Method and apparatus for measuring temperature of a measured area of a surface without contacting the surface. The thermometer apparatus has an optical system which generates a correlative image of an infrared energy detector sensitive area at an image distance from the thermometer. A limiting aperture, having a size and a shape corresponding to those of the generated image, is between a mirror and the generated image. The measured area of the surface is between the generated image and the thermometer in use. With such a configuration, little infrared energy that does not originate from the measured area strikes the detector. Consequently, the energy reaching the detector is limited such that the size of the measured area remains constant, regardless of changes in the thermometer's field of view attributable to differences in the distance between the surface and the thermometer. A scan-and-integrate mode for practicing the invention is disclosed.