Why do astronomers use red lights at night? It is because of the complexity of the human eye. Unlike a digital camera which has pixels of relatively equal light sensitivity, the human eye is composed of two types of light sensitive cells, rods and cones. Cones allow us to see color in bright light. Rods allow us to see dim light, but only in shades of gray. Sometimes astronomers need their night vision and at the same time need to see detail, such as when reading. So how do we do both at the same time?
During the daytime, we need to see detail to maneuver through the environment to accomplish our daily routines and duties. This is accomplished by light stimulating the cones, which are responsible for seeing color in daylight. At night most daytime activities wind down. Biologically, humans don’t need to see the same level of detail at night, and before the dawn of artificial lighting, one of the main concerns of early humans was survival and detecting threats in dim light. This is where the rods come into play.
Rods need some time to achieve maximum sensitivity to dim light. This is mediated by a protein called rhodopsin. Maximum night vision is usually achieved in about 30 minutes or so. Rhodopsin is rapidly degraded when exposed to bright light. If you have spent a half-hour or so getting adapted to the dark, a sudden exposure to a bright light such as a car headlight or flashlight will ruin the dark adaptation and force you to start again. Part of dark adaptation is dilation of the pupil, but the pupil can dilate relatively quickly after bright light exposure.
Red light is different for a couple of reasons. Rods are not sensitive to red light, and red light does not rapidly degrade rhodopsin. So, rods remain able to detect dim light when exposed to red light, and the cones which are sensitive to red light, allow us to see the color.
This is why astronomers use red light flashlights at night to preserve night vision. So, if you go to a star party or any astronomy nighttime event, please be courteous and only use red light.