WARNING: The information in this chapter should NOT be considered a substitute for a comprehensive course in laser safety. Casual reading and common sense precautions may be adequate when dealing with low power visible CW lasers but is totally useless for anything above a few milliwatts and for invisible or pulsed lasers, as accidents will happen. And, if an accident means a beam in your eye, damage may very likely be irreversible. As in permanent. As in, some portion of vision in the affected eye(s) will be gone forever. Only classroom instruction with an associated hands-on laser lab can develop and enforce the required procedures and habits that will apply to a wide variety of laser equipment.
Here we only discuss the hazards with respect to vision. There are other safety issues - such as the danger from the high voltages used to power certain types of laser. These are summarized later in this chapter and dealt with in more detail in the chapters on the lasers for which they apply. There are several reasons that even small lasers which do not represent any sort of burning or fire risk can instantly and permanently damage vision:
A cheap laser pointer also produces a highly collimated beam.
Even at power levels considered relatively safe, one shouldn't deliberately stare into the beam for any reason. For these relatively low power lasers, permanent eye damage is not that likely but why take chances? For these lasers, viewing the spot projected on a white surface is perfectly safe.
For example, at 10 cm from a 100 W bulb (which would be a very uncomfortable
place to be just due to the heat), the power density of the visible light
(assuming 5 total watts) would be only about 0.05 mW/mm2. At 1 m,
it would be only 0.0005 mW/mm2 or 500 mW/m2. Based on
this back-of-the-envelope calculation, a 5 mW laser beam spread out to a
circular spot of 0.1 m diameter (i.e., 1 mR divergence at a distance of 100 m
- without external optics) will appear brighter than the 100 W light bulb at
1 m! And, close to the laser itself, that beam may be only 1 *mm* in
diameter and thus 10,000 times more intense! (And note that the other
invisible radiation that passes through to the back of the eye is still
not nearly as dangerous as the beam from the 1 mW laser because it isn't
focused to a tiny spot by the lens.)
For example, at 10 cm from a 100 W bulb (which would be a very uncomfortable place to be just due to the heat), the power density of the visible light (assuming 5 total watts) would be only about 0.05 mW/mm2. At 1 m, it would be only 0.0005 mW/mm2 or 500 mW/m2. Based on this back-of-the-envelope calculation, a 5 mW laser beam spread out to a circular spot of 0.1 m diameter (i.e., 1 mR divergence at a distance of 100 m - without external optics) will appear brighter than the 100 W light bulb at 1 m! And, close to the laser itself, that beam may be only 1 *mm* in diameter and thus 10,000 times more intense! (And note that the other invisible radiation that passes through to the back of the eye is still not nearly as dangerous as the beam from the 1 mW laser because it isn't focused to a tiny spot by the lens.)
See the section: Laser Safety Sites for links to much more information on general laser safety, laser safety organizations, and regulatory agencies.
And since laser pointers seem to be everywhere these days, consider this: If carefully focused, as little as 5 or 6 mW from a laser is sufficient to produce burn marks on black electrical tape along with wisps of smoke. Think about what similar power levels can do to the delicate tissue at the back of your eyeballs! While laser pointers themselves may not be quite as dangerous as some people (and politicians) may have you to believe, that such macroscopic effects can take place at these relatively modest power levels should provide some additional respect for the damage that can result under just the wrong set of conditions.
A popular graveyard joke in the laser industry is: "Do not stare into the beam with your remaining good eye". Another one is: "How many times can I look into a laser beam?". Answer: "Twice, once left, once right". Or see Peer Pressure in the Laser Lab from David Farley's Doctor Fun Archive. Nonetheless, laser safety is no laughing matter.
Typical 1 mW HeNe laser (or laser pointer):
So the 1 mW laser has the potential to produce an intensity on the retina 167 times that of direct sunlight! But there are many more factors to consider in determining the real risk of damage. In addition to those noted below, the actual focal point when looking at a laser at close range will not be at the retina so the spot size will most likely be much larger than the diffraction limit of the calculation. Even if the spot from the laser beam is smaller, natural eye movements or movement of the source (e.g., some moron waving a laser pointer) will result in it hitting any given point for a shorter time than the larger spot from the Sun (which usually doesn't move very quickly).
But, at least, perhaps you'll now have a bit more respect for that little HeNe laser or laser pointer!
(From: Jim Webb (email@example.com).)
The real problem behind this is that it is assumed that the power density is the significant factor in the thermal damage mechanism. The ability of the retina to dissipate heat is not dependent on the area covered, but the periphery (circumference) of the exposed area! The blood vessels are in the retina and not the sclera (the surface under the retina) - it is the blood flow that dissipates the heat and so can only act on the *edge* not the middle of the exposed area. In circumference terms, the ratio drops to 7 times. Furthermore because the larger spot is less efficient at dissipating heat, the effective power delivered by the laser beam is only about 2 times greater than that of the spot formed by the sun.
(Portions of the following from: Don Klipstein (don@Misty.com).)
The fraction of light entering the eye for a large diameter beam is pupil area divided by beam area.
Assuming a pupil diameter of 1/4 inch (6.3 mm, rather dilated but not fully dark adapted which may approach 1 cm). The portion of the beam entering the eye would then be the square of (1/4)/(48), which is about 27 millionths of the total. Since the 4 foot diameter beam is not uniform but dimmer towards the edges, I would say the eye could get about 35 millionths of the beam near the center or 35 nanowatts (35 nW).
Note that close to the laser, the pupil size is going to be larger than the beam diameter (which is typically less than 1 mm) and pupil size larger than this will not affect the maximum possible power entering the eye (though it will affect the probability of this occurring. (One suggested laser safety practice is to brightly illuminate the laser lab to make your pupils smaller. Even though there are times this will not reduce the severity of the worst case, a smaller target reduces likelihood of this happening.)
However, where the beam diameter is equal to or larger than the pupil diameter, the difference in pupil diameter between bright and dark adapted eyes will be very significant - more than a 30-fold difference in power entering the eye for this analysis.
I calculate that a 4 foot diameter 1 mW 632.8 nm beam appears about as bright as a 100 W bulb does 88 feet away.
