[Radiance-general] Weighting factors used in the irradiance calculation

[Parisa Khademagha]

Hi Jan,

It is true that if these weighting functions are from V-lambda they are to be used only in calculation of photometric quantities. Doing so the summation of the triad irradiance values should be multiplied to 179 efficacy function to give illuminance ( I = 179*(0.265 IR+ 0.670 IG + 0.065 IB)). What I don’t understand is why these weighting functions are used in calculation of radiometric quantities like irradiance ( I = 0.265 IR+ 0.670 IG + 0.065 IB). You said you calculate everything in grey (r=g=b) to avoid this problem. Shall I ask what do you mean by everything? What is the luminous reflectance of the grey you use? Moreover, is it possible to measure the effective irradiance with respect to C-lambda using adjusted weighting functions? Should the summation of the weighting functions be again equal to 1 as they are for V-lambda?

Cheers,
Parisa



On 22 Jun 2015, at 18:24, Jan Wienold <jan.wienold at epfl.ch> wrote:

> Hi Parisa,
> 
> in fact these are weighting functions derived from v(lamda). So you should use them only when  all your quantities are in photometric units. Irradiance "usually" implies the solar spectrum, but in RADIANCE this is  used also only considering the visible spectrum (and then using the weighting). I'm not sure if a color weighting makes sense when talking about radiometric quantities using the solar spectrum. Usually I calculate everything in "grey" (r=g=b) when using the radiometric quantities, so this problem doesn't occur. Be aware, that you usually calculate everything in photometric quantities in RADIANCE, so if you want to calculate the solar spectral behavior you have to take care of using the related values(solar transmittance for glazing, solar reflectance of surfaces and generate sky and sun with radiometric quantities). As far as I know only if you use gendaylit with the -O 1 option, then sky and sun are automatically in radiometric quantities (which means your results are in W/m*m solar spectrum).
> 
> Jan
> 
> Am 6/22/15 um 5:50 PM schrieb parisa khademagha:
>> 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. 
>> 
>> Thank you in advance,
>> 
>> Parisa
>> 
>> 
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