In late 1949 the first colour television was introduced, the need for 3 separate electron guns meant that television technology had to improve, subsequently this TV only had a screen of 11cm.
All of the early televisions used circular dots of phosphors arranged in a triangle (see figure 2, page 5), they were called Delta tubes. In 1972 Sony introduced a screen that used stripes of phosphors (see figure, page 5) which all Television screens now use.Delta gun tubes continue to use the circular phosphor dots rather than the stripes but they are not used for Television any more but they are used in computer monitors as they give a far higher definition when used for a very high resolution, with the use of many dots. The electron guns are positioned in a triangle, perfectly similar to the triangles that appear on the screen. The electron guns are aimed so that the electron beams converge as they pass through the grille.
The grille (see fig 2) is positively charged acting as an accelerator to the electrons and a stencil to make sure that the electrons only hit the desire phosphor dots. After the beams pass through the grille they hit the phosphor dots on the screen. With the delta tube (fig1) each electron beam has a separate set of electromagnets acting on it yet they are needed to travel to the same point.
It seems that Televisions stopped using Delta tubes because all three electron beams had different scanning patterns, rasters, as they had left the electron guns at different angles so would behave differently at different positions on screen. By looking at the diagram below it should be apparent that as the guns are quite far apart the beams will also be far apart so when they hit the screen they will not be the ideal distance apart..
The result of this is shown by fig 3 The Trinitron tube uses three electron beams that are in line, but only one gun.This means that the electron beams travel much closer together and the picture they produce is not made from round dots but the rectangular ones. By only having one electron gun with three cathodes, each beam can be put under the same magnetic field which means there is no need for calibration In the Trinitron tube the magnetic field directs all three beams to a prism which directs the beams, instead of the solenoid electromagnets. Analysis Television was only made possible through comprehension of the phtoelectric effect which allowed scientists to detect light, as an intensity.