Light is an electromagnetic wave whose wavelength range is from a few nanometers (1 nm = 10-9 m) to about 1 mm (mm). The human eye only sees part of it. We call it visible light. The wavelength of visible light ranges from 380nm to 780nm. The wavelength of visible light is divided into red, orange, yellow, green, cyan, blue and violet. The wavelength is shorter than violet. It is called ultraviolet light, and the wavelength is longer than infrared light, called infrared light. Ordinary CCD black-and-white cameras can sense the spectral characteristics of light, and it can not only sense visible light, but also infrared light. This is the basic principle of using the ordinary CCD black and white camera, with infrared light to achieve night vision more economically. The spectral characteristics of ordinary color cameras cannot feel infrared light, so they cannot be used for night vision.