Gamma Correction

Update 28/03/02, using Gericom monitor with on-screen colour management

After researching the net and practical experience this is my understanding of gamma correction. Being a mechanical services engineer I always like going back to basic principles and have a full understanding of what's going on.

All monitors have a non-linear relationship between input voltage and pixel intensity, which for Windows is to the power of 2.5. therefore any difference in the balance of the RGB guns can be small at the bottom of the curve, but extreme at the top.

Gamma correction corrects this curve into a straight line, whose mid-point will be a neutral grey of values R=127,G=127,B=127.  For most Windows monitors a value of 2.2 should be correct. But, if the monitor guns are not in balance to begin with, these values cannot be equal, nor can any amount of gamma correction make them so.

However, to overcome this problem, Adobe and PaintShop Pro gamma correction routines go one stage further, letting you correct for any colour in-balance.  This produces a different gamma correction value for each gun. The problem is that this is very subjective to the viewer and plainly open to error. Don't forget that this error can be amplified by the power of 2.5!

My advice for the average home user is to balance the monitor RGB guns to an equal value to begin with, or choose a colour temperature of 6500k with a reasonable level brightness so you can still see solid blacks.

Install Photoshop software from scratch. 

First, try to obtain an ICC profile for your make of monitor, if you can, and use this to start with. If you haven't got an ICC profile you will need to find out the type of phosphors and the gun intensities from the monitor manufacturer, not always easy.

Ensure that the desktop is set to a neutral mid-grey colour before carrying out Gamma Correction. Carry out the gamma correction routine in Control Panel, ideally using an ICC profile for your monitor and save the resulting ICC profile, giving it a new name such as "Gamma Corrected Iiyama 350". In Photoshop File-Colour Settings-RGB Setup set the RGB Setup to Simplified Monitor RGB, the white point to 9300K, Gamma 2.2, Primaries to Custom and Monitor to Generic EBU 2.2 Gamma Monitor. Finally ensure that the check box is ticked for "Display using monitor compensation".

After running  the gamma correction routine for the first time you should find that the default values for gamma of 2.2 will need no alteration, and similarly no adjustment to the RGB balance points will be needed. ie there will be no error prone adjustment required.  Printing an image using the right ink and paper, with printer settings to match, should produce a perfect result.

My new Gericom monitor gives the following RGB values for the listed colour temperatures, using the on-screen controls:-

Please email me if you have a similar success, as there are many people who argue against such an approach, saying you have to compensate for ambient light etc. I would agree, if the colour correction can be done very accurately with proper (expensive) instruments to measure the correction values. For the average user I maintain it's best to keep things simple.