Hi Andy,
Surely not if behind the light source?
For a Class 4 laser, even if behind the light source:
EYE INJURY HAZARD - DIFFUSE REFLECTION
The scattered light from the laser "dot" as viewed on a surface, can be an eye hazard. Avoid looking directly at the laser dot for more than a few seconds. The light is too bright if you see a sustained afterimage, lasting more than about 10 seconds.
The more powerful the laser, and the closer your eye is to the laser dot, the greater the chance of injury. This can occur during certain actions, such as aligning the beam or trying to hold the laser dot on a fixed location in order to burn a material.
Now a Class 4 laser admittedly has >= 500 mW power, and in this thread, no-one actually suggested to use a laser of that power class.
However, I believe some people recommended "at least 50 mW", and for someone not aware of the dangers associated with lasers, it potentially could seem like a good idea to jump straight to a Class 4 laser if the budget allows - which it very definitely isn't.
The laser safety site also states on Class 4 lasers:
DO NOT USE AS A LASER POINTER
Even if this laser looks like a pointer or flashlight, do not use this laser for pointing purposes. Class 4 lasers are too powerful to be used as pointers. Use a Class 2 (less than 1 mW) or Class 3R (less than 5 mW) laser for pointing purposes.
(Quotes are from
http://www.lasersafetyfacts.com/4/ )
With regard to 50 mW lasers, here's an assessment from the Class 3 safety advice:
At the low end, around 5 to 50 milliwatts, a Class 3B laser poses a moderate risk of eye injury. It is unlikely that a handheld beam aimed from more than a few dozen feet away would cause injury -- laser light could not stay on one spot on the retina long enough for heat to build up to injurious levels. However, the risk is increased if the beam is held steady or if the laser is relatively close to the eye.
For example, if you're looking at your smartphone and someone accidentally shines a 50 mW laser on the reflective screen, that might cause eye injury in a small fraction of a second.
Class 3R lasers of up to 5 mW are considered eye-safe as it takes 0.25 s to reach dangerous heat levels, giving you time to blink and save your eye.
A 50 mW laser, being 10 times as powerful, can damage the eye in a tenth of this time, which means can easily cause eye damage before your blinking reflex can protect you. It also means that even a sweeping beam can cause eye damage.
So for anyone using a laser of > 5 mW, it's probably a good idea to read this page in order to be able to assess the risks and use it as safely as possible:
http://www.lasersafetyfacts.com/3B/
Personally, I would consider it a good idea to make it a rule to never hand over the laser to anyone else who might not be aware of the hazards associated with a powerful laser - for example because he mistakes it for just another low-powered laser pointer like the ones he has used a thousand times.
Regards,
Henning
