Heterophoria--its influence on stereopsis and the importance of cycloplegia in refraction testing of pilot applicants.

Heterophoria, its influence on stereopsis, and the importance of cycloplegia in refraction testing of pilot applicants were studied. Distant heterophoria was found in 94.5-100% while near heterophoria was found in 86.9-98.1% of the cases, depending on the test used. Hyperphoria of over 1 prism diopter was extremely rare. No statistically significant correlation between distant heterophoria and the degree of stereopsis could be demonstrated at a range of up to at least 6 prism diopters. Cycloplegia induced a mean difference of +0.75 diopter in refraction before and during cycloplegia. Eight percent of the applicants failed because of refractive errors (spherical extremes, -0.75 diopter and +4.50 diopters; cylindrical extreme, 1.25 diopters). It is concluded that stereopsis does not seem to be affected much by heterophoria within moderate limits, and that cycloplegia in refraction testing of young pilot applicants must be considered relatively important.