Panrock wrote:So, regarding my original question... Say a CRT grid just cuts off at -10v (negative). If you then put -5v on it, what sort of grey do you get, assuming linear measurement of the spot brightness? A lighter-than-mid or darker-than-mid grey? Although instinct supposes it will be lighter rather than darker grey due to the law of increasing squares, the opposite is actually the case! This is because there is even more expansion of the squares' values toward white, between -5 and 0v, than between -5v and -10v, meaning the mid grey is comparatively pushed down toward black.
Indeed Steve, if -10 volts is cutoff, black on the screen, a brightness of 0 ,
and -0V is a fully open grid hole, white on the screen, a brightness of 1 ,
then -5 volts is a half open grid hole. half the diametre, 0.25 of the circle surface, so a brightness of 0.25 .
Panrock wrote:And also... when you are looking at the grey spot, you are not 'linearly measuring' the spot brightness but are looking at it with your eyes! Your eye-brain system is non-linear, favouring expansion (better gradated perception) of the dark greys. So wouldn't this tend to counter the 'dark grey' rendering of a mid grey signal on a CRT, making the response look approximately linear, and restore the spot to looking mid-grey, without any gamma correction?
Steve O
No Sreve, what happens in our eyes and brains is always in "the chain". There is no difference in looking to a flower directly or on the screen of a TV. At least that is how we want it to be. I reality we even don't know what happens in a human eye and what happens in the brain. This even might be different for different persons. But because they are fully used to the way THEIR vision system works, every body assumes that some one else sees the same as you. We even don't know if everybody sees RED as red (what ever that may be) and GREEN as green. We only call it by the same name because our mother told us so.
In first instance consider the gamma correction in a TV camera just as a compensation for the no linear behaviour of the CRT, without which we should not have television at all. This compensation costed one dual tube extra, which was too expensive to build it in every TV. So just only one in the (/ each) camera. In total the television channel is linear between white and black (= very dark grey).
And indeed, the correction of the gamma in the camera had the advantage that the (radio) transmission channel was better adapted to the equal visibility of noise / digital quantisation in dark areas compared to bright areas. This saved us in transmitter power a factor of 64 (in watts) and in bits 3 bits in word length. In a linear digital channel we should use 11 bits words to keep quantisation in dark parts invisible. But that is not how gamma correction was born.