قناع الظل .. كتاب التلفزيون الملون والعادي

11 - 3 (2) Shadow mask:

It is a very thin metal case

(0.22 mm) located behind the front face of the screen. It contains very small holes, cracks, or windows - the number of which is equal to the number of phosphorous triplets in the surface of the screen, which is about (440,000) holes in the 25-inch screen. Figure (11-4) shows the holes, openings, or cracks in the shadow mask, the three rays of which - and Figure (11)- (8) shows the process of matching the rays and their collision with the specific light points. These holes are distributed in a very precise way - using photochemical techniques - so that it allows Each hole by passing the three rays through it to hit its designated points, i.e. one triple for each hole, while acting as a veil and a shadow from which the other rays do not pass. Thus, we avoid the errors of a ray reaching other than its color points

The same work is done in the mask for the linear dots screen.

(Inline) To tighten the beam, the holes in the center of the mask are completely circular, while their shapes are conical at the edges. The holes are sharp edges to prevent reflection

The light from its edges Figure (10-11). The shadow mask must be installed very accurately - and it is about (1.25 cm) away from the phosphorescent face of the screen - and despite the necessity of having this mask, it has some disadvantages. It blocks most of the light that reaches the screen, so it only reaches about 15% of the rays through holes or openings. This makes the monochrome screen more productive for the same beam intensity - and the color screen requires a higher voltage of more than 25 kV in large measurements.

And the metal of the mask must have a weak thermal factor, because its high temperature may lead to a change in the dimensions of the holes, which deviates each ray from its special points.

The mask can be fixed by means of four regular springs that prevent accidental deflection of the mask, and a belt of aluminum alloys is placed on the inner perimeter of the screen valve, to tighten the seal between the mask and the inner glass wall (Fig. 9-11) to prevent the leakage of scattered electrons affecting the image. - The deviation and convergence in the screen valve are adjusted by the magnetic method - and the beam is focused or focused by the electrostatic method.

Shadow mask magnetization:

The mask is exposed to external magnetic fields, arising from nearby iron or steel blocks and geomagnetism, as well as internal magnetism arising from some coils and elements of TV circuits, and the magnetization of the mask leads to distortion of the image and colors - so the screen covers the back with magnetic shielding materials in addition to wrapping all magnetic circuits in inside the device, whose fields may affect the screen. There are magnets and coils around the neck of the screen whose function is to achieve the static (static) and kinetic (dynamic) convergence process. There is a demagnetization file that is located around the back of the screen - and a variable current of decreasing intensity reaches zero within a few seconds.