In this video, we will be understanding one of the techniques used for displaying colour using CRT - Shadow-mask method. 
The Color CRT monitor displays images using a combination of Phosphorous which emits different coloured light. From the combination of an emitted range of light, colours can be produced from different phosphors. 

There are 2 basic techniques for displaying colour using CRT (Cathode Ray Tube):

Beam-penetration method: It is similar to normal CRT and it uses one electron gun. It displays coloured images by using random-scan monitors. It uses two layers of phosphors, red and green,  which are coated from the inside of the CRT screen. The colour depends on, how far the electron beam penetrates Phosphor layers. This two combination of two colours produces four colours red, green, orange, and yellow. A beam of slowly moving electrons can excite only the outer red layer. A beam of very fast electrons penetrates through the red layer and can excite the inner green layer.

Shadow-mask method: Shadow-mask is a commonly used technique, which uses a raster scan system. It produces a much wider range of colours as compared to Beam penetration. It is mainly used on television sets and monitors.

In this, the pixel is made up of three – colours. Because of three colours it uses three electron guns one for producing each colour, a phosphor dot emits a red colour light, the second emits green light, and the third emits blue light. In this way, the CRT has three electron guns, one for each colour point, and a shadow mask grid just behind the phosphor-coated screen with tiny holes in a triangular shape. It is having metal screen placed before the phosphor screen known as a “shadow mask”.

Working: A shadow mask consists of a metal plate with tiny holes which are present inside a colour monitor.

It directs the beam of the electrons, and the beam hits only at a particular point and displays a resulting picture. The three guns present in the device direct the beam to the shadow mask which allows them to pass. Shadow mask displays a wide range of pictures when compared to beam penetration.