Effects of Positive and Negative Image Polarity VDT Screens

Abstract

Eight experienced typists worked two full days in a VDT workstation using either a hard copy-screen or split screen data presentation and a light or dark character CRT display. Each of the four conditions was run one half day, using a data entry and a file maintenance task. An Applied Science Laboratory 1998 computer controlled eye monitoring system collected eye scanning and pupil diameter data. Keystrokes per minute and errors per file were monitored by an Apple II computer which was also driving the IBM 3101 VDT (with glare filter). At the beginning and end of each 90 minute work session (separated by a 15 minute rest break or a 45 minute lunch break), the subjects answered 9 ocular and 9 musculoskeletal comfort/discomfort questions displayed on the screen. The results of this study show that the percentages of time, the number of dwells per minute, the dwell times of the subjects looking at different areas of the VDT workstation (screen, document, keyboard, outside), and the typing performance are closely the same for both screen polarities and there is no evidence that screen polarity affects any of these measures in a practically and statistically significant manner. There are also no practically and statistically significant differences for the pupil diameters. However, there appears to be a small but consistent trend which indicates slightly smaller pupil diameters when the dark characters with the light (green) background CRT display was used. The subjective discomfort questionnaire results do not reveal any significant differences in either ocular or musculoskeletal discomfort scores while using either the light or the dark character CRT display. Therefore, one might use either a light or a dark character CRT display and can expect very similar VDT operator eye scanning behavior, typing performance, discomfort scores and pupil diameters as long as a sufficient character-background luminance contrast is provided. If one wants to consider engineering factors such as longer tube life one should specify the use of light characters on a dark background VDT screen (negative polarity).