Infrared sensors can be divided into:

(1) The infrared part is converted into heat, and the heat type of the output signal such as the change of resistance value and electromotive force is taken out by heat.

(2) The photoelectric effect of absorbing energy difference using semiconductor migration phenomenon and the quantum type of photoelectric electromotive force effect using PN bonding.

The phenomenon of Thermal type is commonly known as the pyrothermal effect, which is representative of the Bolometer (Thermal Bolometer), Thermopile (Thermopile) and Pyroelectric (Pyroelectric) components.

The advantages of the hot type are: it can be operated at room temperature, wavelength dependence (wavelength difference has a great change) does not exist, and the cost is cheap;

Disadvantages: low sensitivity, slow response (mS spectrum).

The advantages of quantum type: high sensitivity, fast response (μS spectrum);

Disadvantages: must be cooled (liquid nitrogen), wavelength dependence, high price;

Pyroelectric infrared sensors especially use the sensitivity of the far infrared range for human medical examination, the wavelength of infrared is longer than visible light and shorter than radio waves. Infrared makes people think that it is only emitted by hot objects, but in fact it is not so, all objects that exist in nature, such as humans, fire, ice, etc., all emit infrared rays, but its wavelength is different because of the temperature of its object. The body temperature of the human body is about 36 ~ 37°C, emitting a peak of 9 ~ 10μm of far infrared, in addition to the object heated to 400 ~ 700°C, can emit a peak of 3 ~ 5μm of intermediate infrared.