowledgeable person is certainly very valuable, and should be supplemented with additional training materials such as clearly written notes, a laser safety video or DVD, etc.

Non-Technical Measures

Technical measures alone are generally not sufficient to keep safety hazards under control. A number of non-technical measures are therefore very important:

  • The risks have to be carefully assessed before anything bad can happen. They need to be reassessed every time when important circumstances change, e.g. the used devices, applications, staff, details of the room, etc.
  • On that basis, one has to develop reasonable ways of dealing with these risks. This involves the implementation of technical measures (see above) as well as establishing suitable working practices. The results need to be clearly described in written safety regulations.
  • One has to ensure adequate education, so that all people who may be at risk are properly informed about both the risks and the proper ways to deal with them.
  • All responsibilities need to be properly assigned and clearly defined.
  • It is very important to establish a spirit which motivates all staff to take safety issues serious, recognize responsibilities for themselves and for their colleagues, suggest practical solutions, etc.

Laser Safety Regulations

Making laser safety regulations for some production facility is a difficult challenge, and for a research laboratory it is even harder. The reason is that there are partially conflicting goals:

  • Regulations must be clear and understandable for those reading them.
  • The rules should be sensible, ideally under all conceivable circumstances: the implied restrictions should be so that all risks are minimized without being in straight conflict with the actual goals of the work.
  • The set of rules should be compact, so that one can expect people to read them carefully.

It is clear that various trade-offs are inevitably involved, e.g. between compactness and suitability for many different circumstances, or between safety and productivity. Giving absolute priority to maximum safety while ignoring productivity and similar practical requirements will not even serve safety, because it increases the risk of regulations being ignored.

One should therefore very carefully analyze hazards and consider the most practical ways of dealing with them. Also, one should not expect that safety regulations alone will solve the problem.

Common Obstacles

Unfortunately, reasonable laser safety regulations are either not in place or (more frequently) routinely ignored in many places such as research and development labs. Possible reasons (but no good excuses!) are:

  • a lack of general knowledge on laser safety issues
  • the lack of available information on specialized safety issues, e.g. related to potential hazards of ultrashort laser pulses from mode-locked lasers (hazard potential determined only by average power, or also by peak power and pulse duration?), or awareness of risks associated with fumes
  • unexpected effects such as accidentally misaligned beams, vaporization of poisonous substances, defects or poor design of safety equipment, etc.
  • wrong interpretation of labels like "low power laser": a 10-mW near-infrared laser may have a low power, but is still very dangerous to the eye!
  • irrational assessment of risks and inappropriate judgments on working routines ("We have always done it like this!")
  • the general human tendency to underestimate invisible risks, particularly when they occur over long times without apparent effects
  • missing safety devices (e.g. insufficient numbers of laser goggles in situations with visitors)
  • highly inconvenient, uncomfortable or otherwise impractical safety devices, e.g. laser goggles which can not be used over longer times
  • excessive pressure to produce results quickly
  • nonsensical laser safety regulations which undermine the awareness that the adherence to the rules is in the operator's own interest
  • very formal and abstract sets of rules, obviously made primarily for avoiding legal problems for the bosses, rather than providing help in real life
  • the negligence and the bad example of irresponsible supervisors, who sometimes even ridicule more responsible persons

Due to such factors, which are difficult (if impossible) to eliminate altogether, perfect laser safety (making accidents impossible) is probably impossible to reach. However, sensible regulations can greatly diminish the risks without affecting the productivity too severely. To find such sensible regulations can be a highly non-trivial task.

A few guidelines to keep in mind:

  • Thoughtful risk assessment and sensible regulations are required before accidents occur (i.e., before the work begins).
  • Regulations must be practical and convincing, because otherwise they are likely to be breached. Staying on the "safe" side by imposing unrealistic and nonsensical rules on workers will undermine the respect for the regulations, and can thus be very counterproductive.
  • All responsibilities must be clarified for everyone involved. The assignment of a laser safety officer alone (possibly as a scapegoat without sufficient time and powers for enforcement of rules) is not sufficient.
  • Stupid arguments for breaching rules, e.g. of the style "we have always done it like that" or "I know others who also do that", must be banned.
  • Formal adherence to given rules is not sufficient – operators must stay risk-aware during routine work.



[1]International Electrotechnical Commission (IEC) in Geneva, Switzerland, origin of the international laser safety standards: IEC 60825-1 ("Safety of laser products - Part 1: Equipment classification, requirements and user's guide") and IEC-60825-2 ("Safety of laser products - Part 2: Safety of optical fibre communication systems (OFCS)")
[2]American National Standards Institute (ANSI), origin of the American Z-136 safety standard series, in particular the important Z-136.1
[3]Laser Institute of America on laser safety
[4]Technical Manual on Laser Hazards by the Occupational Safety & Health Administration, U.S. department of labor

Dangerous Green Laser Pointers

Posted on 2006-12-16. Permanent link:

Ref.: encyclopedia articles on laser pointers and laser safety

Here is a safety warning which might help to prevent terrible accidents. I just learned that some people are selling quite cheap "laser pointers" via ebay, emitting green light at a power level of the order of 100 mW. Such a power level is of course well above what is reasonable for a laser pointer – about 1 mW should normally be enough. (Less optical power is required for a green laser pointer, as compared to a red one, since the eye is significantly more sensitive to green light.) Indeed it is shown that such lasers can ignite a watch, burn skin, etc.