Although 35 nW is definitely eye-safe, it may look quite bright against pitch black surroundings especially when the eye is fully dark adapted (the pupil is wide open and the combined retinal/neural sensitivity is maximum as it is after awhile when out at night) and may quickly result in a noticeable afterimage. The effect is probably enhanced by the knowledge that the light source is a laser and thus potentially damaging to your eyesight.
As a side note, the 1,710 lumen output of a typical 100 Watt incandescent bulb is about the same lumens as *10 Watts* of 632.8 nm light!
Also see the section: How Much Light Does a 5 W Laser Really Produce?.
Note that even a wavelength considered eye-safe like 1,500 nm (1.5 um) is only safe in the sense that this light won't penetrate to the back of the eye and be focused on the retina. A high enough power density can still obliterate the cornea and/or lens!
(From: Paul Mathews (firstname.lastname@example.org).)
There are a variety of problems with doing experiments to determine safe levels of optical radiation incident on the eye. Here are some:
When you graduate to higher power lasers (e.g., argon ion) rated Class IIIb or more, additional very real dangers are present of both instant damage to vision and with Class IV lasers - the possibility of burning or setting fire to flesh and other things. The smallest CO2 laser is going to be rated Class IV!
Higher power diode lasers (above 5 mW) are becoming more readily available both as surplus or pulls from optical drives and high performance laser printers, and also at not totally unreasonable prices even new. Their small size may lead one to assume that a diode laser can't be dangerous. WRONG! A 100 mW laser diode operating on battery power can blow a hole in your retina as easily as a 100 mW argon ion laser consuming the same electrical power as a space heater! And, higher power laser diodes are more likely to be infra-red (IR) and invisible - and thus more dangerous because the aversion response won't work - you have no idea your vision is being destroyed until it's way too late! (CO2 lasers are also IR but the much longer wavelength will only vaporize the front of your eye since the beam is blocked by the cornea.)
In addition to their vision hazards, gas lasers generally use high voltage or line connected power supplies so there is the added shock hazard resulting from touching or accidentally coming in contact with uninsulated connections. See the document: Safety Guidelines for High Voltage and/or Line Powered Equipment before working on any type of equipment which uses line voltage or produces high voltage. (With diode lasers, you can easily fry the laser diode but the low voltage power supplies don't generally pose much of a shock hazard.)
Furthermore, you may come across a truly high power CO2 or argon ion laser, or even a 100 mW HeNe laser tube. These, rated at the upper end of Class IIIb or Class IV, represent even more significant risks of both instant permanent eye damage even from momentary reflections from shiny (specular) surfaces as well an actual fire hazard. The possibility of electrocution from their power supplies is correspondingly greater as well. You must handle them properly for your own safety and the safety of others around you and your surroundings.
See the specific chapters on each of these types of lasers for additional hazards and precautions Note that other people in the area may actually be more likely to get caught by the beam. The reason? You will be aware of what NOT to look at while they will be looking in the direction of the action not having a clue of what to expect! Don't take chances.
The following very large number is designed to impress: The power density of a 1 mW laser beam when focused to a spot of around 2 um (which isn't difficult with a simple convex lens) is around 250,000,000 W per square meter! Don't let that spot be in the back of one of your own or someone else's eyeballs!
Be extremely careful when working with any laser!
(From: Mike Poulton (email@example.com).)
A 1 mW diode will probably not cause damage if you briefly look into it, but I wouldn't encourage you to try it. While it probably won't do anything bad, it is not good to become comfortable with the idea of checking the operation of lasers by looking into them. If you are a hobbyist who uses lasers quite a bit, there is a good chance you will, at some point, end up with an unmarked diode. It could emit any wavelength at any power level, and how bright the beam appears when you shine it on something has no bearing on the power level. Looking into an unmarked diode just because the beam is dim could (and probably will) have disastrous results. I have a 1 W 808 nm laser diode, and it appears much dimmer than a .5 mW 670nm beam when focused into a .2 mm spot. When focused in that way, it will easily engrave plastic and burn paper and wood (and skin). Just because it looks dim doesn't mean it won't instantly blind you.
(From: Daniel P. B. Smith (firstname.lastname@example.org).)
Be aware that eye damage that is localized to a small area of the eye is not very noticeable. For example, few people ever notice the existence of the large blind spot where the optic nerve enters the eye even though it is rather huge (10 degrees or so) and not all that far from central vision. A laser wouldn't necessarily have to make you totally blind; it could just wipe out a teeny patch here and a teeny patch there. This kind of damage would be very insidious; each time you'd say "Wow! That was bright! lucky I didn't get blinded" - while slowly and cumulatively losing your sight...
(Portions from: Lynn Strickland (email@example.com).)
In addition to laser equipment and laser safety gear manufacturers, large laser surplus outfits often have some minimal selection of laser safety goggles, but those that are available will probably cover the types of lasers you are using. However, they may not have all the regulatory approvals - that's one of the things that boost prices! :) Also be careful whether the eye wear is designed for diffuse viewing only, or will withstand a direct hit from the laser. Know what you are getting - the worst thing is to think you are protected when you are not. Or, to become so disgusted with the reduction in visual acuity and clear view resulting from poorly made or mismatched goggles that you end up not using them at all!
When adjusting or aligning a laser with the covers off, beware of reflections from all optics surfaces. Those inside the laser cavity will have optical power densities much higher than that of the output beam making even a small percentage of reflection significant. For example, an argon ion laser outputting a few hundred mW can have 10 or 20 mW reflected from each Brewster window in two directions. These may be non-existent or weak when you start out but can appear suddenly as adjusting screws are turned. The risks are even more significant with a laser producing an invisible beam. Where possible, put sleeves around the Brewster windows and block reflections from other optics while the laser's innards are exposed.
Keep in mind that what gets reported in the popular press is not exactly what you would call rigorously reviewed for scientific accuracy. And, if it turns out that the outcome wasn't quite as reported originally, any correction for a front page story is usually to be found in fine print buried on page 17! Actual substantiated instances of long term or permanent effects on vision resulting from momentary or unintentional exposure to a laser pointer's beam - or even from prolonged intentional misuse - appear to be all but non-existent. Flash blindness IS possible, but this is temporary and will clear up on its own.
The above applies where the laser pointer has been manufactured and tested to meet CDRH Class IIIa safely limits or below. Note that where these devices originate from countries with less rigorous quality control or where an internal current adjust pot can be twiddled or even if run at very cold temperatures where laser diode output power is greater, to risk of eye damage from intentional abuse, at least, may increase.
