
                 WHY DO IMAGES APPEAR DARKER ON SOME DISPLAYS?

An Explanation of Monitor Gamma



   Robert W. Berger


    rwb@cs.cmu.edu



   Note: All inline images in this document are links to the inline image
   files themselves. This is done to support non-inline web clients, and
   to make it easier to experiment with the images using external
   viewers.

  WHAT IS GAMMA?

   In the early days of television it was discovered that CRT's do not
   produce a brightness that is proportional to the input voltage.
   Instead, the brightness produced by a CRT is proportional to the input
   voltage raised to the power gamma. The value of gamma varies depending
   on the CRT, but is usually close to 2.5. The gamma response of a CRT
   is caused by electrostatic effects in the electron gun.

   A CRT with a gamma of 2.5 has a response like this:

   [LINK] Gamma Chart

   Note that the effect of the CRT gamma is to darken the midtones
   relative to the dark and light regions. An input voltage of 50%
   produces a brightness of only 18%.

  WHAT DO TELEVISION STANDARDS DO ABOUT GAMMA?

   Since most sensors used in television cameras produce output voltages
   proportional to scene brightnesses, a correction for CRT gamma must be
   applied to the camera signal to avoid having midtones in the scenes
   being too dark on the TV set. When television standards were defined
   it was decided to correct for the gamma of the CRT in every television
   set by using a correction circuit in the camera which applies a gamma
   of 0.45 = 1/2.2. This value was chosen to simultaneously correct for
   the CRT gamma of 2.5 and compensate for the apparent reduction of
   contrast which occurs when a TV is viewed against the dim background
   typically found in a living room. Applying the correction at the
   studio was more economical than applying the correction in every TV
   Set.

  WHAT DO COMPUTER DISPLAYS DO ABOUT GAMMA?

   Many computer displays ignore the effect of monitor gamma. The frame
   buffer values provided by the application software are converted
   linearly into voltages that drive the CRT in the display. The values
   in the frame buffer are not proportional to the resulting brightness.
   A frame buffer value of 1/2 the maximum will produce less than 1/2 the
   brightness, as shown by the gamma curve above.

   Some display systems such as NeXT's and SGI's contain hardware lookup
   tables that correct for monitor gamma. On these systems the frame
   buffer values provided by the application are corrected for the gamma
   of the CRT by a lookup table in the display controller, producing a
   display system gamma of 1.0 which linearly maps frame buffer values
   into brightness.

   Macintosh computers contain lookup tables in their display hardware
   which can be used to correct for CRT gamma. However, the default
   values of the lookup tables do not completely compensate for CRT
   gamma. Instead, the CRT gamma is partially compensated for to produce
   an effective display system gamma of 1.8. A gamma of 1.8 closely
   matches the response of a Laserwriter, which has a nonlinear response
   due to dot gain (ink spreading) and other effects.

   A 1 bit display that produces grayscales by dithering between two
   values will have a linear brightness response no matter what the
   monitor's gamma response is. This fact is used in the tests below.

   The lack of standardization in dealing with monitor gamma has caused
   significant problems with systems like World Wide Web which distribute
   images to different types of displays. An image that looks good on one
   brand of display might have the midtones too bright or too dark on a
   different brand, because of the difference in the displays' gamma.
   Lack of gamma correction also affects color hues. For example, if a
   color having a red component of 50% and a green component of 25% is
   displayed on a CRT with a gamma of 2.5 without correcting for gamma,
   the resulting intensities will be 18% red and 3% green. In addition to
   being darkened, the color has been shifted toward red. Dark red
   fleshtones are a common manifestation of failing to correct for CRT
   gamma.

  WHAT IS THE GAMMA OF MY DISPLAY SYSTEM?

   As mentioned above, a display which simulates grayscales by dithering
   adjacent pixels between 0% (black) and 100% (white) will have a linear
   brightness response regardless of the monitor's gamma. This fact is
   used to demonstrate the effect of display gamma in this image:

   [LINK] Gamma Demonstration Image

   The image contains 2 rows of 3 squares, with the value of the squares
   in each row varying from 25% to 75%. The top row uses gray values; the
   bottom row simulates the grays by dithering. On a display which
   corrects for monitor gamma the top squares will have the same
   brightness as the corresponding bottom squares. On a system which does
   not correct for monitor gamma the top row of squares will appear
   darker than the bottom row. Standing about 6 feet from the monitor
   gives the best results.

   The image below allows you to directly estimate the gamma of your
   display system. Stand about 6 feet away and decide which column of the
   image comes closest to having equal brightness in the top and bottom
   halves. The number under this column is the gamma of your display
   system.

   [LINK] Gamma Measurement Image

  WHAT ABOUT COLOR MATCHING?

   As mentioned above, display gamma can have a major effect on the color
   hues of an image by changing the relative intensites of the red,
   green, and blue channels in a nonlinear fashion. Lack of gamma
   correction is probably the major cause of color shifts between
   different displays.

   Other phenomena also affect the color hues produced by different
   displays. The colors of the red, green, and blue primaries can be
   different for different displays. Color can be measured in a device
   independant way using the CIE color model, which is based on an
   analysis of the human visual system. Two color sources with the same
   CIE color coordinates will look the same to the human eye. The color
   of a source is specified in the CIE system as two coordinates, x and
   y. A third coordinate z need not be explicitly stated since x, y and
   z are normalized such that x + y + z = 1.

   A fairly complete characterization of the response of a color display
   can be had by specifying:

    1. The gamma values, or response curves, for the red, green, and blue
       channels.
    2. The CIE colors produced by displaying white, red, green, and blue
       on the device. The first is known as the display's White Point,
       the last three are known as the display's Primary Chromaticities.

   The TIFF file format has provisions for specifying the properties
   listed above.

  FOR MORE INFORMATION

   Charles Poynton has written some excellent FAQ's and articles
   concerning gamma and color correction. I recommend them to those
   interested in more details on the subjects discussed here.

   This page has been accessed [INLINE] times since June 20, 1995.

   Robert Berger's Home Page

    rwb@cs.cmu.edu

