Multispectral color technique
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.
In practice the human eye perceives each color like three numbers: one related to amount of radiation in the red waveband, one related to the radiation in the green waveband and one related to the radiation in the blue band.
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.
A white light is shined on the artwork. A number of separate images, one for each spectral band, are acquired with a scientific calibrated CCD camera and a set of interferential filters.
In this way for each image it is possible to quantify the amount of radiation received. The images can then be used to evaluate the amount of reflection or emission present in each band for each pixel.
The multispectral images can then be recombined into an RGB image, which will show a much more faithful reproduction of what is perceived with a direct observation, since with this method it is possible to approximate the tristimulus curves in a more accurate way, compared with what is done in the standard RGB cameras.
The acquired data can then be processed and allow for a better identification of materials.
Pisa – Museo Nazionale di San Matteo
Particolare della Madonna col Bambino, pannello del Polittico di Santa Caterina di Simone Martini.
Immagini multispettrali in fluorescenza UV acquisite tramite CCD e filtri interferenziali a banda media.
Ricostruzione dell’immagine a colori in fluorescenza UV e nel visibile