Influence of age and spectral power distribution on mesopic visual sensitivity

The ability of the human visual system to detect stimuli at low illumination levels provides awareness of potential risks. The influence of age and spectral power distribution on mesopic spectral sensitivity is analyzed. Two typical light sources are used, a high-pressure sodium lamp (HPS), with a higher content of long wavelengths, and a metal halide lamp (MH), with a higher content of short wavelengths. Two experiments were performed, using a two-channel Maxwellian-view optical system to measure contrast threshold under different experimental conditions. In Experiment 1, three age groups (young, middle-aged and old, n = 2 each), two retinal locations (on-axis and off-axis vision), four background luminances (0.01, 0.07, 0.45, and 3.2 cd/m²), and two photometry systems (photopic and the MES2 systems) were considered. In Experiment 2, contrast threshold measurement was performed with two age groups (young and old, n = 11 each), one retinal location (off-axis vision), one background luminance (0.01 cd/m²), and two photometry systems (photopic and the MES2 systems). In on-axis vision, neither age nor spectral power distribution have an effect on the contrast threshold. In off-axis vision, however, a significant interaction between age and spectral power distribution is obtained, albeit only at 0.01 cd/m² with an MH lamp. Only at this lowest background luminance was the greater content of short wavelengths of this lamp responsible for higher rod stimulation in off-axis vision, with the subsequent improvement in detection performance in young subjects. However, the effect of diffused light inside the aged eye counteracted the benefits of increased rod sensitivity for the MH lamp.