Every object emits radiant energy. The intensity of this radiation is a function of the temperature of the object. The radiance at every wavelength increases as temperature increases, and determination of the radiance at any wavelength can serve to establish the temperature of the emitter. The energy which is emitted by light sources is mainly transmitted by photons. All photons travel at "the speed of light" and can be reflected by appropriate mirrors, as well as their path can be bent and focused with certain lenses. The energy of the photons is inversely proportional to their wavelength. The elements of an industrial infrared thermometer are a collecting optic, the radiation detector and an indicator. The collecting optic focused to the target transmits the photons to the radiation detector which converts the energy of the photons into a electrical signal.One type of detector is the quantum detector which consists of a semiconductor crystal. The incident photon interact with a bound electron within the crystal lattice.

• The temperature increase is not rapid enough within special wavelength-bands. Due to this fact the accuracy in this temperature range is too low.
• Some objects have no recordable emission characteristics in wavelength range.
• The photons are transmitted in the normal atmosphere which always contains carbon dioxide and water vapour. These gases absorb at different wavelength bands. So measurement at this special wavelength will depend on path length and humidity.

For indicating the real temperature the emissivity dial of the instrument must correspond to the target emessivity. This dial is a calibrated gain adjustment which allows the user to trim the instrument to the emissivity of the target. The diagram also shows, that material with a lower emessivity have a less steep slope. The voltage gradient at different temperatures is smaller and therefore small changes in the temperature are more difficult to detect. The transmission of radiation is an important point, as an infrared thermometer determines an object' s temerpature by quantitifely measuring its radiance. The amount of radiant energy recited at the detector is influenced by the transmission path. Transmission losses are caused by objects, particles and even gas molecules. These interlining materials absorb or reflect some of the radiation before it reaches the detector. If this loss is significant the instrument, quite obviously, gives a temperature that is lower than the actual temperature. The transmission errors can be minimised by keeping the lens clean and free of dust, make the transmission path a short as possible and to keep away solid objects as far as possible. In cases where transmission losses are known, it is possible to compensate them. If there wouldn't be any hot objects (like an oven) near the target there would be no background error. This case is unfortunately quite utopic in a foundry, where there is plenty of hot objects everywhere near the target. These objects emit radiation as well and in some cases even radiation at the same wavelength bands the sensor is adjusted to. The radiation that is emitted by sources other than the selected target object is called "background radiation". If the detected radiation includes radiation from the target and an additional component originating from background sources, the indicated temperature is higher than the true temperature of the target. Background radiation does not present a problem to temperature measurement unless it has a significant intensity. The intensity is significant when the brightness of the background source at the detected wavelength is comparable to or even greater than that of the target. Under the following circumstances the detected radiation includes an additional component from background sources, a target which is smaller than the instrument's field of view. In this case the unit sees beyond the target into the background. Another important factor is that even if the lens seems not to focus the background object there could be a temperature error. The radiation emitted by the background object could be reflected on mirrors or could be transmitted through windows. The reflectivity and transmittance affect the size of the contribution made by background radiation to the detected energy. For minimising the effect of hot background a position of the sensing head which avoids every contribution of background radiation should be chosen. Furthermore the instrument should use a spectral region where the target emissivity is high.