[Radiance-general] why "global horizontal illuminance" is smaller than "global horizontal radiation" multiplied by 179 in epw file?

[Lars O. Grobe]

Hi Ji Zhang!

> So, can we put it as:
> The *"global horizontal radiation" *in the epw file is a total solar 
> radiation value*NOT yet *integrated over the visible spectral range 
> (380-780 nm) (from gendaylit man page), so we can't simply multiply it 
> by 179 to get the illuminance value. We need to "break" the *"global 
> horizontal radiation" *into its RGB components and then use 
> (R*0.265+G*0.670+B*0.065)*179 to convert it into a illuminance value.

I would not put too much effort into understanding why it is 179 lm/W in 
Radiance. It usually really does not matter for you. You can use it as a 
constant as long as you are interested in photometric quantities, as in 
such case, you input and your output will be in lumens, not watt. So 
feed in by dividing by 179, read out by multiplying with the same. 
That´s it. This is completely unrelated to the "real" luminous efficacy 
of lamps, skies, stars (aka the sun...), which is not that easy to 
predict. So, whenever possible, I'd use the photometric units for input 
when I need photometric results. This means you are working with 
well-defined conversions.

Cheers, Lars.
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