Understanding and exploiting the abilities of the human visual system is an important part of the design of usable user interfaces and information visualizations. Designers traditionally learn qualitative rules of thumb for how to enable quick, easy, and veridical perception of their design. More recently, work in human and computer vision has produced more quantitative models of human perception, which take as input arbitrary, complex images of a design. In this article, we ask whether models of perception aid the design process, using our tool DesignEye as a working example of a perceptual tool incorporating such models. Through a series of interactions with designers and design teams, we find that the models can help, but in somewhat unexpected ways. DesignEye was capable of facilitating A/B comparisons between designs, and judgments about the quality of a design. However, overall “goodness” values were not very useful, showed signs of interfering with a natural process of trading off perceptual vs. other design issues, and would likely interfere with acceptance of a perceptual tool by professional designers. Perhaps most surprisingly, DesignEye, by providing in essence a simple visualization of the design, seemed to facilitate communication about not only perceptual aspects of design, but also about design goals and how to achieve those goals. We discuss resulting design principles for making perceptual tools useful in general.