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

[Ji Zhang]

I see, thank you very much, Andy!

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.

- Ji


On Thu, Jan 3, 2013 at 3:23 AM, Andrew McNeil <amcneil at lbl.gov> wrote:

> Hi Ji,
>
> 179 is the efficacy of white (equal energy) light over the visible
> spectrum.
>
> Daylight is composed of a broader spectrum, so the efficacy (visible light
> per watt of energy) is lower.  Usually around 90 for the sun and 110 for
> the sky, but changes based on various factors.
>
> 179 is used in Radiance as a convention since we are simulating visible
> light.  So when you're defining you sky using gensky with weather data you
> need to either use the measured illuminance values and divide by 179 to get
> radiometric units for the visible spectrum, or use the measured radiance
> values (for solar spectrum), multiply by an approximate efficacy, then
> divide 179 to get radiometric units for the visible spectrum only.
>
> If you use gendaylit all the conversions are done for you.
>
> Best,
> Andy
>
>
> On Tue, Jan 1, 2013 at 8:36 AM, Ji Zhang <hope.zh at gmail.com> wrote:
>
>> Dear list, Happy New Year!
>>
>> I have a simple question related to conversion from irradiance value to
>> illuminance value, and pls correct me if I'm wrong:
>>
>> Usually we can estimate the *illuminance (lux)* for a given point by
>> multiplying the *irradiance (w/m2)* for the point as simulated via
>> Radiacne by *179 (lm/w)* which is the luminous efficacy used in
>> Radiance, or more strictly (R*0.265+G*0.670+B*0.065)*179.
>>
>> However, it seems that in a epw weather file the* "global horizontal
>> illuminance"* value is not equal to but smaller than the *"global
>> horizontal radiation" value multiplied by 179*.
>>
>> May I ask:
>> 1. why there's such a large discrapency?
>> 2. Will this lead to over-estimation of illuminance when using cumulative
>> sky derived from "global horizontal radiation" ?
>>
>> Thanks in advance!
>>
>> - Ji
>>
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>
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