To make things worse, there are reports that such pointers sometimes even don't have a proper filter to remove the infrared light! Without a filter, there can be an infrared power of many hundreds of milliwatts, as the green light is not directly generated with a laser diode, but rather by frequency doubling the output of an infrared solid state laser (a kind of microchip laser) which itself is pumped with an infrared laser diode.

Even without the infrared light, such devices are rather dangerous: directly looking into the beam, you can destroy your eye within a fraction of a second. Even a weak reflection is strong enough to cause an injury of the retina.

I leave it to the readers to judge how responsible it is to sell such devices as "laser pointers", not even making people aware how hazardous they are.

Thanks to Russell Grew who made me aware of this.

Green Laser Pointer Can Cause Eye Damage

ROCHESTER, Minn. — Mayo Clinic ophthalmologists have found commercially available Class 3A green laser pointers can cause visible harm to the eye's retina with exposures as short as 60 seconds. The findings will be published in the May issue of Archives of Ophthalmology,

Dennis Robertson, M.D., Mayo Clinic ophthalmologist, conducted investigations with a green laser pointer directed to the retina of a patient's eye; the eye was scheduled for removal because of a malignancy. The green laser damaged the pigment layer of the retina, although it did not cause a measurable decrease in the visual function of the patient's eye. Dr. Robertson believes that longer exposures could harm vision, however. He also warns about potential damage from higher-powered green laser pointers.

"With the use of laser pointers that are more powerful than five milliwatts, there would likely be damage to the actual vision," he says. "Functional damage could occur within seconds."

Dr. Robertson does not advocate against use of green laser pointers; rather, he advocates against their misuse. "Green laser pointers are not a public health hazard at this time, but something people should be aware of," he says. "I'm raising concerns that people should be cautious when using green laser pointers not to point them at someone's eye or face. It's like how you use your knife — carefully."

While pointing out risks of green laser pointers, he adds, "This is a potential hazard to people's eyes, but rarely is it going to be a practical hazard because the aversion reflex we have naturally will cause a person to blink or turn away from a laser light."

Green laser pointers are readily available in stores and on the Internet, according to Dr. Robertson. "Kids can buy these," he says. "They're not strictly regulated."

He adds that Class 3A green laser pointers are increasingly being used by amateur astronomers to pinpoint objects in the night sky and by the construction industry and architecture educators to point out details of structures in daylight.

Dr. Robertson conducted the eye exposure test with a consenting patient two weeks before eye removal due to ring melanoma. The patient's vision was 20/20, and the macular retina appeared healthy. Dr. Robertson exposed the patient's retina to light from a commercially available Class 3A green laser with an average power measured at less than five milliwatts: 60 seconds to the fovea, the center of acute vision; five minutes to a site 5 degrees below the fovea; and 15 minutes to a site 5 degrees above the fovea. Dr. Robertson had color photographs taken of the eye before and after exposure to the laser.

Dr. Robertson examined the patient's eye 24 hours after laser exposure. He found retinal damage characterized by yellowish discoloration involving the pigment layer beneath the fovea and at the site of the 15-minute exposure above the fovea. Each of these sites developed a grainy texture within six days. Study of the eye tissue under a microscope also confirmed damage to the pigment layer in the laser-exposed regions.

Dr. Robertson has been interested in the effects of lights on the human eye during his career, testing operating room microscopes, lights used in the clinic, red laser pointers and now green laser pointers.

Previously, he determined red laser pointers to be quite safe. "I tested different powers up to five milliwatts and could not create recognizable damage in the human eye with the red laser pointers," he explains. "So, at least a transient exposure to red laser pointers' light is only of trivial concern."

Dr. Robertson attributes the risk differential between red and green lasers to wavelength. "We know that the retina is infinitely more sensitive to shorter wavelengths," he says. "The green lasers appear much brighter to the human eye because of the shorter wavelength and can cause damage."

Dr. Robertson says Mayo Clinic's investigations have clearly demonstrated that green laser pointers can cause irreversible damage to the pigment layer of the retina.


Diode Pumped Solid State Lasers (green laser pointer)

Diode-pumped solid-state (DPSS) lasers are solid-state lasers made by pumping a solid gain medium, for example, a ruby or a neodymium-doped YAG crystal, with a laser diode.

The most common DPSS laser in use is the 532 nm wavelength green laser pointer. A powerful (>200 milliwatt) 808 nm wavelength infrared laser diode pumps a neodymium doped yttrium orthvanadate (Nd:YVO4) crystal which produces 1064 nm wavelength light. This is then frequency doubled using a nonlinear optical process in a KTP crystal, producing 532 nm light.

