Since all CRTs for color displays work with additive color mixing according to the RGB color model, the three electron beams for the colorsred, green and blue must be guided so that they hit only the color phosphors assigned to them. The electron beam for red must excite only the red-emitting phosphor; the same applies to green and blue.
To achieve this goal, a mask with small holes is placed in front of the color phosphors, the shadow mask, which prevents the electron beam of one color from hitting a neighboring color phosphor and then illuminating it as well, which would result in color distortions. The hole spacing between the individual holes is about 0.2 mm. Hole masks have a similar structure to the slit mask or the strip mask.