Units of Luminosity from Camera into PS

[font color=\'#000000\']Looking for a technically sound reference source that deals with units of brightness as they move from light image on the sensor ( photons per sq mm per second? ha!) to sensor output (? electronic charge per sensor site or millivolts?) that is fed to the A-D converter producing 8 to 12 bit information.

A very basic issue is whether the digital tone scale is indeed linearly related to brightness or logarithmically related to brightness ( as in ev units). i.e if brightness is described by a 0 to 255 scale does each unit represent a constant fraction of an ev unit or a constant increment of sensor voltage/current/charge output? Question to follow on histograms and dynamic range.[/font]