## 4 UV-VIS-IR Radiation Detectors
### Photometric and Colorimetric Detectors  
Photometric and colorimetric techniques are widely used in diagnostic studies and in conservation and restoration programs. The most performed measurements are light characterization and color specification, which use the luminous efficiency function, V(\(\lambda\)), as a weight function, that describes the average spectral sensitivity of human visual perception of brightness [209]. Photometric techniques (limited to the visible range, 380-780 nm) are of great importance in terms of fruition and vaporization of works of art, because they allow for the characterization of the emission spectrum of light sources and thus the evaluation of chromatic light rendering. Photometric measurements consist in the characterization of light sources in terms of illuminance (lx), light intensity (cd) and luminance (cd/m\({}^{2}\)) and are made by illuminometers and spectroradiometers. Colorimetric techniques are crucial for conservation because they allow for evaluating changes in the optical properties of polychrome artworks, such as paintings. As is well known, the color of an object depends on its ability to reflect, transmit and absorb light. The knowledge of the spectrum of reflected light, acquired under the standard conditions defined by the Commission Internationale de l'Eclairage (CIE) [210; 211; 212], enables the color specification and the quantification of the color differences. In color measurements, both colorimeters and spectrophotometers are used to acquire optical information in terms of Spectral Reflectance Factor (SRF%) and color coordinates, useful to the color specification and the color difference quantification [213; 214]. The instruments listed in Table 1 are equipped with silicon photon counters. In photodiodes, the measured current is proportional to the radiant power of the incident light. Recently, PIN photodiodes are also used to increase the speed of response, the responsivity, and the linearity over the pn ones. In photometric applications, photodiodes operating in the photovoltaic mode are the most used because they ensure lower noise signal and are less sensitive to changes in environment temperature. Their spectral response, based on human eye perception, is in the range 380-780 nm, the peak sensitivity wavelength is set at 550 nm and the photosensitivity at approximately 0.24 A/W considering a photosensitive area size of max 3.6 nm \(\times\) 3.6 nm [215; 216]. The color measurements, performed by colorimeters, are carried out by RGB color sensors that are three-element silicon photodiodes specifically designed for red (590-720 nm), green (480-600 nm) and blue (400-540 nm) spectral regions. The peak sensitivity wavelengths are, respectively, set at 620 nm for Red, 540 nm for green and 460 nm for blue with a photosensitivity between 0.2 to 0.5 A/W [217]. Colorimeters are designed to automatically provide the chromaticity coordinates in different color spaces codified by CIE. Colorimeters are generally constituted by a light source, an optical device for illuminating the sample and detecting the emitted light under standard geometries according to CIE recommendations. An optoelectronic device is also present in order to analyze the light signals [218]. Portable spectrophotometers and spectroradiometers for CH applications make use of spectrometers equipped by fixed diffraction gratings, that allow detection of light signals at different wavelengths using different elements of the photodiode array. The silicon photodiode array consists of multiple elements of the same size, formed at equal spacing in one package. In particular, the recent models of spectrophotometers mount dual 40-elements while spectroradiometers have generally a photodetector composed of 512 linear image sensor elements. Only in the spectrophotometers is the lighting system, generally a xenon-flash lamp, integrated on the instrument together with an optical apparatus to illuminate and to collect the reflected light according to CIE standard geometries.