The Monitor calibration and Gamma assessment page

(If you already know all about gamma, and just want to get on with the calibration; go here)

First of all, a quick word about gamma:
Basically, gamma is the relationship between the brightness of a pixel as it appears on the screen, and the numerical value of that pixel.
You probably already know that a pixel can have any  'value' of Red, Green, and Blue between 0 and 255, and you would therefore think that a pixel value of 127 would appear as half of the maximum possible brightness, and that a value of 64 would represent one-quarter brightness, and so on. Well, that's just not the case, I'm afraid.
Cathode-ray tubes, such as the screen you're probably reading this on at the moment, have a peculiar relationship between the voltage applied to them, and the amount of light emitted. It isn't linear, and in fact it follows what's called by mathematicians and other geeks, a 'power law' (a number raised to a power).  The numerical value of that power is what we call the gamma of the monitor or system.

Anyway, you can read up all about gamma here, if you're really interested. What really concerns us is applying some measure of correction for this inbuilt non-linearity.

OK. Having established roughly what gamma is, why is it so important to adjust it correctly?

An uncorrected monitor is perfectly adequate for a bit of word-processing, reading e-mail, or even casual web browsing, but we're here to do some much more serious and demanding work - image processing and editing.

Here's an example of the effect that a change in gamma can have on the appearance of an image.

On the left is the image as it might appear on an un-corrected monitor.
The centre image should look right on a monitor with a gamma of around 1.8, and lastly;
the right-hand image is how a system with a linear response [gamma of 1.0] might display the image.
By the way; if you prefer the right-hand image, your first name's probably Timo. (in joke - sorry!)

Notice how the colour saturation and hue change with the gamma?
What this means is that if your monitor gamma isn't set correctly, then you haven't a hope of seeing colours and tones the way that they'll appear on other people's monitors; and they won't see your images the way that you intended either. You might also have difficulty matching the output of a printer to your monitor if your system gamma is set too dark, because -
You can't edit what you can't see!

Without any correction at all, most images would be displayed far too dark, and no detail would be visible in the shadow areas. At the same time, the highlight tones would be too widely separated.
Because we only have 255 brightness levels to play with, it's important to get the distribution of these levels right.
That way the shadows don't get 'bunched up' and difficult to distinguish, while we waste half of our precious levels defining unnoticeable increments of tone in them.

Even under ideal and dimly lit viewing conditions, an uncorrected monitor will make shadow detail difficult to see, and in a reasonably well-lit room, other factors come into play which make the task of seeing the darker parts of the image more difficult still. Our eyesight accommodates to the light, making it harder to see darker detail, and more light is reflected from the screen, further reducing image contrast.
Partial Gamma correction of the system opens out the shadows and midtones; making viewing them much easier.
I say partial correction, because a totally linear response is undesirable for reasons of colour accuracy and plain eyestrain, apart from other good technical reasons.

So; what gamma value should you aim for?

My personal preference is to set a system gamma of 1.8, even though I use a PC.
I'll explain why I made this decision, and you can choose to agree or ignore me, as you wish.

Reason 1, and most fundamental, is that a gamma of 2.2 is just too damned dark!

Reason 2. A gamma of 1.8 agrees fairly well with the output of most printers.

Reason 3. The majority of graphics professionals and pre-press proofing rooms use a gamma of 1.8, and who am I to argue?

Reason 4. Most monitors, graphics cards and associated gamma correction software can easily cope with a gamma of 1.8.
                  (Try forcing a gamma of 1.0 on an old or cheap monitor, and you'll see why it's wise to stay well within the
                  limitations of your hardware!)

Reason 5. If you go much lighter than 1.8, then you run the risk of highlight detail becoming difficult to distinguish, and
                 of colour matching problems.

Reason 6. It gives Mac users one less excuse to sneer, and they do enough unwarranted sneering as it is.

It's pretty obvious that my recommendation is to aim for a gamma value of 1.8, but you may equally want to stick to the sRGB / PC standard of 2.2, or compromise between the Mac and PC with a value of 2.0, or something else entirely.

