(1) Kirchhoff's law. It points out the relationship between absorption and emission of objects. A good absorber is also a good radiator, and a black body is an ideal radiator with the strongest radiation ability. (2) Stefan's law. That is, the integrated radiation flux density of blackbody is proportional to the fourth power of absolute temperature. (3) Wayne's law. It is pointed out that the product of blackbody temperature and peak wavelength is constant. There are various wavelength components in the energy radiated by a blackbody at a certain temperature, but the radiation energy of each wavelength is different, and the wavelength corresponding to the maximum radiation energy is called the peak wavelength. It has been nearly two centuries since infrared ray was discovered. Although infrared ray was discovered earlier, it took people half a century to understand its nature because of the difficulty of measurement. Although infrared ray has a long wavelength, it has the same physical properties as visible light. At the beginning of this century, people began to use infrared rays. It has been widely used in military, exploration, remote sensing and other fields. It can be used to find military targets and take ground photos at night. Now, the research and application of infrared radiation is deepening, and a new scientific field has been formed.