Near infrared imaging has a special capability to see some materials differently than human eye or cameras working in visible light wavelengths.
Let’s study imaging of two glass bottles with some water in one bottle and some kerosene based liquid in the other one. When these bottles are imaged with NIR camera, we can notice that
- water, transparent with visible light camera, looks very dark in NIR image
- kerosine based liquid, non-transparent with visible light camera, looks transparent in NIR image
Water looks dark in NIR image because water is very absorbent in near infrared wavelengths. Defining moisture content with NIR imaging is based on this same phenomenon.
Water and kerosine based liquid look very different in NIR image and in visible light image.