Depending on its chemical composition, each material reflects visible light in different ways, with a characteristic reflectance curve, therefore generally showing a different visible colour. Generally speaking, we can distinguish materials by observing their color.
Just looking at something, however, it is not possible to understand the exact reflectance curve, since our eyes are only sensible to 3 wavebands: the red, the green and the blue one.

 

However, two radiations having different spectral content, i.e. related to different materials, may be perceived as identical when they have the same amount of radiation in the red, green and blue bands. The eye cannot distinguish between them as it perceives the same 3 numbers.
This phenomenon is known as metamerism.

Using a multispectral system the possibilities of metamerism are much reduced, since the information is acquired on more bands. The Art-Test device employs 8 bands instead of 3.

 

In practise we have 8 numbers instead of 3 to describe each color, and a much improved way to distinguish among materials showing a similar color

 

Through our equipment we are able to distinguish among different materials, with different reflectance curves, even if they present the same colour to the eye.

  

Until recently, the only way to document the reflectance of a painting surface was to use a spectrophotometer; however, this device would only acquire data from a spot. With an imaging device the information is collected for all points of the surface at the same time.