The short wave infrared (SWIR) band (defined here as 750~2600nm) beyond the visible light band is a non-visible light band. Many materials that look very similar under visible light will look very different under SWIR light. For example, four different liquids-water, acetone, trichloroethylene and isopropanol. Under visible light illumination, these four liquids are colorless and transparent; but when imaging under SWIR broadband light illumination, these four liquids will show different colors.
Short wave infrared imaging can be achieved by reflection, in which case the object is illuminated by an external light source or if the object is hot enough. Imaging can also be achieved by measuring the emitted light of the object. When an object is illuminated, the object reflects photons from an external light source to the camera. The illumination angle of the external light source, the wavelength of the light source and its components will directly affect the reflection characteristics of the object. In other words, the object will emit light under the irradiation of light of all wavelengths, but the amount of light emitted depends on the temperature of the object.
As the temperature increases, the light radiation becomes stronger, and the wavelength peak radiation shifts to shorter wavelengths. Since the emissivity is a function of the wavelength and the physical properties of the object, when an object (such as coal) emits red light, the object is obviously very hot and cannot be touched. An ideal blackbody at 500°C emits enough energy in the red light band of 700~750nm to be seen by the human eye. When the temperature is lower than 500°C, although the object may still be too hot to touch, the human eye can no longer see red light. Because the combination of red light emission and emissivity, it makes the amount of luminescence too low to show red corlor.
In summary, the selection of light sources in short wave infrared imaging is critical to achieving a successful imaging solution. The SWIR band can distinguish and detect defects that are invisible to the human eye. In order to have a good imaging quality, the developer must know how to choose the correct light source.