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.
The diagram shows the spectral luminous efficiency for both the human eye and a typical CCD sensor. The human eye is most sensitive to a wavelength of 555nm. As the wavelength grows longer or shorter, the human eye becomes less sensitive. The relative sensitivity of the human eye to light is called the spectral luminous efficiency also known as the V-Lambda curve.
The next diagram clearly shows that a CCD's sensitivity to infrared light is far better than the human eye. It is important to consider this difference in sensitivity when designing illumination systems for machine vision applications, as what a camera sees can be very different to what a human will see.