A strobe is a form of light projector that produces bright flashes of light. They are common in both film projectors and camera flash systems. Because of the way the human eye is built, there are many effects of strobe light on vision, ranging from the appearance of “slow motion” movement to temporary blindness. The effects of strobes have therefore made them very useful tools in photography and the film industry alike.
Our vision is created by the interaction of light with the retina; the back of the inside of the eye, where the light is transformed into electrical impulses that are sent to the brain.
The retina contains two forms of light-receiving organ, cones and rods. The rods become inactive in bright light but are responsible for high visual acuity in low light. When strobe lights are used, the rods get confused and vision can blur.
Human vision is temporarily disrupted by the effects of strobe lights. Cones are the dominant method of seeing in daylight, they are of limited use in low light. Instead, the rods are stimulated for low-light acuity but these take time to reach full ability. A strobe light can turn off the rods in the eye by exposing them to too much light, rendering a person temporarily blind or seeing “spots”.
The Effect of Red Light
Rods do not detect color, only light. Rods and cones also work independently of one-another. Furthermore, red light run on a much lower electromagnetic frequency than other light, so it is less powerful. It is therefore possible for a person to be subjected to a red light strobe and not lose their night vision because although they detect the red light, the rods in their retinas are not affected by it.
Persistence of Vision
When an object is perceived by the retina, a copy of the image it projects will remain on the retina for a fraction of a second, in what is known as “persistence of vision”. When a strobe is set to a low frequency of pulsation, we see objects only a small number of times each second. These brief images merge together in the brain, which is how film mimics motion on a cinema screen.