Re: Excuse me Sir, can you see the laser beam?

in article [EMAIL PROTECTED],
chibitul at [EMAIL PROTECTED] wrote on 12/1/03 8:00 PM:

> Hi,
> 
> I have a laser diode, looks red, probably about 660 nm, but I don't know
> the power. The laser spot is well colimated and easily visible in
> average lit room. Haven't had the chance to try outside in sunlight.
> 
> Now this colleague of mine is worried that after reflexions on several
> mirros (which are optimized for a different wavelength) the intensity
> becomes too weeak and the laser beam is not visible anymore when it hits
> the target. I estimate the reflectivity of these mirrors at the laser
> wavelength to be 40%. So after 2 mirrors, I have 16% left. I would argue
> the beam is easily visible, since the eye has such an incredible dynamic
> range (11 or so???). But he really wants to do the experiment to make
> sure the beam is still visible. Fine. He has a point. But my question is:
> 
> How do you defind the visibility treshold of a laser beam onto a target?
> When is the beam too weak to be considered visible by most observers?
> 
> rant on!

There is no sharp edge to visibility as a function of wavelength. Vision
pigments have long tail response. When I first saw the light of an IR diode
laser it looked orange and in a rather dark room and with the eye reasonably
dark accommodated. My guess is that the green receptive cones had fatter
long wavelength tails than did the red ones.

Bill