Page "Shadow mask" Paragraph 17

The guns, arranged in a triangle at the back of the tube, were aimed to focus on the metal plate and scanned it as normal.

For much of the time during the scan, the beams would hit the back of the plate.

However, when the beams passed a hole they would briefly pass through it to the phosphor in front of the plate.

In this way, the plate ensured that the beams were perfectly aligned with the colored phosphor dots.

This still left the problem of focussing on the correct colored dot.

Normally the beams from the three guns would each be large enough to light up all three colored dots on the screen.

The mask helped by mechanically attenuating the beam to a small size just before it hit the screen.