Using scotopic and photopic flicker to measure lens optical density.

Many applications require knowledge of lens absorption. Measuring lens optical density (OD), however, is often difficult and time-consuming. For example, psychophysical measurement typically requires a long period of dark adaptation (e.g. about 40 min) and assessment of absolute scotopic thresholds. In this study, we examined efficient scotopic and photopic methods for measuring lens OD. In Experiment 1, 30 subjects were tested using a Maxwellian-view optical system. Relative scotopic thresholds were obtained after 15 min of dark adaptation using slow-rate (2 Hz) flicker photometry. A 3 degrees test stimulus, presented at 10 degrees nasal, was used that alternated between measuring wavelengths (420 and 460 nm) and a reference field (540 nm). The results showed that the relative scotopic method produces values that are consistent with published lens spectral curves. In Experiment 2, relative photopic measures (i.e. no dark adaptation period) were also obtained at 406 nm in natural view and compared with lens data obtained in Maxwellian view at 407 nm using the absolute scotopic method. The photopic method compared well with the absolute scotopic values obtained on the same subjects. Taken together, the two experiments showed that a relative method can yield valid lens density estimates. Using a relative rather than an absolute method reduces the time needed for dark adaptation and is an easier task for subjects to perform and may therefore be preferable when expeditious measures are desirable.