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EIA TEP 105-9

1987 Edition, 1987

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Line Profile Measurements in Shadow Mask and Other Structured Screen Cathode Ray Tubes

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Description / Abstract:

The purpose of this test is to measure the profile of a line on the face of a shadow mask or other structured screen cathode ray tube (CRT) in order to estimate the resolution capacity of the CRT. Since the display on a CRT usually consists of a series of lines or line segments forming alphanumeric characters or graphics patterns the line width is an important parameter in the determination of the resolution. For this reason, the test method discussed below measures the horizontal and vertical (and possibly diagonal) profiles of the lines produced by the electron beam or beams. From these profiles, the line widths can be determined. Only one gun of a mutiple gun CRT is measured at a time. Such a test for determining the size of the CRT spot is the first step in any resolution calculation. Note that line profiles can also be determined from measurements of the intensity contours of the CRT spot. This method of determining the line profile is not discussed in this document.

The line profile of the CRT spot is dependent on a number of factors in addition to the focused electron beam distribution at the screen. Some of these factors are:

1. Electron scattering in the screen and support structure

2. Light scattering due to the granular nature of the phosphor and the aluminum backing of the screen

3. Saturation of the phosphor

4. Internal reflections and absorption of light in the faceplate and panels mounted onto the faceplate

5. Optical coatings, etches, etc., on the faceplate or panel

6. Static or dynamic convergence of the beams in multiple beam tubes

External influnces on the resolution of the CRT, including ambient light, are not discussed in this method. These include the the transfer characteristics of the viewing system, in particular, the nonlinear response of a human observer.

Determination of the resolution of a structured screen CRT is a complex problem which is not completely solved at the present time. The limiting resolution of the tube can, in some cases, be estimated by referring to the conclusions reported in recent papers. For a shadow mask color CRT, the resolution is strongly dependent on the pitch of the shadow mask. Kojima1 and Barten2,3 suggest that for a dot screen CRT the spot diameter at 5% of peak brightness should be 2.5-2.6 times the shadow mask pitch( the center to center distance between nearest neighbor mask holes ). The same relation seems to hold for line screen tubes where the pitch is the horizontal distance between the mask slots. For beams smaller than this, moire patterns are produced. In addition to the references above, a short bibliography is attached. The reader is urged to consult these references and the current literature before making any conclusions about the resolution of a structured screen display.