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

[Ji Zhang]

Noted with many thanks, Lars! and Happy New Year!
- Ji


On Fri, Jan 4, 2013 at 6:50 PM, Lars O. Grobe <grobe at gmx.net> wrote:

>  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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