With respect to direct personal danger, potential damage to vision is the only real consideration - there is no risk from radiation or enough power in a beam of less than 5 mW to burn anything. However, from a public policy and regulatory perspective, there are actually three areas of concern:
I am in favor of tough laws to make (2) and (3) crimes and require at least full restitution (maybe even 2X or 3X) for any resulting damages in addition to disciplinary action or jail time. Such behavior should not be tolerated. However, in the remainder of this section, I only really want to address the vision issues (1).
While I absolutely agree that intentionally aiming a laser of any kind into someone's eye is basically stupid (unless you are having laser eye surgery), one must be careful in interpreting the meaning of press reports that describe momentary exposure to the beam from a laser pointer waved around an auditorium resulting in instant total loss of vision in all three eyes. One would have to direct the beam into the pupil of the eye from a close distance for a few seconds or more without either the eye or pointer moving, twitching, or blinking. Distance is significant both because even laser pointer beams diverge (especially cheap ones) so less energy is able to enter the pupil of the eye as the source moves further away and it is harder/less likely for it to remain stationary and centered on such a target a few mm across. This is not really possible by accident and even takes significant effort to do intentionally since the eye's natural pupil contraction, blink, and aversion reflexes will prevent the beam from focusing on a single spot on the retina with a sufficient concentration of energy for more than an instant - not enough time for damage to result. There would have to be cooperation which can only really happen in a game of chicken - but it is hard to protect people from their own stupidity. This does mean, however, as if it isn't already obvious, that laser pointers should be kept from infants - period, and away from children unless adequately supervised. Adults, on the other hand, presumably know not to stare into painfully bright lights and some may even read the warning labels!
Though momentary exposure may indeed result in temporary flash blindness, disorientation, multiple afterimages, and a headache, such effects, while not to be minimized in importance, should not be permanent. And, as the distance between the eye and the pointer increases, their severity and duration diminishes greatly. To suggest any long term eye injury from a pointer's beam originating on the other side of a football stadium is simply not plausible.
In fact, despite the great amount of press coverage lately - and such reports resulting in the passage of laws in some places banning laser pointer sales to minors (or to anyone), there are very few if any confirmed reports of permanent vision damage attributable to these things. The irresponsible aiming of a laser pointer at a person that might result in tragic consequences from distraction or misinterpretation of intent is far more likely to be a problem in today's world - and justifiably so.
Laser pointer manufacturers and resellers make all sorts of claims about power levels and there may be deliberate (power is, after all, a major feature) or unintended (due to poor quality control) sale of devices with power even beyond the approved safety limits and these could indeed be much more dangerous. However, simply enforcing existing regulations could go a long way toward reducing this possibility. But, of course, the prices would likely go up if more sophisticated laser power control circuitry were required and every unit had to be more fully tested, adjusted, and certified to be compliant.
To further minimize the chance of vision damage, I think a maximum power limit of 1 mW would be more than adequate for most purposes with the newer 635 nm pointers. These appear 5 to 7 times brighter than previous 670 nm models and green laser pointers (which should be available at affordable prices in the not too distant future) will appear even brighter by another factor of 5 or so. Staring into the noonday Sun would result in the same order of magnitude of power focused on the retina as a 1 mW laser pointer against your eyeball and we don't even bother to regulate THAT! :)
Don't get me wrong - I am definitely NOT recommending that laser pointers be treated as toys and handed out to all the neighborhood kids as party favors. They can still be dangerous and at least a niusance even if eye injury isn't the primary risk. I fully agree that any use of such a device in a way that annoys other people or puts them at risk - even if it is a small risk - is valid grounds for confiscation and possible severe disciplinary action.
For that matter, how come no one has banned butane lighters or matches? :-) They are cheaper, more readily available, and certainly result in more injury, death, and destruction in the hands of kids than laser pointers! Or, how about cigarettes.... Sorry, I will get off my soap box now....... No, I don't expect an answer. :-)
For more information on laser pointer safety, check out the following:
The second paper describes research that was unable to find any permanent damage to the retina from Class IIIa exposures of up to 15 minutes. This was on volunteers who were to have their eyeball removed due to cancer.
And for someone advocating a total ban (includes some useful links):
Also see the other links in the section: Laser Safety Sites.
(From: Gregory Makhov (firstname.lastname@example.org).)
As chair of the ILDA (International Laser Display Association) laser safety committee, I have been carefully following the thread on laser pointer safety (in the sci.optics newsgroup - search via Google Groups (formerly Deja.com/Dejanews) for the complete saga). I have seen most of the articles in the press on laser incidents/accidents in the UK. If you have a source of factual evidence concerning these 'injuries', I would greatly appreciate the information. My own experience with laser pointers would indicate that a level of 5 milliwatts and below is unlikely to cause injury unless self-inflicted and for a substantial duration (several seconds). I say self-inflicted, as it is unlikely that another person could direct the laser accurately into someone's eye at any significant range. Almost immediately after the initial exposure to the beam, the pupil shrinks to a very small size (a few millimeters) which is an awfully small target to illuminate from a distance of even a few meters.
However, if there is any medical evidence of these injuries, and some documentation of how they occurred (laser power, range, duration, etc.) I am most interested.
You can tell the difference between the types of scanners by the color of the light. The beam of the diode laser based scanners will appear a much deeper red than that of a HeNe laser based unit. (If you are into lasers, this is one of the 'rites of passage' so to speak - to check out the local groceries and supermarkets!) Of course, the other way to tell is that if your store installed checkout scanners when the UPC was new technology, and hasn't upgraded since, they are almost certainly based on HeNe lasers. (Barcode scanners of all types, shapes, and sizes are often available from surplus outfits as well as on-line auctions like eBay. At the right price, they represent an excellent source of laser and optics related parts - even if you don't want to use the unit for their intended purpose.)
For wavelengths within the visible spectrum and near IR where the cornea, lens, and vitreous of the eye are transparent, 1 mW is the same amount of power whether it is near IR, red, or green. There will be slight differences in damage threshold depending on wavelength (spot size on the retina, absorption) but green is really not more dangerous than red, mW per mW for a beam that reaches the back of the eye. Since green light at 555 nm *appears* about 30 times brighter than red light at 670 nm, the green laser may actually be slightly less of a hazard since you will likely respond to it faster (and, in the case of laser pointers in particular, a lower power unit may be adequate).
