You Only Received One Set
of Eyeballs?
Lasers have tended to be highly glamor devices popular
with hobbyists, experimenters, entertainers, and serious
researchers alike. However, except for very low power
lasers - those with less than a fraction of a mW of
beam power - they do pose some unique hazards particularly
with respect to instant and permanent damage to vision.
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:
 The
output of many lasers is a nearly parallel - highly
collimated - beam which means that not only is the energy
concentrated in a small area but the lens of the eye
will focus it to a microscopic point on the retina instantly
vaporizing tissue in much less than the blink of an
eye. A collimated beam represents the rays from an object
at infinity so if your eye is focused for distance,
the laser will be in focus as well. Even a common helium-neon
laser without external optics will approximate a point
source a .5 meter or more behind the exit window of
the laser. If you are working in a small room, this
approximate distance would likely be where your eyes
are focused. While purists might argue that the lens
of the eye isn't perfect and will not produce a diffraction
limited spot on the retina, this won't save your vision!
The power density in a sub-optimal spot can still be
astronomical.
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.
A 100 W light bulb puts out about 5 to 7 W of visible
light and another 35 to 40 W in the near-IR which is
also relevant since it passes through glass, water,
and the anterior structures of the eye can be focused
on the retina. The rest is mid to far-IR and heat with
a small amount of UV tossed in. All of this radiation
is more or less uniformly distributed in every direction.
However, at any reasonable distance from the light bulb,
the power density (e.g., W/mm2) entering the eye is
much lower than for a collimated laser beam of even
very low power. And, it takes significant effort to
produce any sort of truly collimated beam from such
a non-point source such as is present with even the
filament of a clear light bulb. For a frosted light
bulb, insert another factor of a thousand or so. :)
Without collimation, even the portion of that additional
35 to 40 W of near-IR that enters the eye isn't going
to cause damage. However, for a helium-neon laser, the
collimation is such that the entire beam (total power
output of the laser) will still be small enough to enter
the eye even at a distance of several meters.
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 50 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.)
 As
another point of reference, the mid-day Sun at the Earth's
equator on a clear day has a power density of about
1 kW/m2 or about 1 mW/mm2. It would not take very long
staring into the Sun to burn out your eyeballs!
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