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

[Ji Zhang]

Hi, Rob, ... already put into the "Radiance Bible" ...

On Thu, Jan 3, 2013 at 3:55 AM, Guglielmetti, Robert <
Robert.Guglielmetti at nrel.gov> wrote:

> Can we have this reply laminated onto cards? =8-)
>
> Thanks Andy!
>
> Rob Guglielmetti
> National Renewable Energy Laboratory (NREL)
> Commercial Buildings Research Group
> 15013 Denver West Parkway MS:RSF202
> Golden, CO 80401
> 303.275.4319
> robert.guglielmetti at nrel.gov
>
>
>
> On 1/2/13 12:23 PM, "Andrew McNeil" <amcneil at lbl.gov<mailto:
> 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<mailto:
> 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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