Is there a relation between absolute and relative luminance with RGB pixel values?

I think the display devices shows the luminnce values ranging from 0-255. I have got the relative luminance from r,g and b values using the equation L=k*(.2127*r+0.7151*g+0.0722*b); But I am interested in getting absolute luminance.Can somebody help me out. Thankyou

[Mojtaba]

I have the same question. Is there any answer for it?

Answer 1

[Walter Roberson]

"I think the display devices shows the luminnce values ranging from 0-255"

Not really. Some monitors work in YCrCb colorspace with 8 bits per channel per pixel, but others work in sRGB with 8 bits per channel per pixel, and some use xxYCCC, and some permit 10 or 12 or 16 bits per channel per pixel. http://www.eizoglobal.com/library/basics/displayport_to_d-sub/index.html . And, of course, analog monitors have input voltages and so are continuously variable in theory (high end graphics boards used to support high resolution of voltages.)

Even in cases where the software is controlling at 8 bits per channel per pixel, the software value will be put through gamma correction both by the graphics board (a correction that is plausibly queryable) and gamma correction at the monitor (which often cannot be queried.) The post-correction value would be in the same range -- but this means that you probably do not know what any particular relative value will be mapped to.

After gamma correction, you still face the problem that you probably do not know your monitor's absolute brightness, which is something that varies a lot with different monitors and which the user often has control over at the monitor.

... So, in order to know the relationship between relative luminance and absolute luminance, you need to use a calibration tool... that is valid for that particular combinations of settings on that particular monitor.

Did I mention that my monitor has automatic brightness adjustment, turning down its output in dimmer light, since the human ability to read a display depends no as much on the absolute brightness of the display and more on the relative brightness compared to ambient light? Your screen is harder to read when the sun is shining on it, so monitors like mine automatically turn up the brightness when they detect that.