DPSS lasers have advantages in compactness and efficiency over other types, and high power DPSS lasers have replaced ion lasers and flashlamp-pumped lasers in many scientific applications.


The wavelength of the laser diodes is tuned by means of temperature to produce an optimal compromise between the absorption coefficient in the crystal and energy efficiency (low as possible pump photon energy). As waste energy is limited by the thermal lens this means higher power densities compared to high-intensity discharge lamps.

High power lasers use a single crystal, but many laser diodes, arranged in strips (multiple diodes next to each other in one substrate) and stacks (stacks of substrates). This diode grid can be imaged onto the crystal by means of a lens. Higher brightness (leading to better beam profile and longer diode lifetimes) is achieved by optically removing the dark areas between the diodes, which are needed for cooling and delivering the current. This is done in two steps:

1. The "fast axis" is collimated with an aligned grating of cylindrical micro-lenses.
2. The partially-collimated beams are then imaged at reduced size into the crystal. The crystal can be pumped longitudinally from both end faces or transversely from three or more sides.

The beams from multiple diodes can also be combined by coupling each diode into an optical fibre, which is placed precisely over the diode (but behind the micro-lens). At the other end of the fiber bundle, the fibers are fused together to form a uniform, gap-less, round profile on the crystal. This also permits the use of a remote power supply.

Some numbers

High power laser diodes are fabricated as bars with multiple single strip laser diodes next to each other. Each single strip diode typically has an active volume of
1 µm 2 mm 100 µm
Height Depth Width
fast axis optical axis slow axis

and depending on the cooling technique for the whole bar (100 to 200) µm distance to the next laser diode.

The end face of the diode along the fast axis can be imaged onto strip of 1 µm height. But the end face along the slow axis can be imaged onto a smaller area then 100 µm. This due to the small divergence (hence the name: 'slow axis') which is given by the ratio of depth to width. Using the above numbers the fast axis could be imaged onto a 5 µm wide spot.

So to get a beam which is equal divergence in both axis, the end faces of a bar composed of 5 laser diodes, can be imaged by means of 4 (acylindrical) cylinder lenses onto an image plane with 5 spots each with a size of 5 mm x 1 mm. This large size is needed for low divergence beams. Low divergence allows paraxial optics, which is cheaper, and which is used to not only generate a spot, but a long beam waist inside the laser crystal (length = 50 mm), which is to be pumped through its end faces.

Also in the paraxial case it is much easier to use gold or copper mirrors or glass prisms to stack the spots on top of each other, and get a 5 x 5 mm beam profile. A second pair of (spherical) lenses image this square beam profile inside the laser crystal.

In conclusion a volume of 0.001 mm³ active volume in the laser diode is able to saturate 1250 mm³ in a Nd:YVO4 crystal.


Green Laser Pointer

Green laser pointers[2] appeared on the market circa 2000, and are the most common type of DPSS lasers (also called DPSSFD, diode pumped solid state frequency-doubled). They are much more complicated than standard red laser pointers, because laser diodes are not commonly available in this wavelength range. The green light is generated in an indirect process, beginning with a high-power (typically 100-300 mW) infrared AlGaAs laser diode operating at 808 nm. The 808 nm light pumps a crystal of neodymium-doped vanadate (or Nd:YAG or less common Nd:YLF), which lases deeper in the infrared at 1064 nm. The vanadate crystal is coated on the diode side with a dielectric mirror that reflects at 1064 nm and transmits at 808 nm. The crystal is mounted on a copper block, acting as a heatsink; its 1064 nm output is fed into a crystal of potassium titanyl phosphate (KTP), mounted on a heatsink in the laser cavity resonator. The orientation of the crystals must be matched, as they are both anisotropic and the Nd:YVO4 outputs polarized light. This unit acts as a frequency doubler, and halves the wavelength to the desired 532 nm. The resonant cavity is terminated by a dielectric mirror that reflects at 1064 nm and transmits at 532 nm. An infrared filter behind the mirror removes IR radiation from the output beam, and the assembly ends in a collimator lens. The output power of most green laser pointers is on the order of 5 mW. In 2007, the Guinness Book of World Records published a new world record called "Most Powerful Handheld Laser" which was awarded to Shanghai-based Wicked Lasers for their 200-300 mW Spyder series lasers. [3]

Nd:YVO4 is replacing Nd:YAG and Nd:YLF due to lower dependency on the exact parameters of the pump diode (therefore allowing for higher tolerances), wider absorption band, lower lasing threshold, higher slope efficiency, linear polarization of output light, and single mode output.[4] For frequency doubling of higher power lasers, LBO is used instead of KTP. Newer lasers use a composite Nd:YVO4/KTP crystal instead of two discrete ones.

Some green lasers operate in pulse or quasi-continuous wave (QCW) mode, to reduce cooling problems and prolong battery life.