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

[Ji Zhang]

Dear list, any comments as a celebration for avoiding the avoidable "fiscal
cliff" ? ;-)

- Thanks! Ji



On Wed, Jan 2, 2013 at 12:36 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
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.radiance-online.org/pipermail/radiance-general/attachments/20130102/01ea4490/attachment.html>