A compromise value of 2.0, midway between the Mac and the PC, is probably a good choice if you're preparing images solely for web publishing.

Anyway, it's your choice, and if you find it's not working out, you can always recalibrate, or revert to your previous gamma setting.

Calibration time!

Of course, you need some way of adjusting the gamma of your monitor before you can proceed, and there are several ways of doing this.

If you have a full version of Photoshop from version 3 onwards, it comes with a little utility called 'Adobe Gamma'.
You can use it in conjunction with my adjusment squares, and just ignore the imprecise Adobe test patterns. In particular, Adobe's instructions for setting the brightness level lead to a very poorly defined black level.
One good feature that Adobe gamma does have, is the ability to save settings, allowing you to quickly switch between gamma values, colour temperatures, etc.

If you don't have the Adobe gamma applet, then many graphics cards have the facility in their driver sofware for adjusting gamma. Cards which have this feature are made by ATI, Matrox, 3dfx, Nvidia and quite a few others.

Then there's always an independent little application called Powerstrip, which gives you the ability to alter the gamma and colour on a whole range of graphics cards.

How to use the "Gamagic" test patterns
Ensure that your monitor has been switched on for at least half an hour to stabilise before you make any adjustments. Even the best monitors are liable to drift slightly during their warm up period.

My unique 'Gamagic' colour test patterns are a very sensitive and accurate way to set up the gamma of your system, even though using these patterns is very easy.
You simply adjust the gamma setting until all the squares match up with their backgrounds as closely as possible.
The gamma curve is checked at three intensities, and this shows up any incorrect setting of the monitor's black level (brightness control).
If you can't get the darkest set of squares to display as neutral grey, then you need to adjust the black level setting of your monitor.
The brightness control is the usual way of setting the black level, but monitors vary considerably between makes and models.
"Your mileage may vary".

Best viewing method is to half-close your eyes and sit well back from the screen to make the patterns a little blurry.
When the gamma is correct the patterns will blend to an almost uniform grey.
Away from the target gamma the colour of the inner squares shift away from neutral grey, assuming red/cyan, green/magenta, and blue/yellow hues. This allows a very sensitive and accurate setting of the target gamma.

I've been able to differentiate changes of gamma as small as 0.05 using these patterns.
Not bad for a purely visual system, even if I do say so myself.


Please choose a target gamma from the following list:

Gamma 1.4
Gamma 1.6
Gamma 1.8
Gamma 2.0
Gamma 2.2
Gamma 2.4

When you've finished, you may want to verify the gamma with an independent 'second opinion'.
I recommend this ingenious little applet from Hans Brettel, who seems to be one of the few other people to realise that gamma should be assessed across a range of brightnesses in order to be accurate.

There are a number of gamma checkers and so-called 'calibration' patterns out there on the web that will give an entirely false reading or setting of your system gamma.
They're all based on the same fallacy: That of using a dithered pattern which is too small to properly represent the correct brightness value(s). (For the technically minded; the problem is caused by the risetime of most monitor hardware not being sufficiently fast to turn from full black to full white in the space of a single pixel, or even two, in some cases.)
Luckily, most of the purveyors of these misleading patterns also seem to think that a gamma of 1 is a good idea too. Couple this with the fact that their faulty gamma patterns always indicate a gamma that's darker than intended, and to some extent, the two misconceptions cancel. The end result is a gamma setting of anywhere between 1.2 and 1.8, depending on the quality of your monitor hardware and the resolution it's set to.
Basically, if the dithering of a gamma pattern is too small to see, or appears to be made up of any sort of checkerboard pattern, then be extremely suspicious, especially if coupled with a recommendation to work at a gamma of 1.

I'm on a mission to stamp out these useless gamma checkers, and I'm prepared to 'name and shame' the worst offenders. Some of them are even incorporated in big-name software.
For a start, the 'monitor calibration' in JASC's PaintShop Pro uses these bogus patterns, and closer to home, the gamma setup of Acer's Miraphoto has exactly the same fault.

You have been warned.


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