Beyond the visible - IR and UV - there are other issues. UV laser light, like UV Sunlight can indeed have effects beyond just those due to the power density. Fortunately, there aren't likely to be UV any laser pointers any time soon even if there were a use for them (phosphorescent white boards?)! :-) Most other UV lasers (excimer, helium-cadmium, frequency quadrupled YAG, etc.) are not that common either (at least not that the typical hobbyist will acquire). However, should you consider building the nitrogen laser (among the easiest of home-built lasers), its output is at 337.1 nm which is near-UV (UV-A range).
Near IR is perhaps the most dangerous since it progressively less visible the longer the wavelength starting at about 1/250th visibility compared to 555 nm and going down to 3E-14 visibility (estimated) at 1,064 nm. Yet, until well beyond this (maybe 1,500 nm), the light can still pass through the anterior structures of the eye to reach the retina and will focus reasonably sharply despite not being visible. There will be no blink or aversion reflex so damage can be done even for modest power lasers without any immediate symptoms. Only later, will the pretty patterns engraved on your retina(s) become evident (since your brain will initially tend to fill in and mask their effects). And, they won't go away - ever!
At mid IR, the beam can still penetrate to the lens, heating it, which may produce a cataract. Far far IR such as the 10.6 um (10,600 nm) from a carbon dioxide (CO2) laser is effectively absorbed and blocked by the cornea of the eye - and it can be damaged in a similar way. And, almost all CO2 lasers produce enough power (a few W to 10s of kW) that they are also hazardous with respect to burning things (including other types of flesh) as well as actually setting fires.
The long and short of it is that there is a threshold of laser power that will be dangerous in various ways at ANY wavelength and no laser can be treated as totally safe until the detailed specifications of the laser and its optical system are known.
Unless your laser is set up for harmonic generation, there will be no higher frequency radiation in the beam. The only accidental source of harmonics that could pose a risk would be for the beam from a high peak power pulsed laser (most likely it would need to be Q-switched) to pass through a non-linear material that has good optical quality AND for the reflection from some surface downstream to hit you in the eye. Materials with both those properties do not occur naturally. Aiming the beam at common plastics, glasses, and crystals won't produce significant, if any, harmonics. Finding a household material that does so could be an important discovery. :)
To be doubly sure, you can buy goggles that protect for both the fundamental and doubled outputs (e.g., 1,064 and 532 nm, harmonics above SHG would be virtually impossible.) But they will be darker than those for the fundamental along and thus less desirable. They are also more expensive. If you intend to experiment with SHG, etc., or acquire a such a laser, that could be a worthwhile investment.
A great deal of laser safety is in good work practices (outlined elsewhere in this chapter). Goggles are just the last resort when everything else goes wrong. However, for pulsed lasers where the entire beam path isn't enclosed, they really are essential.
(From: Don Klipstein (don@Misty.com).)
While thumbing through some gel filter sample packs, it has occurred to me that there are neutral density gel filters - and that they are not truly neutral. Both Gam and Rosco ones are somewhat neutral through to about 700 nm - and become more transparent as wavelength increases through the low and mid 700's. They are nearly transparant above about 750 nm.
They also have a slight peak at 380 nm, where they are a bit more transparent than they are to visible light. Transmission at 380 can exceed the average visible transmission for darker grays.
This is because these filters are made gray with some kludge of dyes rather than something truly neutral-density. They also do not equally attenuate all visible wavelengths; they have transmission peaks around 480 (greenish blue) and 600 (orange), and absorption peaks around 450 (mid-blue) and the mid 500's (yellowish green). Different brands may have some differences, as well as having some similarities. They probably have some but not all dyes in common.
I do not know whether the infrared transparency is an unavoidable consequence of dying plastics/gels, or something intentional to reduce filter heating. I do know that the colored filter gels are also nearly transparent to most wavelengths from the upper 700's (sometimes low 700's) through probably at least around 1500 nm.
Because of this, dark filter gel combinations are probably unsafe for directly viewing the sun, and are probably unsafe for attempting to protect eyes from infrared lasers.
I have one of those $50 video cameras that are sold by various electronics distributors. This particular one is listed for IR and comes with 4 IR LEDs (IREDs) for illumination (which I removed). It works fine except that there is no way to defeat the automatic gain control so it gets confused with very bright sources like lasers. I have also been given a very nice digitally controlled color CCD camera. This has a Windows interface and provides full control of gain, offset, and other parameters. For low power lasers, this can be used without a lens viewing the beam diractly. Where there is a risk of damage to the CCD, the beam is projected on a screen.
(From: Dave (email@example.com).)
I use a CCD video camera with the proper filters in line to balance the sensitivity to the laser lines (e.g., 808 nm pump, 1,064 nm IR beam, 532 nm green beam) and mount this outside of a cardboard box(helmet) with a 9" LCD flat-panel display mounted on the inside. With this contraption over my head I can see everything clearly and without any worry of eye damage and have both hands free. My first version was my autofocus digital camera in a box but the screen was too small for long duration work.
I would have to say that proper eye protection is much more important than any laser component. This cannot be stressed enough. There have been some interesting demonstrations performed showing the effects of high optical power densities on meat (think lots of smoke and some flame). The ones I've seen on videotape were spectacular, and were more than enough to convince me that proper beam blocks and eye/body protection are mandatory.
When in doubt, be overly safe.
You should have enough pairs of laser goggles for everyone in your laser lab! After all, what's the fun of a laser if you can't show it off to your friends? ;)
Now, about the ratings of goggles in terms of optical density.
Optical densities are reasonably easy to understand. To determine the fraction of optical power transmitted through a material of optical density D, divide 1 by 10 raised to the D power. Or, if D is an integer, just write a zero followed by a decimal point, followed by D-1 more zeros, followed by a 1. This is the fractional transmittance of the material. Multiply by 100 if you want a percentage.
