Definitions for "Red-eye"
Keywords:  dimly, retina, flash, lit, dilate
noun: the appearance of a red glow in the eyes of people in photographs. this effect can be reduced by camera settings
In darker conditions, the pupils of eyes of all living creatures dilate (widens) to take in more light. When the flash is used at this time, light reflects off the insides of the eye, making it appear red in photographs. The red-eye reduction system utilizes a flash of light which causes the pupils to constrict before the photo is taken, reducing the chances of red-eye in the final image.
In dark conditions, an effect is caused by light from a camera's flash reflecting off the blood vessels in the black of the eye. It can be reduced by making the room brighter, by using an off-camera flash, or by utilizing the red-eye reduction mode that some cameras and flashes offer.
travel on an overnight flight; "The candidate red-eyed from California to the East Coast the night before the election to give a last stump speech"