This photo includes two infrared images. The top one shows a man on a bed, while the lower image shows the warm imprint of his body after he has risen from the bed. Infrared photography gives a map of the heat distribution of an object from its radiation. These images are color coded from white for the hottest parts through to blue for the coldest.
Image (c) Dr. Ray Clark and Mervyn Goff, Science Source/Photo Researchers