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?

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.

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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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On 2018-06-09 06:39 AM (-4), clipka wrote:
> 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).

Thanks!  I am making those assumptions.

>> 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.

This is what my https://en.wikipedia.org/wiki/Lux has:

   0.0001            Moonless, overcast night sky (starlight)
   0.002             Moonless clear night sky with airglow
   0.05 - 0.3        Full moon on a clear night
   3.4               Dark limit of civil twilight under a clear sky
   20 - 50           Public areas with dark surroundings
   50                Family living room lights (Australia, 1998)
   80                Office building hallway/toilet lighting
   100               Very dark overcast day
   150               Train station platforms
   320 - 500         Office lighting
   400               Sunrise or sunset on a clear day.
   1000              Overcast day; typical TV studio lighting
   10,000 - 25,000   Full daylight (not direct sun)
   32,000 - 100,000  Direct sunlight

   ... The unobscured Sun provides an illumination of up to 100 kilolux
   on the Earth's surface, the exact value depending on time of year and
   atmospheric conditions. This direct normal illuminance is related to
   the solar illuminance constant Esc, equal to 128000 lux (see Sunlight
   and Solar constant).

https://en.wikipedia.org/wiki/Sunlight says:

   Direct sunlight has a luminous efficacy of about 93 lumens per watt of
   radiant flux.  ...

   Multiplying the figure of 1050 watts per square metre by 93 lumens per
   watt indicates that bright sunlight provides an illuminance of
   approximately 98 000 lux (lumens per square meter) on a perpendicular
   surface at sea level.

The solar constant page focuses on wattage and does not address illuminance.

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