Re: Infrared-Digest V #291

[David Sprinkle <david-sprinkle@xxxxxxxx>]

Don Bryant wrote:
> 
> Gary,
> 
> > Midday gets the lowest EI because of the
> > lower ratio of IR to visible.
> 
> Could you explain that statement a little more fully. How did you arrive at
> this conclusion?
> 
> Empirically? Just curious.
> 
> Thanks,
> 
> Don Bryant
> 

If you'll take a look at Lauri White's book, you'll find a brief
discussion of the color temperature for daylight at different times of
the day.  It turns out that the light is "bluer" during midday and tends
to include more red light as the day progresses.  This effect is pretty
intuitive -- the additional red light is the reason sunsets produce that
beautiful golden-orange glow.  IR radiation (being red light, after all,
just invisible) is also much more prevalent as the day progresses.

-dave
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