VDT design factors

	Overview   |   AC component   |   Calculating the AC component   |   Flicker and VDT design   |   Image polarity   |   Display brightness/contrast measurements   |   Resolution   |   Color

Display brightness/contrast measurements
The photometric techniques used to characterize a display image should measure the contrasts of the details that the visual system must detect.

There are several ways to measure display luminance and contrast. One practice is to use large area measurements and assume that they represent the characteristics of the details to be detected by the visual system. Larger area measurements are made by first measuring the luminance of the screen in the off condition, then taking a second measurement from an area of the screen where all the lines, or pels (sometimes called pixels), are turned on. This is an extreme type of measurement and there are electronic and optical reasons why this method may not represent the actual luminance levels that define the symbol/background contrast. For example, consider the following measurements taken with a microphotometer from a display that was capable of presenting positive and negative image segments at the same time.

Positive is defined as dark characters on a light background; negative as light characters on a dark background.

Positive and negative image sections.

The measurements were taken with both image sections presented on the screen at the same time and with the brightness setting in a fixed position. The measurement locations were as follows:

A, 105 cd/m²: this measurement was taken in the center of a 10 cm wide bright background area. The dark characters on either side of the measurement area were more than 1 cm away.

B, 59 cd/m²: the center of the dark stroke of one of the capital "X"s that was presented on the bright background.

C, 13 cd/m²: from an area of the dark background of negative image section that was more than 2 cm away from an illuminated point.

D, 16 cd/m²: from the center of the nonilluminated area between the vertical strokes of adjacent "M"s in the negative image area.

E, 85 cd/m²: from the center of the illuminated stroke of one of the capital "M"s in the negative image area.

Comparing these measurements results in the following:

The A to C comparison derived from wide area photometry. That would indicate a contrast ratio of 8:1. Because of internal light scatter, etc., however, that contrast ratio is reduced to l.8:1 between the actual symbol and its background in the positive image section (A to B). The E to D comparison is the actual contrast, or modulation, that the eye must use to detect the separation between the two "M"s.

It is clear from the above that techniques and conditions can lead to different figures characterizing the same display. While the example assumes a CRT-like display, the situation is even more complicated for Liquid Crystal Displays.

The alignment of the directors is the polarizer, liquid crystal and analyzer vary with direction of measurement, further confounding the numerical results. When making a judgment about displays, it is important that numbers provided are established from the same protocol.

Back to top