https://academic.oup.com/biomedgerontology/article/75/9/e49/5863431 >Twenty-four healthy participants of both sexes were used with University College London ethical approval. They ranged from 28 to 72 years. The cutoff point between younger and older groups was >38 years, with age as the only significant variable. Different participants were used to measure rod (scotopic) thresholds and color contrast sensitivity (CCS), which were undertaken at different times. There were 12 individuals in each group. In the CCS group there were six younger (five female and one male) and six older (four female and two male) participants, and in the scotopic threshold group there were six younger (four females and two males) and six older (four female and two males) participants. 670-nm light devices were based on simple commercial DC torches with ten 670-nm LEDs mounted behind a light diffuser embedded in a tube that was 4 cm in diameter. Energies at the cornea were approximately 40 mW/cm2 which often resulted in a mild green after image for approximately 5–10 seconds. Participants were asked to use the light to illuminate their dominant eye every morning for 3 minutes and to repeat this daily for 2 weeks. These metrics were selected because they fell within the range used in a large number of animal experiments. 670-nm illumination was largely confined to the central retina comprising the peaks in rod and cone density.