Color Purity

To get true color on the screen, we have to start with the three primary colors. Red, Blue and Green.  From these three primary colors are derived all other colors displayed on the screen.  While many believe that the amount of colors displayed on any one screen is dependant on the monitor, this would be a wrong assumption, unless dealing with TTL or Digital signals, more notably know as CGA and EGA (Color Graphics Adaptor and Extended Color Adaptor respectively).  Today we deal with, for the most part Analog video cards, which are able to output more colors depending on the amount of Video ram as well as what scan rate you’re using.

Now this being true, a monitor can only display what it is told to, by the video card installed in any one system.

Still the basic purity of color on any given monitor is due to how well the alignment or landing of the electron beam on the correct phosphor at any given point of scan. Remember, that at any given time of the picture being presented on the screen, only one phosphor per electron beam (there are three in a CRT – Red/Blue and Green) is illuminated. Therefore if we turn off the blue and the green guns, and display what should be a white raster, with only the Red gun left on, the image displayed on the screen should be equal on all parts of the screen. If it is not, it is said not to be pure.  This is because the Red Electron Beam is landing on more than it should.  The same would be true with either Blue or Green on only.

CRT purity is normally set up by the CRT manufacturer at the time of manufacture, but can be affected by outside influences such as electrically generated magnetic fields, FANS  and such, as well as magnets mounted to speakers. 

All monitors manufactured incorporate an Automatic Degaussing circuit that actuates when the monitor is first turned on.  This eliminates stray magnets fields imposed on the CRT by outside sources as described above.

Most monitors manufactured today also contain either a menu item named Degauss or a mechanical button named the same. This allows the owner to manually degauss or De-Magnetize the CRT.  This is generally not needed, however in some cases, the owner may tilt or swivel the monitor to another position while on.  This places it in a slightly different orientation to the Earths magnetic fields.  Manually degaussing the CRT normally corrects this.

In some cases, such as someone placing a magnet to the screen, the automatic and or the manual degauss will not correct the problem. This is because the coil, which in mounted around the CRT inside the monitor, does not have sufficient strength to eliminate the magnetized metal mounted inside the CRT.  The monitor should then be taken to a service shop to be degaussed. It should take only a few minutes and most service shops charge only a small fee to perform the procedure, as it does not require the servicer to open the monitor.  The servicer also knows how long, and how close to have the demagnetizing field to the CRT.  Too close and too long could warp the metal inside the CRT, making the only option CRT replacement.