Near infrared is the closest to visible wavelength of light, and together with heat radiation and visible light, it forms part of electromagnetic radiation. Humans are not able to see it or sense it as heat.
Near infrared spectral imaging is a useful tool in identification of different materials. First, the object is lit by a near infrared light and then a spectral camera can be used to examine how photons reflect off the surface of the object. Reflection and absorption of the photons on the surface of the object vary in different wavelengths. In fact, spectra of diverse materials are a bit like fingerprints – there are no two identical ones.
Spectral imaging is a method to identify a spectrum of a material. In other words, the intensity of the reflected light as a function of the wavelength is the focus of attention in this case. Materials that are object to this imaging process can be detected and recognized specifically by their spectra.