We investigated the effects of pixel density, anti-aliasing, and stroke width on the subjective image quality of characters displayed on high-density liquid-crystal displays (LCDs). We used 6×4.5 photographs to simulate high-density LCDs. The simulated LCDs had 70 different display conditions that consisted of seven pixel densities (100 to 400 ppi stepped by 50 ppi), five stroke widths, and two font types (anti-aliased fonts and bi-level fonts). The character size was fixed at about 3 mm. The display luminance was set to 150 cd/m2 and a contrast ratio of 30:1 was used. At a 30-cm viewing distance, 30 subjects assessed the character image quality of the 70 simulated LCDs. The results indicated that subjective evaluation of character image quality became saturated at about 250 ppi for anti-aliased fonts and 350 ppi for bi-level fonts. Therefore, the required pixel density for computer displays is 250 ppi for anti-aliased fonts and 350 ppi for bi-level fonts.