For our O.D. 5+ goggles above, this means that less than 1/100,000 of the incident power will pass through the goggles, the remainder either being reflected or absorbed. For a 100 Watt laser with a 1 square centimeter beam (power density 100 W/cm2), the transmitted power density should be 0.001 W/cm2 (or 1 milliwatt per square centimeter). I have to locate my safety sheets to see what the exposure limit is for eyes and skin under a 1 mW/cm2 beam at 10.6 microns. I suspect it is eye and skin safe, but without a good reference, I'm not betting my body parts on it. ;)
Keep in mind that the O.D. is rated AT A SPECIFIC WAVELENGTH OR RANGE OF WAVELENGTHS! Deviations from this wavelength will results in completely different O.D. values. If the goggles use some type of interference coatings, then at some wavelengths the coatings may have an effective O.D. of ZERO, meaning they are completely transmissive. Don't expect CO2 goggles to protect you from an argon laser beam.
During the 1st Trimester of the laser program, long before the student is allowed near so much as a HeNe laser, the students are shown "the monkey film." This is one of the films that would drive PETA nuts. All you see is this eyeball, which we are told belongs to a monkey that is strapped down and anesthetized. We are told that an IR beam of a certain power has been turned on, and is striking the eyeball. After an eternity (a second or 2), a small spot appears on the bottom of the eyeball. Then, the spot rapidly expands, forming an ulcer that covers much of the bottom of the monkey's eye. I don't know if that was in slow motion or not.
We also read accident reports from the field. There was a technician who was working on an extremely powerful laser. He had removed his eye protection, and walked across the room. There was a weak stray specular reflection that struck one of his eyes, immediately causing permanent damage to that eye. He did not lose all of his sight in the eye, but he did lose part of his field of vision.
From the first time that the laser students operate a HeNe laser, they are required to treat the beam as lethal. Under no circumstances are they permitted to break any beam of any power with any part of their body. Our little HeNe beams could not cause damage to skin, but we had to act as if they would cut off our arms. The student is also responsible to ensure that he knows where all the parts of the beam go, and to block the beam appropriately.
The HeNe laser labs had curtains across the doorways, which we closed before beginning experiments. The Argon Ion and Nd:YAG labs had solid doors, and there were sensors in the doors that would cut off the power to the lasers if the door were opened. There was also a red warning light that was to be turned on when the laser was in operation.
Normally in the laser show world, you deal with eye injury, lasers up to about 5 watts or so, typically only a few hundred mW though. A few hundred mW on your skin simply "looks cool", while 30 watts will quickly blow a hole right through your whole hand and out the other side! The worst thing about this is it is absolutely painless. Not black burned skin, but white ablated skin. Blows a hole right through, not even smoke is left behind. You don't feel the pain from such an injury for many minutes AFTER, then it's excruciating! I won't even go into what would happen if you took 30 watts into the eye directly. Also, most of us have seen bright laser spots on white surfaces (projection screens) up close, and know how "blindingly bright" they are, but also that the plain-air beams are invisible. Imagine a laser where the plain-air beams hurt the eye to look at! :) The spot on a surface is so bright as to light a room up as though it was bright sunlight (in green) given that the spot was expanded to around 30 mm so as not to burn a hole in the surface.
Several years ago there was a long thread on the USENET newsgroup rec.guns where people posted their stories about all the accidents or near accidents they had experienced with firearms. These were all seemingly intelligent people like computer programmers and scientists and engineers. Still, while dealing with a simple device with only a few knobs, they managed somehow, sooner or later, and while trying to obey all the safety rules, to blast a hole in something or someone. This was very educational reading.
There was a really good story recently posted on sci.optics. Some guy was working with a laser and then took off his goggles, and blew out some of his eye. Dumb. Then, rather than realizing that the goggles don't work if they are not worn, he decided that he just wouldn't wear them at all, and he would Be Real Careful. This is called "People Who Don't Learn From Their Mistakes". Let's hope he doesn't take up firearms.
When you have goggles on (assuming that they are the right kind, and you should make damn sure they are), you have very good protection against loss of vision. When you take them off, you don't. A movement of a mirror, lens, or baffle can cause a specular reflection, total internal reflection, or refraction right into your eye. This isn't something to anticipate -- that's why it is called an accident. Even with all the appropriate precautions, accidents can still happen.
Imagine that you are working with the laser off, aligning some mirror, no goggles, and you spill your coffee over the on-off switch to the laser power. Oops. Collect insurance.
The best discussion of the various classifications, plus general treatment of the topic, is a book by Sliney and Wolbarsht, "Safety with Lasers and Other Optical Sources", Plenum Press, New York. While they will agree with each other in most respects, some differences will result in a particular laser changing classes depending on which standard is used. The major criteria are summarized below.
The following is based on material from the University of Waterloo - Laser Safety Manual.
All lasers are classified by the manufacturer and labelled with the appropriate warning labels. Any modification of an existing laser or an unclassified laser must be classified by the Laser Safety Officer prior to use. The following criteria are used to classify lasers:
Lasers are generally classified and controlled according to the following criteria:
Here is another description, paraphrased from the CORD course: "Intro to Lasers". (Cord Communications. Lasers.) It relates the laser classifications to common laser types and power levels:
Maximum power less than 0.4 uW. This will not cause damage even where the entire beam enters the eye and it is being stared at continuously.
Maximum power less than 1 mW for HeNe laser.
HeNe laser power 1.0 to 5.0 mW.
Visible Argon laser power 5.0 mW to 500 mW.
The classifications depend on the wavelength of the light as well and as noted, there may be additional considerations for each class depending on which agency is making the rules. For example, the NRPB (British) adds a requirement for Class IIIa that the power density for a visible laser not exceed 25 W/m2 which would thus bump some laser pointers with tightly focused beams from Class IIIa to Class IIIb. For more information on laser pointer safety and the NRPB classifications, see the NRPB Laser Pointer Article.
In the US, start with the Center for Devices and Radiological Health (CDRH), part of the Food and Drug Administration (FDA). See the section: Regulations for Manufacturers of Lasers and Laser Based Equipment for more info on how to find the relevant guidance documents.
For additional information on laser safety and laser safety classifications, see the section: Laser Safety Sites (May Also Include Other Laser Information).
