From: SFPhysics@aol.com
Date: Thu Nov 25 1999 - 02:47:14 PST
From: SFPhysics@aol.com Message-ID: <0.12ddfff6.256e6db2@aol.com> Date: Thu, 25 Nov 1999 05:47:14 EST Subject: UV vs. IR eye damage!
Hello Clem,
We focus heat in the form of infra-red waves onto the retina all the time.
We think nothing of looking at a white hot incandescent lamp filament for
short durations or even looking at red hot space heaters. The heat allowed
through the iris is very small and the mass of the eye with all of the water
based liquid surrounding the retina makes this safe. The majority of the IR
is absorbed before the focal point on the retina. Some old time slide
projectors had heat absorbers made in the form of a water cell so the thin
glass slides would not expand and crack. The liquid in the eye is it's own
heat absorber.
The ultra-violet is the damage producer. Even a small UV source such as a
mercury germicidal lamp will burn the retina permanently. The welder's glass
used in arc welding is different from the glass used in gas welding. It has
much more UV absorption or the user would quickly go blind. Welders are
careful to know which is which. Arc sources such as the carbon arc used in
some Physics classroom demonstrations may only be consuming 100 Watts of
power but they are very dangerous because they produce a prodigeous amount of
UV that a similar 100 Watt filament bulb puts into heat (IR). That is why
good manufacturers of sunglasses label their product as blocking UVA and UVB.
You would not see one labeled a heat or infra-red blocker.
Best to the list,
Al Sefl
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