# Why 'more or less stopped using ISO'?

Started Feb 19, 2014 | Questions thread
Re: Maybe this will help.
1

jackdan wrote:

Iliah Borg wrote:

Another thing I need help with is that in RGB color space I understand brightness can be quantited.

Only if you know how to convert to XYZ, where Y is luminance. CIE states that brightness is relative luminance. Relative luminance is defined as photometric luminance normalized to 0..100 in such a way that reference white is 100.

So, even though I don't know how to do it, brightness can be quantitated as luminance using RGB values.

No, only relative luminance.

If I understand correctly, and I wouldn't be surprised if I don't, that in RGB color space, whatever that is, brightness is a quantity that is proportional to the magnitude of the pixel value from 0 to 255 is it, for each of the three values for R,B, andd G.

There is no need to use 255, you can set the upper limit as you wish, it is a question of normalizing. 0..1 is often used. Conversion of RGB to XYZ is often through a matrix, see http://brucelindbloom.com/Eqn_RGB_to_XYZ.html and http://brucelindbloom.com/WorkingSpaceInfo.html#Specifications The Y values for primaries R, G, and B allow to calculate the overall Y of a given RGB triplet by a simple weighted sum

Y = Yr*R+Yg*G+Yb*B

However RGB must be in a gamma=1 space. If it is some other gamma it needs to be linearized before anything.

So, brightness as luminance

No, relative luminance.

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