Infra red

Any object will emit infra-red radiation. The hotter the object, the higher the intensity of the infra-red emitted will be, and the shorter the wavelength.

Infra-red thermometer uses an infra-red sensor to collect the infra-red radiation emitted from a distant object, by determing the wavelength at which the highest intensity occurs, the temperature can be determined.

The most simple form of infra-red thermometer is to connect an infra-red sensor with a sensitive galvanometer. But the main problem lies on the calibration of your instrument.