Monitor Parameter Adjustment
How to visually determine your monitor's gamma parameter
The following table contains columns of color triples at different
intensity levels for each of the three color channels. The center
patches and the two surrounding patches may look different. The
following steps may be used to determine a monitor's gamma parameter:
There are some more video test images contained in the
CVD home page.
- Turn off the room lights if you want "colorimetric" settings. Use
your normal working environment if you want "working environment"
settings (whatever that means).
- Switch your browser to full screen display.
- Adjust your monitor's brightness
setting such that black areas are black and the lowest intensity color
patches are barely visible. You may use a CVD low
intensity test image which contains very low intensity steps.
Nine of the ten low intensity patches should be visible.
- Adjust your monitor's contrast setting as high as possible. You
may use the CVD high intensity test imagewhich contains very high intensity steps. Nine of the ten patches should be
different from the highest intensity white.
- For each color
channel scroll the table horizontally until you find that column where
the visual difference between the center and the surrounding patches
is smallest. The respective number printed in that column is your
monitor's gamma parameter for that channel. If you need a single
gamma parameter for all three channels choose the column with the
smalles over all difference between center and surrounding patches for
all color channels.
How does it work?
Suppose the gamma function is L = E**g with E being input
voltage and L being luminance output. Then the input voltage necessary
to create pixels with luminance L/2 can be computed by
L/2 = (E**g)/2 = (E/(2**(1/g)))**g using the gamma
parameter value g. Thus a patch where each pixel has input voltage
(E/(2**(1/g)))**g has halve of the luminance of a patch where each
pixel has input voltage E. A patch with halve of the luminance of the
homogenous patch with input voltage E, however, may also be created by
only controlling every second pixel with voltage E and setting every
other pixel to zero luminance. If the gamma parameter g is correct
then both patches should look equal if the viewing distance is large
enough. Also, it is crucial to avoid crosstalk between pixels and thus
the usual method is not to control every second pixel but to control
every second video line by input voltage E and every other line by
The method above assumes that there is no incident light from
secondary illuminant sources on the screen. This usually is a wrong
assumption. To take this into account one has to use a different gamma
function with more than a single parameter.