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Research Topic: Grayscales & Gamma

By Ian Mander, 30 November 2003 "to help a friend", updated 1 June 2003.

Question: How do I know if my computer monitor is correctly adjusted?

Answer: Assuming you have your monitor displaying millions of colours, when viewing a stepped series of grey (from black to white), only the black should look completely black, and only the white should look completely white. When viewing a smooth blend from black through to white, the mid-tones should be in the middle of the blend and not have any colour tinge.

See below for more info on that. (This isn't the most accurate of test material, I know, but it was enough to prove a doubting friend's monitor was seriously out of calibration.)

Greyscale

The boxes below are evenly shaded from black to white. On an ideally adjusted computer monitor a viewer should be able to distinguish all the shades, with only box 0 black and only box 16 white.

0	1	2	3	4	5	6	7	8 ^	9	1 0	1 1	1 2	1 3	1 4	1 5	1 6

If you're game for more, try this one. You should be able to tell between boxes 0 and 1, and between boxes 31 and 32.

0	1	2	3	4	5	6	7	8	9	1 0	1 1	1 2	1 3	1 4	1 5	1 6 ^	1 7	1 8	1 9	2 0	2 1	2 2	2 3	2 4	2 5	2 6	2 7	2 8	2 9	3 0	3 1	3 2

Questions to ask yourself:

• Do the boxes look like even steps across the page?
• How many are coloured black on the left end?
• How many are coloured white on the right end?
• Is the middle box (indicated by a ^ symbol) about what you expect mid-grey to look like or is mid-grey closer to one end than the other?
• Do any boxes have a colour tinge?

Gamma:

Gamma is basically how relatively bright or dark the midtones are. Compare these blends, produced with different gamma values:

Very dark				Looks much darker than normal, mid-grey is shifted far towards white. Looks normal when viewed with a gamma of 0.25.
Dark				Looks darker than normal, mid-grey is shifted towards white. Looks normal when viewed with a gamma of 0.5.
Normal				Looks normal, mid-grey is in the middle. Looks normal when viewed with a gamma of 1.0.
Light				Looks lighter than normal, mid-grey is shifted toward black. Looks normal when viewed with a gamma of 2.0.
Lighter				Windows gamma. Possibly looks normal when viewed on a PC with a gamma of 2.4.
Very light				Looks much lighter than normal, mid-grey is shifted far toward black. Looks normal when viewed with a gamma of 4.0.

For some strange reason Windows-based PCs use a very high gamma (2.4) which means that normal pictures have a tendency to look rather dark when viewed with a PC.

If a computer display's gamma is slightly different for the three primary colours (red, green, blue) slight colour tinges will be seen in plain gray blends. In this simulated exagerated example, note how the mid grey tones look purple:

This means that one of the primary colours is operating with an effectively different gamma from the others. It may be due to the display software on the computer, or the monitor itself having a different sensitivity for one of its phosphors. As far as I know this sort of problem can only be fixed by running monitor setup and calibration software with gamma setup for each of the primary colours individually. Good computers (that is, computers with good system software) will allow you to run a series of display tests so that you can tweak each primary colour to get gamma and brightness as good as possible.

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