Here are some excerpts from the Center for Devices and Radiological Health (CDRH) regulation 21 CFR 1040.10 and 21 CFR 1040.11, the standard classification for lasers are as follows with some additional comments by Wes Ellison (firstname.lastname@example.org):
No known biological hazard. The light is shielded from any possible viewing by a person and the laser system is interlocked to prevent the laser from being on when exposed. (large laser printers such as the DEC LPS-40 has a 10 mW HeNe laser driving it which is a Class IIIb laser, but the printer is interlocked so as to prevent any contact with the exposed laser beam, hence the device produces no known biological hazard, even though the actual laser is Class IIIb. This would also apply to CD players and small laser printers, as they are Class I devices).
Power up to 1 milliwatt. These lasers are not considered a optically dangerous device as the eye reflex will prevent any occular damage. (I.e., when the eye is hit with a bright light, the eye lid will automatically blink or the person will turn their head so as to remove the bright light. This is called the reflex action or time. Class II lasers won't cause eye damage in this time period. Still, one wouldn't want to look at it for an extended period of time.) Caution labels (yellow) should be placed on the laser equipment. No known skin exposure hazard exist and no fire hazard exist.
Power output between 1 milliwatt and 5 milliwatt. These lasers can produce spot blindness under the right conditions and other possible eye injuries. Products that have a Class IIIa laser should have a laser emission indicator to tell when the laser is in operation. They should also have a Danger label and output aperture label attached to the laser and/or equipment. A key operated power switch SHOULD be used to prevent unauthorized use. No known skin hazard of fire hazard exist.
Power output from 5 milliwatts to 500 milliwatts. These lasers are considered a definite eye hazard, particularly at the higher power levels, which WILL cause eye damage. These lasers MUST have a key switch to prevent unauthorized use, a laser emission indicator, a 3 to 5 second time delay after power is applied to allow the operator to move away from the beam path, and a mechanical shutter to turn the beam off during use. Skin may be burned at the higher levels of power output as well as the flash point of some materials which could catch fire. (I have seen 250 mW argons set a piece of red paper on fire in less than 2 seconds exposure time!) A red DANGER label and aperture label MUST be affixed to the laser.
Power output >500 milliwatts. These CAN and WILL cause eye damage. The Class IV range CAN and WILL cause materials to burn on contact as well as skin and clothing to burn. These laser systems MUST have:
A key lockout switch to prevent unauthorized use Inter-locks to prevent the system from being used with the protective covers off, emission indicators to show that the laser is in use, mechanical shutters to block the beam, and red DANGER labels and aperture labels affixed to the laser.
The reflected beam should be considered as dangerous as the primary beam. (Again, I have seen a 1,000 watt CO2 laser blast a hole through a piece of steel, so imagine what it would do to your eye !)
Any laser system that has a power output of greater than 5 milliwatts MUST be registered with the FDA and Center for Devices and Radiological Health if it has an exposed beam, such as for entertainment (I.E. Laser light shows) or for medical use (such as surgery) where someone other than the operator may come in contact with it. (This is called a 'variance' and I have filled them out and submitted them and they ARE a royal pain in the backside!)
Sometimes, you will come across a laser subassembly that has a sticker reading something like: "Does not Comply with 21 CFR". All this means is that since the laser was mounted inside another piece of equipment and would not normally be exposed except during servicing, it does not meet all the safety requirements for a laser of its CDRH classification such as electrical interlocks, turn-on delay, or beam shutter. This label doesn't mean it is any more dangerous than another laser with similar specifications as long as proper precautions are taken - such as adding the missing features if using the laser for some other purpose!
(From: Johannes Swartling (Johannes.Swartling@fysik.lth.se).)
It is not the laser in itself that is given a class number, but the whole system. A system which is built around a very powerful laser can still be specified as Class I, if there is no risk of injury when operating the system under normal conditions. For example, CD players are of class I, but the (IR) laser diode may in itself be powerful enough to harm the eye. CD players are designed so that the laser light won't escape the casing.
When it comes to laser safety and exposure levels the regulations are fairly complicated and I will not go into details. Basically, there are tables with 'safe' levels of exposures. The exposure has to be calculated in a certain way which is unique to each case, depending on among other things: laser power, divergence, distance, wavelength, pulse duration, peak power, and exposure time.. Although it is true that near infrared lasers are potentially more dangerous than visible because you can't see the radiation, it is incorrect to say that it must be, say, Class III. The level of exposure may be so low that it can be a class I (note that Class II lasers are always visible though, so infrared lasers are either of Class I or Class III or higher).
You can probably get a copy of the regulations from a university institution that works with lasers or from the government agency in your country that is responsible for safety at workplaces.
The following brief summary is just meant to be a guide for personal projects and experimentation (non-commercial use) - the specifics for each laser class may be even more stringent:
Note: I don't claim that these signs will meet CDRH guidelines in terms of type font or style - though there doesn't seem to be any real standard that is evident looking at commercial laser products!
Edit the labels for your specific laser if necessary. If your equipment is just a laser and not something containing a laser, remove the word 'product'. These were created with MSPAINT. I like to use LVIEWP for format conversion (e.g., .gif->.bmp and vice-versa), filtering, and other simple processing of graphics and pictures. (The version of LVIEWP I used is shareware but may no longer be available from the major download sites. There is now a much expanded commercial product which I haven't tested.) The font is: Arial Bold. Each of the labels is about 800 x 525 pixels. The result will be about 1.33" x 0.87" on a 600 dpi color printer or 2.66" x 1.74" on a 300 dpi printer. To use a 300 dpi printer to produce the same size labels, processing the image with a 2x2 averaging filter and then subsampling (resizing or scaling) by 2:1 works fairly well.
The following is a generic label to put near your output aperture:
And for anyone building their own power supplies or complete lasers, here are a pair of high voltage danger labels that can be edited for your needs:
Many laser companies and some laser organizations sell laser warning, danger, and aperture signs. One example is: Laser Institute of America on their Laser Area Warning Signs and Equipment Labels Page.
Coherent, Inc. currently has an offer of free personalized Class IV laser danger signs if you just go to their Danger Sign Request Page and fill out the form. (If this link doesn't work, go to Coherent's homepage, "Products", "Lasers", and there should be a link from there.) It is certainly worth taking advantage of this offer but please don't abuse the privilege by requesting too many! I requested one and it arrived in about 6 weeks - a most spiffy 8" x 11" plastic laminated card and the perfect addition to any laser lab!
