# Weighting factors used in the irradiance calculation

Hi Parisa,

Yes, these values are the so-called V-lambda or photopic weighting function, and yes these three will always equal 1 (naturally your derived result will generally be > 1).

*Be advised*, your formula as printed in your post is incomplete; when converting irradiance as computed by Radiance (the software) to illuminance, you need to multiply the whole thing by 179 which is the luminous efficacy value to be used _with Radiance_. In other words:

I = 179 * (0.265 IR+ 0.670 IG + 0.065 IB)

There are scores of posts in the archives about this value and how it came to be, and why it works even though it's different from any luminous efficacy value you've ever seen in a textbook on light. =)

- Rob

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On 6/22/15, 9:50 AM, "parisa khademagha" <[email protected]<mailto:[email protected]>> wrote:

Dear all,

I have two questions regarding the weighting factors (0.265, 0.670, 0.065) that are used in the formula ( I = 0.265 IR+ 0.670 IG + 0.065 IB) with which one can convert the spectral irradiance triad to irradiance. My first question is: where these weighting factor come from? Do they incorporate the spectral sensitivity of the human eye (so called V(λ)) in the irradiance calculation? My second question is: should the summation of these weighting factors be always equal to 1.