Graph 2 - Gamma:
Gamma (or more accurately the gamma correction value) controls the overall brightness of an image or how fast the brightness curve comes out of black. The higher the gamma value the slower the signal comes out of black as the brightness increases. An average gamma value that is too high will result in a picture that is brighter with deeper blacks but also show less information in dark scenes. Our previous Luminance graph is proof of this. An average gamma value that is too low will produce an image that is less bright image with too much brightness in the dark scenes which gives a flat/washed out look.
Here is our graph of gamma from 0 to 100 IRE:
The target value for average gamma we are trying to achieve for our display is 2.2. The ColorHCFR software tries to target 2.2. The ideal target is to match a properly calibrated (telecine) mastering reference monitor used by the movie studios. These monitors are designed for a ruler flat 2.20 response.
A gamma value of 2.2 gives us a perfect balance: Plenty of brightness and excellent shadow detail. If your room is pitch black with very little light reflection and you have a display with a very high contrast ratio (such as a CRT projector) then you may find values up to as high as 2.5 to be acceptable (the debate of 2.2 vs 2.5 gamma is a huge raging debate amonst calibrators). In most cases however closer to 2.2 is likely going to look better and is what you should try and aim for. Remember that lower numbers mean more gamma/brightness. Later on when you remeasure your greyscale I'll suggest that you try and target 2.2. For what it's worth, I've tried both 2.5 and 2.2 and prefer something closer to 2.2 in my light controlled home theater powered by a CRT projector. If you have a means of adjusting gamma (see below), I recommend you try both and decide yourself. Either way you should not go over 2.5 or below 2.2.
A gamma that is too high like mine means that our overal brightness will be better and we'll have deeper blacks, but at the expense of losing details in the darker scenes. In our example, my gamma for each point from 0 to 100 IRE can be seen by the yellow line and ranges from something so high (beyond 3) it's off the graph, down to 2.27. The average gamma (cyan line) is 2.64 which is too high compared to the 2.2 target. The especially high values in the dark areas mean that in my setup I'm completely losing shadow detail which is typical of all CRT displays. This needs to be fixed by adjusting the gamma control within the display or by adding a a gamma boost box to the signal as most displays do not have an internal gamma adjustment option. (No CRT projector has the required type of gamma boost adjustment that we require built in).
As mentioned previously, I use an RTC2200
external box to add a gamma boost which helps me achieve the proper target numbers. This box as well as many high end scalers (Lumagen, Crystalio), Moome HDMI cards and converters
, and the X-Vue Box1020
RGB/Component converter products all have adjustable gamma boost features built in. If all you want is gamma boost in the least expensive package, then check out the GammaX
dongle. It's designed by people behind the popular HDfury line of HDMI add-on converters
Videophiles look for this gamma boost feature as it is a critical one to have in order to obtain a proper gamma curve. Without one of these devices in your signal chain, many displays will not track gamma properly and you'll end up with a dull/lifeless image with a lack of shadow details. More information on how gamma boost works and before & after screenshots showing why it's needed (especially on CRT displays) can be found in this thread:Gamma Correction: What is it? Why is it needed?
Later on we'll add the RTC2200
box back into the signal chain to see the difference it produces and explain how and when you should be adjusting your gamma (if you have such an adjustment available).
If your gamma is above 2.2 you'll want to look at adding one of these devices with adjustable gamma boost to your setup.