Or, if you really want people to get the point, try Big Scary Laser Warning Sign. Given the graphic, you might want to edit it to read something like: "Do Not Allow Laser Beam to Contact Remaining Intact Parts of Body". :)
I'm not a lawyer, but this is what I have learned in doing laser shows for quite a while.
Please don't give the legislators ideas. Sales of lasers are unregulated except for medical and laser show systems, and a few systems under export controls. For all other systems, you just have to register as a manufacturer if you're making them for public sales and submit your product for compliance, and maintain records of who it was initially sold to in case there is a need for a recall. Nothing now on the federal books prohibits sale unless it is:
If it's manufactured in the USA and domestically certified laser of any class, you can own it no matter what. The violation occurs if you do a public display or show with it, regardless if for profit or not, and it exceeds Class IIIa. I can legally buy a megawatt laser if it's off the shelf technology and other then the manufacturer having to have the initial customer's name for recalls and modifications required to meet safety specs, no further paperwork is required, except in 4 states that require registration - Texas, New York, New Jersey, and Massachusetts. If I'm not in one of these states, I can then sell my megawatt laser to anyone, even a underage kid. I dont have to notify anybody either. I can toss it into the trash or disassemble it. I can build all the prototypes I want as well, provided I do not put them into commercial sale. The manufacturer will sell me a laser with all the CDRH requirements met, what I do with it after that point is my business, as long as I do not resell it as a commercial product.
If someone wants to send his prototype laser to the USA, he can do it legally provided: (1) the receiver and he fill out a bonding document that says its not compliant, coming in for test, be held by the receiver as if it were under customs bond, tested by the receiver, then it is leaving to go back where it came. It will be tracked by Customs and CDRH and fees may need to be paid. It may not be used as a public display device during this period. It's here for technical test and evaluation, period!
Or he may simply register as a manufacturer, file the paperwork certifying he's built the unit to 21 CFR 1040 (CFR = code of federal regulations) compliance specs, then assign it a serial number, keep the required records and sell it as a laser product. It takes about 3 to 6 months for the paperwork, but actually just costs the postage to get the forms and send in the documents. He can then sell as many as he likes, provided he does the documentation. If after introduction to sale, it is inspected and a fault is found, then he needs to declare his customer list and provide a scheme to modify it, retrofit it, or recall it. The paperwork isn't that bad, it just takes a bit of time for a tiny overworked federal agency to do its duties. Once you get a compliance or registration number based on your documentation proving you meet the rules, you can start selling.
If you already have a unit that has a compliance or registration number, you're fine. All other units are supposed to have a OEM sticker on them, saying its intended for use in a approved application device only. Possession of those for private experiments is not a issue.
When you sell a used unit or advertise it for sale, you are required to include a copy of the warning sticker in your brochure or Web site, even for class I or class II lasers, and warn the potential buyer of its hazards in writing, and include any manuals or other safety related gear that was manufactured for that laser.
You're also supposed to inspect your units for CDRH compliance at least once a year or when you buy it, and correct any problems you find, but you don't have to report doing that.
So basically no rules about anything that is not medical, military or foreign.
NOW, where the crap hits the fan is at the state level. New York records the serial number of all lasers and requires licensed operators, transferring a laser in NY above class II to another citizen of NY without reregistering the unit is an offense. Transporting a laser through NY or selling it out of state from NY is not however a offense. Texas and Arizona have user fees to pay for their states radiologic safety programs, etc. I'm told by a friend that AZs fees are quite steep, on the order of $1,500 a year for large industrial lasers and that AZ inspects laser shows rather thoroughly. Other states may vary, but generally unless they have made misuse of pointers a issue, there are no worries except in NY and AZ. Possession is not illegal and they don't deny permits to register in those states. However, they may disqualify a person who fails to pass the test.
As far as the national fire code goes, there is one line in the code requiring a evaluation of the system so the beam doesn't terminate on anything that could cause a building fire.
I'm sure a few other states have rules, but most don't, and law enforcement types have much better things to do with their time unless its medical and misused or malfunctioning, misused as a weapon or in a unsafe manner in public or the workplace, used in a outdoor laser show going into airspace, or exported to a foreign country where it would enhance their military or technology programs.
In all fairness, New York's licensing system includes one heck of a safety program, including retinal photos of high power laser operators as part of an ongoing safety study. I doubt they care about what you do in your basement, provided you're the only one who can be exposed to laser energy.
As long as you don't use the laser for public displays or shows, put it where the general public can access more then the Class I exposure limits, don't do anything that risks illuminating an aircraft or other vehicle and don't practice medicine (including general surgery and dentistry!) with it, the sky is the limit. You can legally use some Class II lasers and some Class IIIa lasers in public, as long as you comply with the safety instructions that came with the device. This is for the US. However, if you wish to do laser shows, other rules apply. See the sections starting with: Some Basic Info on Light Show Lasers for more info.
So have fun and don't worry, unless your aiming it outside or doing public laser shows or playing doctor or are exporting.
(From: Steve Roberts (email@example.com).)
"CDRH has fangs and will use them. I have a friend who sold helium-neon laser power supply kits without stickers, certification, and registering. His legal bills alone were $8,000, not to mention the hefty fine his company paid."
CDRH will gladly send out a complete copy of the guidelines for manufacturing laser systems free of charge if you request it. Ask them to throw in the laser show stuff as it makes even more interesting reading. The hardcopy from CDRH will include the exposure tables and how to calculate MPEs, etc. It's free to all US citizens and probably free to overseas corporations as well.
Many if not all of these documents are now available on-line (although some are just the scanned paper documents in .pdf format). Go to the CDRH Guidance on Electronic Products which Emit Radiation page. This has all the categories of radiation emitting devices and some of the detours may prove interesting. If you have enough discipline to ignore them, click on "Lasers, Including Light Shows" which takes you to the Laser Products, Including Laser Light Shows and Displays page with its list of available documents. The one you probably want to begin with is: Performance Standard - Lasers and Products Incorporating Lasers. This includes the always very popular detailed information on laser classifiactions! :) Various forms can be found at the FDA Forms Distribution Page for CDRH along with a contact for each - in most cases an actual person (yes, a real living human being!).
For a more interactive experience, spend an afternoon (more or less) starting at the CDRH Device Advice page. According to their blurb: "Device Advice is set up with pages that describe these procedures and link you to the appropriate documents on the CDRH Homepage such as guidance documents, databases, and manuals that will both assist in meeting marketing requirements and answer many questions you may have."
