Luminance of projection focimeter target with oblique viewing

In order to test the capacity of an optical projection focimeter for simultaneous use by several observers gathered around the instrument, the luminance of the projected target was measured for 3 projection focimeters (model Axil from Essilor, model LP2 from Nikon, and model LM-P5 from Topcon) when the photometer axis was perpendicular to the instrument screen and at oblique angles up to 75 degrees in 5 degrees increments. As expected, the luminance of the target decreased as obliquity of viewing increased; however, the rate of reduction is characteristic of each focimeter model. The repeatability of dioptric measurements with the Axil focimeter was assessed in the usual viewing position when the luminance of the target was reduced by optical filters (neutral density varying from 0.2 to 1 in 0.2 steps). The repeatablity remains within +/- 0.05 D for optical densities < or = 0.8. Our results show that a useful luminance is maintained over an observation angle that extends to 45 degrees. Depending on the model of focimeter, several observers may simultaneously observe and judge the focusing of the instrument.