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Thursday, 18 April 2019

Fluorescent Screen in CRT


The Fluorescent Screen in CRT


Fluorescence is the property of some crystalline materials like phosphor or zinc oxide to emit light when stimulated by light radiation. 

Phosphorescence is the property of fluorescent materials to continue light emission even after the source that exited is withdrawn. 

Persistence of the phosphor is the time duration for which the after glow or phosphorescence occurs.

The screen of the cathode ray tube is coated with material called phosphor that will emit light when bombarded by electrons. There are many possible phosphors that differ in colour, persistence, efficiency etc. One of the commonest is willemite. This is a composition of zinc orthosilicate, ZnO + SiO2, with traces of manganese as an "activator". This produces the greenish trace of small general purpose cathode ray tubes. Compounds of zinc, cadmium, magnesium, and silicon are the other useful materials for the screens. Presence of metals like silver, manganese, copper and chromium in proportions as small as 1 part in 105 will increase the light output of the screen by a factor 10 to 100 and will also affect the colour. When used these materials are called activators.

Phosphors are prepared by grinding, crystallizing, regrinding etc. and are then deposited on the end of the cathode ray tube by settling over of a liquid suspension.

The light output of a fluorescent screen is proportional to the number of bombarding electrons that is the beam current and increases approximately as the square of the anode voltage. The spectral distribution of the light depends upon the base material and upon the activator. A great variety of colours is available.

The luminance, (light intensity from the screen) depends on the following factors.

1. The number of bombarding electrons, (i.e., beam current) per second.
2. The energy with which the bombarding electron strikes the screen. This is determined by the accelerating potential.
3. It is a function of the time the beam strikes a given area of phosphor. Hence the sweep speed will affect the luminance.
4. It is a function of the physical characteristics of the phosphor itself. 

A table showing the characteristics of some of the commonly used phosphors is given below Table.

The choice of phosphor for particular application depends on number of factors. p31 phosphor, has high luminance and medium persistence. It is used in general purpose oscilloscopes.

PHOSPHOR DATA CHART

Phosphor Type
Fluorescence
Phosphorescence
Relative Luminance
Decay to 0.1 %(ms)
Comments
P1
Yellow - Green
Yellow - Green
50%
95
General Purpose; replaced by P 31 in most applications.
P2
Blue - Green
Yellow - Green
50%
120
Good compromise for high  and low - speed applications
P4
White
White
50%
20
Television displays
P7
Blue
Yellow - Green
35%
1.500
Long decay : observation of low – speed phenomena.
P11
Purple - Blue
Purple - Blue
15%
20
Photographic applications
P31
Yellow - Green
Yellow - Green
100%
32
General – Purpose; Brightest available phosphor.



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