The visual display terminal (VDT) is an integral part of the modern office. An issue of concern associated with the use of the VDT is maintaining privacy of on-screen materials. Privacy filters are products designed to restrict the viewing angle to documents displayed on a VDT, so that the on-screen material is not visible to persons other than the VDT operator. Privacy filters restrict the viewing angle either by diffraction or diffusion of the light emitted from the VDT. Constrained posture is a human factors engineering problem that has been associated with VDT use. The purpose of this research was to evaluate whether the use of privacy filters affected: 1) the restriction of postures associated with VDT use, 2) operator performance, and 3) subjective ratings of display issues, posture, and performance.

Nine participants performed three types of tasks: word processing, data entry, and Web browsing. Each task was performed under three filter conditions: no filter, diffraction filter, and diffusion filter. Participants were videotaped during the tasks using a camera mounted above the VDT workstation. The videotape was analyzed and horizontal head deviation was measured at 50 randomly selected points during each task. Horizontal head deviation was measured as the angle between an absolute reference line, which bisects the center of the VDT screen, and a reference point located at the center of the participant's head. Standard deviation of head deviation were evaluated across filter type and task type. Accuracy- and/or time-based measures were used to evaluate performance within each task. Participants used a seven-point scale to rate the following: readability, image quality, brightness, glare, posture restriction, performance, and discomfort.

The results indicated that the interaction between task and filter type affected the standard deviation of horizontal head deviation (a measure of the average range of horizontal deviation). The standard deviation of horizontal deviation was significantly larger within the Web browsing task under the no filter and diffusion filter conditions as compared to the diffraction filter condition.

Filter type affected subjective ratings of the following: readability, image quality, brightness, posture restriction, and discomfort. The diffraction filter resulted in lower readability, image quality, and brightness ratings than the diffusion and no filter conditions. Participants reported that the ability to change postures was significantly decreased by the use of the diffraction filter as compared to the no filter and diffraction filter conditions. The diffraction filter resulted in an increase in reported discomfort as compared to the no filter condition. The interaction between filter and task type affected subjective ratings of performance. Participants reported a decrease in the rating of perceived performance under the diffraction filter / Web browsing condition as compared to the no filter / word processing, diffusion filter / Web browsing, and diffusion filter / data entry conditions. A decrease in the rating of perceived performance was reported in the diffraction filter / data entry condition as compared to the no filter / word processing and diffusion filter / Web browsing conditions. Neither diffraction nor diffusion filter affected performance within any of the tasks, based on the objective performance measures used in the experiment.