In taking a photograph of an object, injuries and various effects on the art object are apt to be neglected. The author, therefore, undertook an investigation on injuries caused by the increase of temperature through illumination with spotlights. Attention was given only to cracks in the surface layer of an art object. The object chosen was Ema , a kind of votive picture painted with mineral pigments. Increases in temperature on its surface due to spotlight illumination were measured. The relation between the intensity of illumination and occurrence of cracks in the pigment layer was made clear. The panel painting was most susceptible to heat. This experiment provided us with a standard for the safe limit of illumination for photograph or motion picture taking.