What Makes a Great CRT

A good CRT needs to perform well in all of the tests outlined in the next section. But to earn the distinction of a Great CRT it must excel in each of the critical tests below. Note that all of the test patterns mentioned below are proprietary DisplayMate test patterns and that most are found only in the professional DisplayMate Multimedia Editions. For background information and general guidelines see How to Test and Evaluate Displays.

1. High Video Bandwidth Index:
The DisplayMate Video Bandwidth Index measures the visibility of high frequency fine detail in an image. A value of 100 is perfect and indicates that high and low frequencies, corresponding to broad and fine detail, are identically reproduced. Values near 100 are not normally possible because of limitations in analog electronics and the CRT's finite beam size. Using one of the recommended Video Boards in the DisplayMate Best Video Hardware Guide the Index should be in the range:

85-95   for   1024x768     at   75 Hz
80-90   for   1280x1024   at   75 Hz
75-85   for   1600x1200   at   75 Hz
70-80   for   2048x1536   at   60 Hz

2. Slight Moiré:
With the display's Moiré reduction enabled no more than one test pattern in DisplayMate's Moiré test suite should show a weak Moiré. All of the other patterns in the suite should appear Moiré free.

3. Excellent Color Registration:
Mis-registration in the central area of the screen should be less than 0.1 beam width and less than 0.5 beam width in the corners. Use DisplayMate's Blinking Color Registration Test patterns.

4. Low Geometric Distortion:
Less than 1.0 mm of distortion anywhere near the edges of the screen. Any distortion should be smoothly varying with no kinks or bending visible, so that the distortion is not apparent. Even when the outer edges are true, there can still be many forms of interior distortion. Use any of the DisplayMate Geometry and Distortion test patterns. For quantitative analysis use the Interactive Screen Distortion Measurement suite in DisplayMate Professional for DOS.

5. RGB High Frequency Balance:
The Video Bandwidth Index (item 1) for each of the primary Red, Green and Blue channels should be identical. If not then there will be a shift in color going from broad to fine image detail. An easy way to check this is to view the Video Bandwidth Index test pattern in White. Adjust the Index slider so that the high and low frequency blocks are as closely matched in intensity as possible. There should be no visible difference in color tinting between the blocks.

6. No Color Tracking Error:
The intensity of the Red, Green and Blue channels should vary identically with signal level. If that isn't the case then a pure gray-scale will show color tinting in one or more areas. This often occurs near black at low intensities and also near peak intensities due to unbalanced saturation between the RGB channels. Use the DisplayMate Color Tracking and White-Level Saturation test patterns. No change in color tint should be visible. For a more precise determination use the DisplayMate Window Pattern and a color analyzer.

7. Weak Screen Reflections:
Only very weak internal and external reflections from the screen should be visible. This is accomplished with high quality anti-reflection coatings and high-absorption glass. The internal reflections within the CRT glass and phosphor layers reduce the very important local contrast. Use the DisplayMate Dark Screen, Screen Halos, and Internal Light Scatter test patterns to search for stray light. View these patterns in the darkest possible environment.

8. Focus and Sharpness:
CRT image sharpness depends on many factors including beam size, focus, color registration, and video bandwidth. Use the DisplayMate Focus Matrix, Resolution Matrix, Scaled Fonts, Page of Text, and Defocusing, Blooming and Halos test patterns to evaluate sharpness over the screen. Sharpness is generally best in the center of the screen and worst in the corners. To eliminate the effect of color registration use only one primary color, typically Green.

9. Accurate Gamma:
The shape of the gray-scale as it increases from black up to peak white is described in terms of a mathematical power-law function with an index called Gamma. The Gamma for most CRT displays is in the range of 2.2 to 2.5. A Gamma of 2.2 is the standard for NTSC TVs and 2.8 for PAL TVs. The Gamma affects the accuracy of the gray-scale and color mixtures so specific values are necessary when high accuracy is required. Use the DisplayMate Gamma Correction Measurement test pattern to accurately measure the Gamma interactively on-screen. This pattern requires that the Video Bandwidth Index (item 1) be set first. For a more detailed analysis, use the DisplayMate Window Pattern and a photometer to measure the entire brightness curve. Because of APL Shift, see item 16 below, this will vary with the size of the Window Pattern. Use both 5% and 100% sizes. Plot the measurements on log-log graph paper and make sure that you obtain a perfectly straight line with a slope of the desired Gamma value.

