Standardization of the tangent screen examination: some neglected parameters.

Usually two factors are not rigidly standardized in the routine tangent screen examination: (1) the stimulus characteristics of the carrier that fixes the spot to the wand, and (2) patient instructions about what aspects of the stimulus they should attend to. Differences in these factors within the range of what is commonly done in the tangent screen test can lead to significant differences in the locations of the isopters. There is also an interesting interaction between these two factors in establishing isopter size.