The design of a compact spectrometer for analysis of artworks is presented. Its operation is based on the use of a variable
transmission filter associated with an array detector. The instrument allows the measurement of the spectral reflectance
factor and combines the acquisition of data in a continuous spectrum with the small dimension that is of primary
importance for in-situ spectral imaging.
Generally, the colour of the non-luminous objects in nature is due to absorption, diffusion and refraction of light. The colour of the optical coatings, as that of some kind of bird feathers, soap bubbles, butterfly wings, some insects, etc. is due to interference and therefore is named interference colour. This kind of colour belongs to the gonio-apparent or special-effect colours. Generally, industrial colorimetry does not deal with interference colour and the usual colorimetric instruments are inadequate to measure it. Only recently, with the new mica-pigment coatings, colorimetry is considering the measurement of the interference colour and new multiangle spectrophotometers are produced. This work is a general introduction to the ground of colorimetry and, at the end, deals with interference colours. A short overview is given of the Physiological Optics and of the Colorimetric Standards of the "Commission International de l'Eclairage" (CIE): particularly, Psychophysical Colorimetry, Psychometrical Colorimetry and Measurement Geometries are summarised. The colorimetry of gonio-apparent colours is considered. For a complete and detailed optical characterisation of interference colour the measurement of bidirectional transmittance and reflectance is needed. Particularly, basic elements for the colorimetric analysis of the interface between isotropic non-absorbing media and for thin monolayers are given.
Two different transformations between the (X10, Y10, Z10) coordinates and the (LOSA, g, j) ones of the Uniform Color System of the Optical Society of America (OSA- UCS) are given related to two possible different geometrical structures. Both transformations are logarithmic functions obtained by integration of Weber fractions of ratios of suitable color stimuli. The first structure is related to the color opponency proper of the (g, j) coordinates of the OSA-UCS) system and the second one to a mixing of the (g, j) coordinates. The second transformation has a simpler and highly symmetrical structure, and the regularity of the OSA- UCS lattice is higher.
Recently, chromatic-response function with uniform scales are obtained as logarithms of proper cone activation ratios (CAR hypothesis) and can be considered as Cartesian coordinates for equiluminant-color palettes. Particularly, a properly combined choice of the 'fundamental' primaries, that are at the basis of these functions, and of the RGB primaries of the display give equiluminant-chromatic 'rectangular' palettes, that with the Weberian-lightness define a color space with good scale uniformity and suitable for the color-signal coding. All the transformations are logarithmic. The couple of 'fundamental' primaries laying on the alychne and a third one of pure luminance are considered for trichromatic cameras.
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