Reading performance and visual comfort with scale to grey compared with black-and-white scanned print

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A particular problem in the display of scanned text is the fact that, given current technology limitations, images can be scanned at a much higher pixel density than they can be displayed on the monitor. Therefore one pixel on the monitor must be used to represent a block of pixels in the scanned image. Most commonly a reduction algorithm is applied to the scanned image to produce a set of black and white pixels on the screen (BW). Another algorithm approach (‘scale to grey’ or STG) is to display a pixel whose grey scale value is calculated from the mix of white and black pixels in the scanned image. STG produces characters which appear more visible — similar to anti-aliasing which can be used for designed text. We tested the effects of BW and STG displayed text upon reading performance and visual comfort. Twenty-one subjects performed six 20 min reading trials under different display conditions. These six conditions consisted of three different monitors each with BW and STG. The three monitors were selected to provide different display resolutions. Reading performance was measured as the number of lines read, and symptoms were measured with a questionnaire at the end of each trial. ANOVA analysis showed reading performance to be significantly related ( p<0.0001) to the monitor and also to the display algorithm ( p<0.0001). Reading performance was better on higher resolution monitors, and for each monitor the performance was better with STG (by 19.9, 7.3 and 4.1% for low to high resolution monitors). The symptom score was significantly related to the display algorithm ( p<0.0032), symptoms were lower with STG. STG text resulted in significantly faster reading speed and significantly lower symptom scores compared to the same text displayed in BW. The STG advantages were greatest on monitors with lower resolution.