Originally Posted by laric Gamma Calculation Methods
To have a properly calibrated projector it is required to achieve the best luminance ratio between different parts of displayed picture.
This very characteristic is commonly reduced to a numeric value: Gamma.
The higher Gamma is (above 2.22) the more contrasted image you'll get but you may then have poor black level in dark scenes.
Lowering the Gamma (below 2.22) allows an increase in black details, but picture can then be too bright.
This ends by a compromise in settings that calibrator should achieve, it depends on projection conditions (room, lights...) as well as screen, source and, obviously, projector characteristics.
2.22 Gamma is usually the best compromise and generally admitted as reference value.
The default Gamma computing method we use (second option below) is the generally admitted one (ColorFacts, ICC generation softwares, etc... see references below).
Anyway, there are many passionate debates on the "best" way to compute Gamma.
That is why we introduce different scheme in v1.21. These options are for the one wanting to go a bit further or unsatisfied by default settings. We can later on pursue that very debate, but so far we think it is not required to go more in depth to be able to calibrate a projector.1) Display gamma
The gamma is compute in light to human eyes perception.
After approximation brightness (L) is expressed according to the signal (V) using the equation:
 L = V^gamma
Standards recommend to us a 2.5 Gamma target (2.45 after math) in this mode.2) Display Gamma with Black Compensation (Default)
In practice one can never achieve an absolute black and to properly setup a projector (or screen) it is necessary to take in account of the black you can really obtain.
This is usually done by computing Gamma after removing the "absolute" black from every measure.3) Camera Gamma (Standard Offset)
To have an accurate reproduction, some interpretations say that you must apply a symmetrical treatment to the encoding one. The reference equation being:  Y = ((V+offset)/(1+offset))^gamma
The "offset" is a parameter use at black encoding stage (and reproduction using this method). It's value is 0.099 on all video standard. (0.055 in sRGB)4) Optimized (Regression)
This calculation method is based on the fact absolute black recorded by camera is the absolute black projector can produce.
But offset in standard is roughly for 200:1 contrast ratio, a lot lower than current ration projector can achieve.
The optimized method tries to find the Offset AND Gamma parameters that best match the measures, black included.
Pictures as contrasted as in method 2 can be achieved using this method with even a better black details level.
But the compute method -- and so the resulting projector settings -- are more dependant of proper and accurate low level probes readings.References:[Poynton] Gamma FAQ
(Chapters 4 - what is lightness, and 6 - what is gamma correction)[Pascale] A review of RGB color spaces
(Paragraph 2.1.6 gamma and Table 5 definition of different RGB color spaces)[sRGB] A standard default color space for the Internet
(Paragraph "Gamma and the desired CRT gamma of 2.2")