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Measurement of Beam Landing Errors on Cathode Ray Tube

IP.com Disclosure Number: IPCOM000050391D
Original Publication Date: 1982-Oct-01
Included in the Prior Art Database: 2005-Feb-10
Document File: 3 page(s) / 54K

Publishing Venue

IBM

Related People

ORourke, R: AUTHOR [+2]

Abstract

Using a microscope for measuring purity and other beam landing errors on color cathode ray tubes (CRTs) is a time consuming, tedious and not very accurate process. In the method proposed below, the use of a microscope is kept to a minimum and by using an XY plotter of only A4 size, an effective linear magnification of at least 500 times can easily be realized, the best practical magnification with a microscope being about 120 to 150 times.

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Measurement of Beam Landing Errors on Cathode Ray Tube

Using a microscope for measuring purity and other beam landing errors on color cathode ray tubes (CRTs) is a time consuming, tedious and not very accurate process. In the method proposed below, the use of a microscope is kept to a minimum and by using an XY plotter of only A4 size, an effective linear magnification of at least 500 times can easily be realized, the best practical magnification with a microscope being about 120 to 150 times.

In Fig. 1, optical fiber 1 carries light from the screen phosphor of the CRT under test via an appropriate filter (red, green or blue) to photomultiplier 3. Digital radiometer 4 amplifies the signal from photomultiplier 3 which is then transmitted to XY Plotter 5's 'Y' input as a voltage level. The fiber-optic probe tip used has a diameter of about 4.5 mm and therefore "looks" at about 100 phosphor dots of one color. This averages out errors due to unequal brightness of dots and other screen imperfections.

Spot wobbler coils 6 are temporarily fixed on the front of the CRT 2 in the position of the required measurement. By applying alternating current to the coils from wobbler drive unit 7, the beam is "wobbled" up and down for vertical purity- beam landing error (BLE) measurement or side to side for horizontal purity-BLE measurement.

Any BLE present can be compensated by adjusting the DC OFFSET in the coil drive waveform in the horizontal (H) and/or vertical (V) coils such that the "hourglass" shape produced by the shadowed beam being wobbled is perfectly centered in both the H and V axes. The DC offset current-voltage is used in conjunction with a very slow (for example, 5 sec/cycle) wo...