CRT monitors have at least one feature in common - they all have a cathode ray tube. The tube is a vacuum casing containing several elements. A cathode ray emits electrons when it is heated. The electron gun bunches the electrons and fires them towards the anode, which draws the beam to the front of the tube. As it leaves the gun, two coils containing an electrical current deflect the beam. One is for vertical deflection, the other for horizontal. The more positive coil attracts the negative electron beam, more or less. So, the basic principle of the tube implies fewer mobile parts and ensures good reliability. For monitors displaying color images, there are three electron guns, each handling a basic color - red, green and blue. This is known as additive color technology. Shades on the screen are made up of the three colors in varying degrees of intensity, and are brought out by drops of phosphor at the surface of the tube. These drops are very close together and make the eye perceive the result of the three impacts as a single pixel.
Theoretically, this results in identical colors, but it is mainly in the way the beam strikes the mask in front of the screen that the technologies differ from manufacturer to manufacturer.