The obvious effect of light entering the eyes is to allow the visual system to operate but light entering the eyes affects many aspects of human physiology beyond vision. These effects of exposure to light are driven through what is becoming known as the non-image-forming system. The justification for this rather clumsy name is to emphasize the difference between these effects of light exposure and the visual system. The visual system is essentially an image-processing system. The non-image-forming system is not.