Electrical Engineering ⇒ Topic : Beam Target or Screen
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Lalan
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Beam Target or Screen The screen converts the K.E. of electrons into light energy. For this, special chemical coating is done on the face of the tube which may be flat or slightly convex, round or square. The screen in CRO is normally small (10 cm x 10 cm). Screen is also called windows. The material used is phosphor or zinc oxide. There are different types of phosphor having different colours. They are also rated as per persistence, short (l/e of excitation is less than 1 m in a second), medium (decay is less than 2 m in a second), and long (decay may take minutes or more). The colour that is generally used for oscilloscopes is green, given by P31 with medium persistence. For radar, P33 of orange colour with long persistence is used. For computer graphics, P39 of green colour with medium to medium long persistence is used. For photographic records, Pll of purple blue with medium persistence is used. For B and W TV, P4 of white colour with medium short persistence is used. For colour TV, P22 with three-colour dot pattern (red, blue, green) with medium persistence is used. The phosphor coating exhibits both properties of phosphorescence (light emission even after excitation is removed, that is, afterglow) and fluorescence (light emission on excitation or stimulation). Persistence is the length of time during which, afterglow lasts after the stimulation is removed. When the electron spot impinges on the screen, light is emitted and heat is given out at the expense of K.E. This causes burn out of the phosphor coating. To avoid this, the spot should not be focussed at a particular point for the long time. The screen is aluminized on the inside to act as a heat sink, to prevent scattering of light, to prevent building up of charges on the phosphor, and it increases the brightness on striking the phosphor. Secondary electrons are emitted by the phosphor on bombardment and these low velocity secondary electrons are conducted away by the aquadag coating on the slope of the tube to the anode. The anode and aquadag coating are electrically connected. In magnetic deflecting tube, the aquadag coating acts as the post-accelerating anode. In a CRT, the first three regions up to the neck are called the gun. | |
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Lalan
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Beam Target or Screen The screen converts the K.E. of electrons into light energy. For this, special chemical coating is done on the face of the tube which may be flat or slightly convex, round or square. The screen in CRO is normally small (10 cm x 10 cm). Screen is also called windows. The material used is phosphor or zinc oxide. There are different types of phosphor having different colours. They are also rated as per persistence, short (l/e of excitation is less than 1 m in a second), medium (decay is less than 2 m in a second), and long (decay may take minutes or more). The colour that is generally used for oscilloscopes is green, given by P31 with medium persistence. For radar, P33 of orange colour with long persistence is used. For computer graphics, P39 of green colour with medium to medium long persistence is used. For photographic records, Pll of purple blue with medium persistence is used. For B and W TV, P4 of white colour with medium short persistence is used. For colour TV, P22 with three-colour dot pattern (red, blue, green) with medium persistence is used. The phosphor coating exhibits both properties of phosphorescence (light emission even after excitation is removed, that is, afterglow) and fluorescence (light emission on excitation or stimulation). Persistence is the length of time during which, afterglow lasts after the stimulation is removed. When the electron spot impinges on the screen, light is emitted and heat is given out at the expense of K.E. This causes burn out of the phosphor coating. To avoid this, the spot should not be focussed at a particular point for the long time. The screen is aluminized on the inside to act as a heat sink, to prevent scattering of light, to prevent building up of charges on the phosphor, and it increases the brightness on striking the phosphor. Secondary electrons are emitted by the phosphor on bombardment and these low velocity secondary electrons are conducted away by the aquadag coating on the slope of the tube to the anode. The anode and aquadag coating are electrically connected. In magnetic deflecting tube, the aquadag coating acts as the post-accelerating anode. In a CRT, the first three regions up to the neck are called the gun. | |
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