GB2188425A

GB2188425A - Measuring resolution of a cathode-ray tube

Info

Publication number: GB2188425A
Application number: GB08701762A
Authority: GB
Inventors: Ian Douglas Macarthur
Original assignee: Fujitsu Services Ltd
Current assignee: Fujitsu Services Ltd
Priority date: 1986-03-26
Filing date: 1987-01-27
Publication date: 1987-09-30
Also published as: GB8607592D0; GB8701762D0

Abstract

A method is described for measuring the width of a line (16) displayed on a cathode ray tube. The line is viewed through a parallel-sided prism (10) having a slot (11). The prism is rotated until the line as viewed through the prism appears to be displaced by one line width relative to the line as viewed directly, through the slot. The degree of rotation of the prism is a measure of the line width. The prism is rotated by a knob (14) having a calibrated scale around its edge which can be read relative to a pointer (15). <IMAGE>

Description

SPECIFICATION Measuring resolution of a cathode-ray tube This invention relates to a method and apparatus for measuring the resolution of a cathode-ray tube (CRT), i.e. for determining the width of a spot or line produced by the electron beam on the screen of the CRT under working conditions.

One conventional method of measuring CRT resolution is to use a travelling microscope, with a calibrated graticule. This method is quick and easy, but is also very inaccurate because of variation in operator perception of where the spot actually ends.

Another known method is to reduce the size of a raster pattern on the CRT screen until the individual scan lines just merge. However, this method is not suitable for use in testing a complete video monitor system, since it involves disturbing the picture which it is desired to test.

Other methods involve the use of electronic equipment for measuring the spot size, but such equipment is very expensive and can take a long time to set up.

One object of the present invention is to provide a method of measuring CRT resolution which avoids these problems.

Summaries of the invention According to one aspect of the invention there is provided a method of measuring the width of a line displayed on a chathode ray tube, comprising: (a) viewing the line directly and at the same time through a parallel-sided prism, (b) rotating the prism about an axis parallel to the line until the line viewed through the prism appears displaced by one line width from the directly viewed line, and (c) measuring the displacement of the line as a function of the degree of rotation of the prism.

According to another aspect of the invention there is provided apparatus for measuring the width of a line display on a cathode ray tube, comprising: a parallel-sided prism; a support; means for enabling the prism to be rotated relative to the support about an axis parallel to the line; viewing means mounted on the support for viewing the line directly and at the same time through the prism; and measuring means for indicating the degree of rotation of the prism; the apparatus being effective when the prism is rotated to measure the displacement of the line viewed through the prism in relation to the line when viewed directly, as a function of the degree of rotation of the prism.

Brief description of drawings One method in accordance with the invention, and apparatus for performing the method, will now be described by way of example with reference to the accompanying drawings.

Figure 1 shows the apparatus.

Figure 2 illustrates the operation of the apparatus.

Figure 3 shows how the apparatus is supported when in use.

Description of an embodiment of the invention Referring to Fig. 1, the apparatus comprises a parallel-sided prism 10 of glass or acrylic material, having a slot 11 extending through it.

A viewing lens 12 is positioned (by means not shown) over the slot 11. The prism is mounted for rotation on a shaft 13, having an attached measuring knob 14, calibrated with a scale which can be read off against an index pointer 15.

As shown in Fig. 3 the shaft 13 is rotatable in a container 16 having an open end. The viewing lens 12 is carried in a holder 17 which is positioned in the other end of the container 16.

In use, the container 16 is mounted on a tripod 18 which supports the apparatus in a fixed position over the screen 19 of the CRT to be tested, with the axis 13 parallel to a line 16 displayed on the CRT screen. The knob 14 is set at a zero reading on the scale, and in this position the face of the prism 10 lies parallel to the CRT screen.

The line 16 is then viewed through the lens 12. The observer will see three images of the line: a central image through the slot 11 in the prism, and a further image on either side of the slot, through the material of the prism.

These three images should all be in line, as shown in Fig. 2a: it may be necessary to move the eye to achieve this.

The measuring knob 14 is then rotated, so as to rotate the prism 10. Because of refraction through the prism, this causes the images of the line viewed through the prism to be displaced relative to the central image viewed through the slot. The amount of displacement will depend on the degree of rotation. The knob is rotated until the top of the central image lines up with the bottom of the other two, as shown in Fig. 2b. The reading on the scale is then noted.

The procedure is then repeated with the knob 14 rotated in the opposite direction until the bottom of the central image lines up with the top of the other two images. The reading on the scale is again noted: this should be the same as the first reading.

To make reliable measurements, it is preferable to make a number of readings, ten for example, half with the central image above and half with it below the other two images, and then to take an average of the readings.

The scale on the measuring knob 14 can be calibrated directly in terms of line width.

Various modifications can be made to the apparatus described above. For example, better optical arrangements can be made to enable the apparatus to be positioned some distance from the CRT, enabling measurements to be made without casting shadows and in corners that might otherwise be inaccessible due to surrounding casework. Also, some means may be included for superimposing the images, so as to permit easier comparison.

Furthermore, the rotation of the prism may be measured by electrical means to provide a readout to permit computer logging of the readings.

Claims

1. A method of measuring the width of a line displayed on a cathode ray tube, comprising: (a) viewing the line directly and at the same time through a parallel-sided prism, (b) rotating the prism about an axis parallel to the line until the line viewed through the prism appears displaced by one line width from the directly viewed line, and (c) measuring the displacement of the line as a function of the degree of rotation of the prism.

2. A method according to Claim 1 wherein the line is viewed directly by way of a slot in the prism.

3. A method according to either preceding claim wherein the prism has an attached measuring knob, calibrated in terms of displacement distance.

4. A method according to any preceding claim, wherein the prism is first rotated so as to displace the line viewed through the prism by one line width in a first direction, and then rotated so as to displace the line by one line width in the opposite direction.

5. Apparatus for measuring the width of a line displayed on a cathode ray tube, comprising: a parallel-sided prism; a support; means for enabling the prism to be rotated relative to the support about an axis parallel to the line; viewing means mounted on the support for viewing the line directly and at the same time through the prism; and measuring means for indicating the degree of rotation of the prism; the apparatus being effective when the prism is rotated to measure the displacement of the line viewed through the prism in relation to the line when viewed directly, as a function of the degree of rotation of the prism.

6. Apparatus as claimed in Claim 5, wherein the prism includes a slot through - ow which the line is viewed directly.

7. Apparatus as claimed in Claim 5 or 6, wherein the means for enabling the prism to be rotated comprises a shaft on which the prism is secured, the shaft being rotatable relative to the support.

8. Apparatus as claimed in Claim 7, wherein the measuring means comprises, a measuring knob having a calibration scale thereon and secured to the shaft, and a fixed index pointer positioned so as to enable the scale to be read.

9. Apparatus as claimed in any preceding claim, in which the viewing means includes a viewing lens mounted in the support and aligned with the prism.

10. Apparatus as claimed in any preceding claim, wherein the support comprises a container mounted on a tripod to thereby enable the apparatus to be supported in a fixed position relative to the cathode ray tube.

11. A method of measuring the width of a line displayed on a cathode ray tube, substantially as hereinbefore described with reference to the accompanying drawings.

12. Apparatus for measuring the width of a line displayed on a cathode ray tube substantially as hereinbefore described with reference to the accompanying drawings.