Typically, CRT manufacturers today do not specify the image performance such as focus, convergence, and geometry, out to the edges of the tube. As a result, you have to evaluate these parameters for yourself. Also, users typically do not want their images overscanned as in TV displays, especially when using GUI's. If monitors overscanned, parts of the image near the edges of the screen may not be visible. Thus, the useable image size of a monitor will be smaller than the maximum useable phosphor area which may be specified for the FTC. VESA has already established and published a standard for useable image size in a computer monitor.

Comparing and Testing Monitors

First make sure that the monitor(s) has warmed up for at least ten minutes. The heat escaping from the rear of the monitor should not be much more than that generated by a colour television. Some monitors are now coming with fans installed for positive ventilation. Next, adjust the brightness so that the illuminated part of the screen has the same brightness as the unilluminated border. Increase contrast to a reasonable level (fairly high) and reduce screen glare as much as possible. Now you're ready to check the following:

Focus: It is important that the electron gun be focused in the center of the screen and near the corners. The corner areas are typically problematic. Look at bright text on a dark background in the center, and in the corners of the screen. Letters should be quite legible, and pixels shouldn't bleed into each other at the screen edges. Bill Nott suggests looking at lower case e's and m's to see if they're readable everywhere.

Convergence: Look closely at white lines on a black background. If the lines are white along the edges, convergence is good. If, however, a band of another colour is visible along the line, then colour reproduction of small objects such as characters or lines may be poor. Even if color banding is present, the monitor may still be within the manufacturer's specification. If you can see distinct differently colored lines, chances are the monitor does not meet the specification, but color fringing, while possibly considered objectionable, is likely to be present in almost every monitor built.

Pincushioning: Hold something straight (like the edge of a piece of paper) up to the edge of the screen image while viewing the display straight on, from a typical viewing distance. If the image edges bow away from the straight edge, the monitor is exhibiting pincushioning or barreling. Barreling occurs when too much pincushion correction is applied, such that the display bulges outward. Some monitors provide a pincushioning adjustment, but if one is unavailable and pincushioning is severe, significant geometric distortion is likely. Check the pincushioning for different screen addressabilities/refresh rates, as it may vary.

Geometric Distortion: Move an object of consistent size ( a window works well) around the screen and measure its height and width with a ruler. Significant variations in the size at different locations indicate geometric distortions that may not be correctable.

Colour Purity: Display pure red, green and blue and for each look for colour inconsistencies in the display that may indicate poor colour reproduction.

[From: Sam Goldwasser (sam@stdavids.picker.com)]

White Purity: Display a totally white screen. The brightness should be reasonable uniform and there should be no objectionably obvious coloured or tinted splotches.

Color Bleeding: Display bright primary colored object - red, green, and blue. There should be no colored trails off to the right of the bright areas.

Moire: This will depend on resolution and size. There should be no objectionable contour lines visible in the background or smooth areas of the image.

[From: Andy Laberge (tic-toc@wolfe.net) and Michael Scott (scott@bme.ri.ccf.org)]

Overall Impression: Is the image clear, bright and sharp? Remember that you will be looking at the monitor for hours at a time, and that a minor flicker may become irritating over time. When possible, look at the specific monitor you want to buy, as each monitor has undergone a calibration procedure and some may be better than others - even of the same model. This is one big advantage that local stores have over mail order companies - you can look at the monitor before paying.