"670nm light devices were based on simple commercial DC torches with ten 670nm LEDs mounted behind a light diffuser embedded in a tube that was 4cm in diameter. Energies at the cornea were approximately 40mW/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. 670nm illumination was largely confined to the central retina comprising the peaks in rod and cone density."