Monitor Adjustment: Measuring Gamma

Here is a method to measure the gamma of a display (monitor and associated computer) using an inexpensive home made “photometer” and simple test targets displayed in Photoshop. Gamma can be measured for the greyscale and for red, green and blue seperately if desired. The measurement is absolute, in the sense that it does not depend for its accuracy on the calibration of the “photometer”. Only numbers displayed by Photoshop’s info window are used to determine gamma.

First check your monitor’s brightness and contrast settings using the test cards previously described.

The “photometer” consists of a photoresistor (light dependent resistor) connected to a digital multimeter. This indicates the resistance value which depends on the level of light. By mounting the photoresistor in the base of a plastic pudding basin (about 6 inches (150mm) in diameter) which rests against the screen it “sees” a fixed area of the screen from a fixed distance. This is not strictly a “photometer” as it is not calibrated, but because it is sensitive to very small changes in luminance, it can produce a very accurate “match”, which is all that we require. From now on I will refer to it as a “sensor”.

How to use it – to set gamma to 2.2

Two test targets are provided : stripes.png a pattern of equal black and white stripes (24 pixels wide to minimise edge effects) – this will give an average luminance of 50% of the white level – and gamma22.png a grey target which will have exactly the same luminance if the gamma is 2.2.

You might choose to use this to adjust the gamma correction provided in the graphics card. The method is as follows.

  1. Open the stripes.png target and the gamma22.png in Photoshop – assign the sRGB profile in each case.
  2. Position the sensor so that it is over the target, select the stripes and read the resistance from the meter.
  3. Switch to the grey target and adjust the gamma value in the graphics card control panel to obtain the same resistance reading on the meter.
  4. Repeat steps 2 and 3 to be sure that they match (an exact match may not be possible).

How to use it – to measure gamma

If you have calibrated and profiled a system by some other method you may wish to check the value of gamma obtained.

  1. Open the stripes.png target and the gamma22.png in Photoshop – assign the sRGB profile in each case.
  2. Open the info window and select the dropper from the tools pallette – the black stripes should read 0,0,0, the white stripes 255,255,255 and the grey target 186,186,186.
  3. Add a brightness and contrast adjustment layer to the grey target and set the opacity to about 10% . This will provide a fine adjustment of the brightness (ignore contrast).
  4. Position the sensor so that it is over the target, select the stripes and read the resistance from the meter.
  5. Switch to the grey target and adjust the brightness slider to obtain the same resistance reading on the meter.
  6. Repeat steps 4 and 5 to be sure that they match (an exact match may not be possible).
  7. Use the dropper on the grey target and read the RGB value from the info window.
  8. Read the corresponding gamma value from the table below.
  9. When closing the target files do not save changes. It is a good idea always to check the grey target with the dropper – the info should read 186,186,186 corresponding to a gamma of 2.20.

How to set gamma or measure gamma for the Red, Green and Blue channels seperately

The methods described above can be used for measurements on the separate red, green and blue channels.

  1. Select the stripes and in the layers window open the channels tab.
  2. Select the grey target and do the same.
  3. In each case, click on the “eye” to “switch off” the green and blue channels – this produces a reference target of red stripes and a red target of 50% luminance for gamma of 2.2.

Repeat this process for green and blue, by “switching off” the unwanted channels. Remember: do not save changes when closing the files.

RGB value gamma RGB value gamma
177 1.90 187 2.24
178 1.93 188 2.27
179 1.96 189 2.31
180 1.99 190 2.36
181 2.02 191 2.40
182 2.06 192 2.44
183 2.09 193 2.49
184 2.12 194 2.54
185 2.16 195 2.58
186 2.20 196 2.63