Wavelength of light
In addition to the visible spectrum, both ultraviolet and infrared light can be used to reveal specific features on or in the work piece to be inspected, that are not visible using the visible spectrum.
It is often believed that the wavelength or colour of light is only important in polychromatic applications such as print inspection or coloured component recognition. There are, however, monochromatic applications where coloured illumination can play a crucial role in producing a good quality image - dramatically simplifying the machine vision task.