Dear All

As a part of my Ph.D research I am doing a study about the influence of the colors of surfaces on the illuminance at any arbitrary point of the room under daylighting condition. So I need to know about the exact relationship between the color of a surface and its luminance. For that reason I am going to do some simulation with radiance. As I have just common knowledge of using radiance, sorry if my questions are too basic.

The calculation method used in “Radiance” for computing the luminance value is:

Luminance = 179 * (0.265*R + 0.670*G + 0.065*B) (1)

Where the 179 value has been defined so: The value of 179 lm/w is the standard luminous efficacy of equal energy white light that is defined and used by Radiance specifically for this conversion.

1. Unfortunately I could not find this value at any other document about lighting and colour measurement. Does anybody know how the value of 179 has been derived? I assume it is the average of all the luminous efficacies in visible light spectrum (380 – 780 nm). Isn't it ?

2. As I know, the method for calculating the luminance is:

L = 683 ∫ I (λ) * ρ (λ) * V (λ) dλ (2)

Where: 0=<V (λ) < =1 and has a maximum of 1 at 555 nm and I (λ) is the Spectrum power distribution at any wavelength of arbitrary spectrum. If I have understood correctly, in the file “src/common/color.h” the value of (0.265*R + 0.670*G + 0.065*B) will give us the tristimulus value “Y”, which is the same as: Y= ∫ I (λ) * ρ (λ) * ‾y (λ) dλ (3)

Since ‾y (λ) = V (λ) for computing the luminance value (according to the methods (2), (3)) what we should do is to multiply the Y value at 683 which gives:

Luminance = 683 * (0.265*R + 0.670*G + 0.065*B)

I suppose if we use the method (1), it means that we have calculated luminance so: L = 179 ∫ I (λ) * ρ (λ) * V (λ) dλ

That means the value of 179 lm/w has been used as maximum luminous efficacy instead of 683 lm/w at 555 nm??

And if 179 has been used as average, I can not understand why we shoud average again , whereas behind the 683 ∫ V (λ) dλ (380nm- 78nm) there is the meaning of averaging the luminous efficacy over the visible light spectrum.

I hope I could make my questions understandable.

If I have gotten the concept wrong, I would be thankful if somebody could tell me where my mistake is, it would be really important and helpful for the result of my research.

Thank you in advance

Niloofar

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Niloofar Moghbel

Chair for Building Physics

University of Stuttgart

Pfaffenwaldring 7

70569 Stuttgart, Germany

Tel.: ++49 (0)711-685-6301

Fax: ++49 (0)711-685-6583

Niloofar.Moghbel@lbp.uni-stuttgart.de