Testing and Evaluating CRTs

To thoroughly evaluate a CRT display you should examine it using all of the DisplayMate test patterns. Refer to the Test Information screen for each test pattern for details on the purpose of the pattern and what to look for in the image. In addition to the test patterns discussed in What Makes a Great CRT above a number of important proprietary DisplayMate patterns are also featured below:

10. Screen Uniformity and Color Purity:
The screen is generally brightest at the center, with lower intensities in the corners. On a high quality display this will not be apparent visually. Use one of the DisplayMate Screen Uniformity test patterns to check the uniformity for both bright and dim intensities using a White screen color. Use a photometer if measurements are required. Cycle through Red, Green and Blue to look for intensity and color variations that indicate Color Purity and Beam Landing errors. Note that some displays include controls to at least partially correct these problems.

11. Really Black Black-Level:
The black-level for a properly adjusted CRT should appear to be true black except in very dark environments. Verify this visually using one of the DisplayMate Black-Level Adjustment test patterns. A sensitive photometer may be used to measure the residual brightness of the black-level.

12. High Peak Brightness:
You can visually compare the relative brightness between two CRT displays but it's best to use a photometer to measure this value. Before measuring peak brightness you need to properly adjust the Brightness and Contrast controls. The Contrast control should be set as high as possible without producing blooming, halos or screen regulation problems, see items 8 and 15. Because of screen regulation the brightness will depend on the area of the screen being illuminated. Use the DisplayMate Window Pattern. Areas of 5% and 15% are frequently used. Compare those values to that for a 100% area. The closer the measured values the better. DisplayMate also includes the ANSI brightness test pattern with target positions for measuring ANSI standard brightness.

13. High Contrast:
Display contrast is the ratio between the peak white intensity and the black-level intensity. The higher the better because visibility and readability depend on contrast. While contrast can be compared or estimated visually it's best to measure it with a photometer. Unfortunately the peak white-level and black-level can be specified in many different ways. As mentioned in item 12 above the peak white-level depends on the Window area used: 5%, 15% or 100%. It will be highest for the 5% window and lowest for the 100% window. It also depends on the settings of the Brightness and Contrast controls. The black-level depends on the room lighting and will be lower in a pitch black room. It will also be lower if the entire screen is set to black. If portions of the screen are illuminated the intensity of the black areas of the screen will increase due to internal reflections within the CRT. There are two common measures of contrast: full-field sequential and ANSI. The ANSI method uses a carefully defined procedure and a 4x3 checkerboard pattern that partially takes into account screen regulation and internal reflections. The sequential method is non-standard but generally uses a 5% window with the Contrast control set at maximum regardless of blooming and regulation effects followed by a completely black screen in a pitch black room. This is the contrast that manufacturers generally publish. It can be a factor of 2 or more greater than the ANSI value. The DisplayMate Multimedia Edition includes test patterns to measure the contrast both ways.

14. No Ringing and Overshoot:
Poor signal processing or over-peaking will produce Ringing and Overshoot, which is visible as extra light or dark edges or waves around fine image detail. Use the DisplayMate Ringing and Overshoot and Sharpness with Contrast test patterns. The Reverse Video Contrast and Color Streaking patterns are also excellent. No Ringing or Overshoot should be visible.

15. Good Screen Regulation:
The high voltage circuits in the CRT are affected by the beam current needed to produce the image brightness. On most CRTs the image expands in areas of the screen that are bright and contracts in areas that are dim. The entire screen may be affected or just portions of it. Use the several DisplayMate Screen Regulation test patterns to check for these effects. On high quality displays no effect will be visible. You can measure the peak deflections with a cloth tape measure.

16. No APL Shift:
The brightness in one part of the screen may be affected by the brightness in another part of the screen and by the average brightness over the entire screen. This is due to another form of regulation. It is evaluated in terms of the Average Picture Level or APL of the image. Use the DisplayMate APL Black and Gray-Level Shift test patterns to visually evaluate this effect.

17. Weak Streaking:
Streaking is visible as positive or negative shadows that trail image detail on the right. When there is severe streaking it will wrap around and appear to precede the image detail. Streaking is most apparent in high contrast situations. Use any of the DisplayMate Streaking test patterns to look for this effect.

18. White-Point Color Temperature:
The color temperature of the screen is generally set at the factory to 9300° for most computer monitors and televisions. This corresponds reasonably well to the color of "Bright White" paper in office lighting. Many monitors include end user presets for other values, typically 5000° and 6500°, which are useful for graphic arts and photography respectively. Use a color analyzer to verify the accuracy of the settings. Note that measuring correlated color temperature alone is not sufficient to determine the white-point. The actual Chromaticity Coordinates must be measured and compared.