Am 09.06.2018 um 02:30 schrieb Cousin Ricky:
> I'm trying to calculate the lux of a 1100 lumen light source at 2
> meters.  I have the total area of a sphere at that radius at 50.27 sq.
> meters.  Dividing, I get 21.88 lux.
> 
> Is my derivation correct?

It is, provided the surface on which the light shines is perpendicular
to the direction of the light, and the light source is isotropic (i.e.
emits light in all directions with the same brightness).

> I'm trying to wrap my head around the disparity between indoor lamps and
> solar illuminance, which apparently can be as much as 100,000 lux.

My English and German Wikipedias can't agree on that figure. The English
one claims that the illuminance of full daylight on Earth is "only"
10,000 lux. The German one has a lot more entries, assigning 10,000 lux
to "Im Schatten im Sommer" (in the shade in summer), and listing a
bright sunny day as 100,000 lux.

The human visual system has an incredible dynamic range, especially when
it comes to "static contrast": Given enough time to adapt, the human
visual dyanmic range is about 10^9; and you need a factor of about 10 in
(overall) illuminance to even be able to notice any difference.

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