Thermogram of the hands of a person suffering from Raynaud's disease, a condition of unknown cause in which the arteries of the fingers are unduly reactive and enter spasm when the hands are cold. Thermography maps the heat distribution over the body surface, providing a colour-coded image of temperature variation in 1 degree celsius steps (see code strip at bottom). A reference disc, calibrated at 30 degrees celsius, can be seen at the right of the frame.