Display quality and the speed of visual letter search

Abstract

Previously it has been suggested that visual search tasks can be used to evaluate the perceptual quality of display devices with respect to their ability to convey alphanumeric information. The purpose of the present study was to examine how character size and contrast used in a visual search test affects its sensitivity to differences in the quality of display devices. The task of the observer was to search for, and identify, an uppercase letter from a rectangular array of characters in which the other items were numerals. Threshold search time, that is, the duration of stimulus presentation required for search that is successful with a given probability, was determined by using a multiple-alternative staircase method. Eye movements were recorded simultaneously by using a fast video eye tracker. Threshold search times were measured as a function of character size at two contrast levels using a CRT and an LCD display. For all experimental conditions, threshold search time decreased with increasing letter size. This was accompanied with a decrease in the number of eye fixations per search as well as a decrease of fixation duration. At high contrast ( C Michelson≈1), no statistically significant difference was found between the two displays. However, at the lower contrast used ( C Michelson≈0.2) and at small character sizes, threshold search times for the CRT display were clearly longer and the number of fixations per search was higher than for the LCD display. In conclusion, visual letter search is a more sensitive method for display evaluation at small contrasts and with small character sizes.