Three human performance experiments were conducted to examine the pixel structure requirements for graphical text displayed on flat panel systems. The results of the experiments demonstrate an asymmetric effect of horizontal and vertical pixel width and separation upon several different measures of image quality. The first experiment demonstrated that pixel structure is undetectable with horizontal fundamental spatial frequency components greater than 0.167 cycles per mil (6.68 c/mm) and vertical spatial frequency components greater than 0.100 cycles per mil (4.0 c/mm). The second experiment replicated this effect using a subjective image-quality-rating procedure. The third experiment extended the replication to character legibility performance. The asymmetry effect stems from the structure of the characters used in the experiments; that is, the spatial frequency spectrum of the characters used was broader in the horizontal dimension than in the vertical dimension. Moreover, these results suggest certain visual principles applicable to design alternatives for high-quality flat-panel display systems.