Why is using a flashlight covered with red cellophane recommended for observers maintaining dark adaptation?

Red light minimally affects the rod cells responsible for night vision

Effective night vision relies heavily on the rod cells in the retina, which are highly sensitive to low levels of light but are responsible for monochromatic vision. This sensitivity is lost rapidly if the rods are exposed to bright light, a process known as bleaching or reversing dark adaptation. Red light, due to the specific physiology of the human eye, has the least impact on these sensitive rod cells compared to brighter colors like white or blue light. Therefore, using red illumination allows an observer to check maps or navigate briefly without severely compromising the 20 to 30 minutes required to achieve maximum light-gathering sensitivity.