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

[Andrew McNeil]

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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