An apparatus for measuring the optical density of human ocular media to blue light

The paper describes an apparatus constructed by the authors and aimed at evaluating the optical density of human ocular media by a non-invasive psychophysical method described previously (Sample et al. 1988). The device has two light sources within the wavelength bands of 437 +/- 6 nm and 557 +/- 6 nm, respectively. They are a square-wave alternated at 1 Hz. The radiance of each can be controlled by the subject and adjusted at visibility threshold or at subjective equiluminance. Upon scotopic vision, the ratio between the two thresholds (or ratio of radiances at equiluminance) is a function of the ocular optical density at the short wavelength. The psychophysical procedures of adjustment of the 437 nm source at the absolute threshold and at equality with the 557 nm source had been tested with volunteers aged between 24 and 67 years. The dependence of blue-light sensitivities on age, obtained with the two procedures, suggests, in agreement with the literature, two main sources of sensitivity decline with age: senile myosis and ocular-media density increase at the short wavelength end of the visible spectrum.