Also, see the chapters on 'Control Measures' in the laser safety manuals at the Laser Safety Links Web site and the other laser safety links in the chapter: Laser Information Resources of this document.
(The following was first posted to the USENET newsgroup: alt.lasers.)
(From: Adam Burns (firstname.lastname@example.org).)
Disclaimer: Long-winded post follows. I do not work for the CDRH. I have no hidden agenda to either praise or pan the government. I am not a laser expert. Do not read post while operating heavy machinery. Not intended for children under the age of 3. no deposit, no return.
With all the discussion about CDRH rules concerning lasers in schools, laser shows for demonstration purposes, etc., I decided to try calling the CDRH directly. (Insert ominous music here!) I called 1-800-638-2041 and got transferred to Walter Snefko at extension 120. Unfortunately, he was unclear on several of my questions, and admitted that the real laser device gurus were only reachable by dialing the non-toll-free number. (Figures!)
So with no concern for daytime long distance rates I took his suggestion and dialed 1-240-276-0120. Having been primed with information from Walter, I then asked for Jerry Dennis at extension 135. Jerry was available, and was *AMAZINGLY* helpful. We spoke for about 15 minutes, and he encouraged me to call again if I had further questions. He also said that two others in the office, Dale Smith (extension 147) and Frank Mackson (extension 145) were even more familiar with CDRH rules as they applied to laser shows. He said that Dale Smith, in particular, was a very valuable resource.
OK, here's what I asked:
Answer: No. If you are under 5 mW you do NOT need a variance and you can do whatever you want. (This was correctly pointed out by others here on alt.lasers before. I was really just warming up with this question.) However, he *DID* caution me to remember that 5 mW can still be an eye hazard. Basically he said to avoid sending static beams into the audience. I pressed him about scanned beams and the like, and he temporized. I gather that he knew that they really don't have jurisdiction here because of the power level, but he wanted to discourage blatantly stupid behavior nonetheless.
Answer: Yes. As long as the beams that exit the device (projector, housing, etc) are each less than 5 mw, and the beams are separated far enough apart such that it is not possible for multiple beams to enter the same pupil, then the entire device is exempt. Thus, you can have a 20 mw laser putting out, say, 8 beams from a diffraction grating, and as long as none of the beams are over 5 mw the entire setup is exempt.
As an aside, he went into considerable detail as to how these beams would be measured for intensity. The basic standard is to use the standardized pupil diameter (listed in the CDRH main document - I forgot to write it down) and measure the power at a minimum distance of 20 centimeters (!) from the apparent source of the laser light. (i.e., the window on the projector housing, the aperture of the scan head, or the output coupler of the laser if the other two do not apply.) The pupil size used for the calculation changes if the environment is such that it would be likely that audience members would be using binoculars. Binoculars?!? I asked?: "Yeah, like in a large arena - folks typically bring a set to make it easier to see the stage." He floored me with that one, but it makes sense if you think about it.
Answer: No. The CDRH deals with commercial products only: either laser systems that are assembled to be sold to the public, or laser shows that are created (designed) to be "sold" as a "product" to an audience. Laser shows that are not part of a commercial endeavor are NOT subject to CDRH rules, no matter what power levels are involved. This was AMAZING to hear as it runs counter to just about everything I've read here and elsewhere, so I questioned Jerry at length about it.
Basically, he said that there are white areas, black areas, and grey areas. The white would be what you do in your basement for you and your family, friends, or neighbors. This includes classrooms (see below). The black would be when you set up a show at a local auditorium and charge admission. The grey area is when you have a show that could be considered commercial in nature, even if you choose not to charge admission. Examples include doing a laser show for free at some other event, when it would normally be customary to pay to gain admittance to that event. However, he did make it clear that in order for the CDRH to have legal authority, the situation must involve COMMERCE of some sort. Thus, volunteering to show off your class IV lasers in the church basement after the service is over (call it your Sunday afternoon laser club?) would be an exempted laser show.
Displays in a classroom were one case that I specifically asked about. I asked about the case of a private citizen volunteering to do a display for the class, as well as the case of a teacher doing a demonstration for his class. In either case, because there is no commerce involved, the CDRH does not have jurisdiction and a variance is not required NO MATTER WHAT POWER LEVEL LASER IS USED.
Now, before you start thinking that this is a gaping legal loophole, Jerry did inform me that there is an entirely different set of guidelines that are normally applied to classrooms.
The American National Standards Institute has released a document dealing with what are considered acceptable practices for using lasers in school environments. The standard is ANSI Z 136.5, and evidently it is available from the Laser Institute of America for a nominal fee. (No, I haven't checked yet.) Nearly all college environments dealing with class 3B and class IV lasers are expected to adhere to this standard according to Jerry.
He also pointed out that compliance with the ANSI standard is not required by the CDRH. (Though it might well be required by the school district, as well as local or state laws dealing with lasers) It is simply an example of what a legal court of law would accept as a "reasonable standard of conduct" for the safe operation of a laser in a classroom environment. (Translation: if you do not follow the standard, no one from the CDRH will shut you down, fine you, or take you to jail, but if anyone ever gets hurt at your show and decides to sue you, they have a better chance of winning if you failed to follow the guidelines set down in the standard.) Short answer is that I'm going to at least have a look at that standard before I consider taking any of my lasers into a classroom.
I have to admit that I was a bit concerned about calling the CDRH. I wondered if they would take the time to talk to an "enthusiast" that really didn't have much intention of ever going commercial. I was afraid I'd be accused of wasting their time. I was also a little afraid that they would read me the riot act for even asking some of these questions.
What I found out is that the folks there are VERY helpful. I'm not kidding, this Jerry Dennis fellow sounds like a great guy! He took the time to answer my questions at great length, and always pointed out that even if certain circumstances were not under CDRH control it would still be foolish to ignore the safety precautions they promote. He really was easy to talk to, and was quite supportive of my hobby. Despite the fact that he was clearly used to speaking to engineers and Ph.D.s about laser physics, he was able to speak at my level about everything we talked about. He also had some good information about laser safety. (Hey, how about that? A government employee that actually gives great customer service!) I'll probably call him (or maybe Dale Smith) again in the future.