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

[Ji Zhang]

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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