From: Mike Poulton
Subject: Re: Make infrared goggles inexpensively (like $10!!!)
Date: 17 Sep 2002 13:41:13 GMT
Organization: MTP Technologies
References: <firstname.lastname@example.org> <4Aoh9.15391$1C2.email@example.com>
On 17 Sep 2002, Jonathan Kirwan said:
> On 16 Sep 2002 19:50:14 GMT, Mike Poulton
>>At 1mW/cm^2 full spectrum, the sun is not that much of a hazard to
> The incident flux is some 1387 watts/m^2 across the Earth.
> Anyway, 960 watts/m^2 is 96 mW/cm^2. This includes the full spectrum.
> I've no idea where you got the figure of 1mw/cm^2 for the full
> spectrum, but it doesn't seem reasonable to me. It seems off by
> perhaps as much as two orders of magnitude. Can you tell me where
> that figure came from?
The top of my head. Where else would a number come from? :-) Some time
after my previous posting, I began to think that the value was 1mW/mm^2,
not /cm^2. Clearly, that's the case. Regardless, the actual irradiance
doesn't make much difference -- it's a comparison of filtered to
> Once again, I don't know what you imagine "small fraction" to be, but
> the IR portion of sunlight getting through the filters, through the
> cornea, and to the retina is closer to 40% or so of the available
> solar radiative power.
If your stack of filters has 0.1% loss, then they are very special
filters. A stack of glass sheets would loose more than that! I can't
imagine that, across the entire range of 700-1600nm, a stack of several
filters could pass more than 90%. I'd bet it's much less than that.
> Anyway, this "left with a small fraction" comment seems to underplay
> the reality, a bit.
Okay then, call it a "reasonable fraction less than 1/2". It can be
called whatever you want -- what matters is that it is far less than
what you get from staring directly at the sun with no filter. Since we
can probably agree that short glances at an unfiltered sun do not cause
instant damage, we can reasonably infer that somewhat longer glances at
filtered sun will not cause damage, either.
>>glances of the sun without a filter aren't too damaging, brief glances
>>with the filter can't be very bad, even if your pupils are a bit
> A comparison I'm uncomfortable with, but I'll leave this one for
> others to think about.
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