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

[Andrew McNeil]

Hi Aksel,
Unfortunately I don't have sources for this other than Greg's emails in the
vintage radiance digests (1990's).  Search these pages for "179", there are
several emails on this topic):
http://radsite.lbl.gov/radiance/digests_html/v2n5.1.html
http://radsite.lbl.gov/radiance/digests_html/v2n9.html
Best,
Andy

On Fri, Jan 4, 2013 at 10:34 AM, Aksel Groß <aksel at gobo.io> wrote:

> Hi Andy,
>
> Interesting rule. Could you recommend further reading for my interest on
> how to get to the number 179? I would like to get a tiny grasp on the
> underlying principle to better understand.
>
> Thanks a lot,
> Aksel
>
>
> Am 02.01.2013 um 20:23 schrieb Andrew McNeil <amcneil at lbl.gov>:
>
> > 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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>
> --
>
> Aksel Groß
> Dipl.Ing.Arch., Dipl.Szeno.
>
> Electric Gobo
> Schönhauser Allee 182
> 10119 Berlin, Germany
>
> T +49 30 559 531 75
> M +49 179 394 30 92
> aksel at gobo.io
>
> http://gobo.io
>
>
>
>
>
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