Analysis of these questions leads into deep waters of sensory physiology and signal analysis, but the problem can be adequately solved. A preferred optotype is the 'Landolt-C', an annulus with a gap in one of 8 major directions. The entire procedure lends itself well for computer implementation, where graphic presentation of annuli with gaps derserves special attention. One could increase observer distance, but that would require huge screens under pathologic conditions. This term has been popularized in flyers for CD players and is increasingly used in computer graphics.
Spatial resolution is improved with the help of intensity resolution.
So a small input box is necessary for the subject, and an external key pad for PowerBooks proves to be an ideal solution. In the ophthalmic department of Freiburg University it is permanently installed at several sites. Children use it like a video game. Visual scientists use it for rapid screening of visual function. Many options in the program allow customization for various situations.