No abstract

Because commonly used organic pigments are highly responsive to visible light, high-light-sensitivity art is vulnerable to irreversible illumination-induced damage caused by radiation from light sources. With application of the four primary white light-emitting diodes (fp-WLEDs) in museums, it is urgent to evaluate the illumination-induced damage to high-light-sensitivity art caused by the fp-WLEDs. Four narrowband LEDs with different peak wavelengths of 450, 510, 583, and 650 nm that constitute the spectra of the fp-WLEDs were used to irradiate three commonly used organic pigments: safflower, gamboge, and indigo. Based on the fundamental reason for the illumination-induced damage, that is, photochemical reactions, Raman spectroscopy was introduced into the study. The Raman spectra of pigments were measured before and after illumination. The characteristic Raman peaks corresponding to the functional groups that determine color and structure of pigments were selected, and the variations in their peak intensities were calculated. The illumination-induced damage coefficients of four narrowband LEDs on three organic pigments were obtained, providing a data basis for illumination-induced damage evaluation equation proposed in this study, which was expected to further realize museum admission evaluation of the new fp-WLEDs.

Traditional Chinese paintings (TCPs), which are characterized by enormous storage and high value, are prone to suffering from radiation damage from museum illumination, such as color fading, discoloration, and color vanishing, especially for the most light-sensitive traditional Chinese paintings that are painted with organic pigments (op-TCPs). Thus, the development of light sources that minimize the damage to TCPs is essential. Meanwhile, the color quality of light sources is also of great importance, and there is a lack of methodology for obtaining light sources with optimized spectra to simultaneously achieve the protective effect and ensure the color quality. Here, changing curves of the CIE DE2000 color differences of pigments were shown as a function of the exposure duration by calculating the periodically measured color parameters in a long-term illumination experiment under four monochromatic lights. Relative spectral responsivity functions of the op-TCPs were deduced by fitting the experimental data and obtained the corresponding equal-illuminance relative damage formulas. Then, damage laws to the op-TCPs and the lowest color damage spectral power distributions that satisfy the color quality requirements of a four-primary white LED model were obtained. Our results can provide the theory and application basis to manufacture white LEDs that are suitable for illuminating op-TCPs; the method can be further used in preparing white LEDs for other cultural relics.

As the substrates of traditional Chinese painting and calligraphy, paper and silk are susceptible to optical radiation in museum illumination, resulting in mechanical damage, the origins of which are changes in microscopic molecular structure. However, there is no effective method of quantitatively evaluating the microscopic molecular structural changes caused by irradiation. In this study, Raman spectroscopy was introduced for illumination research on the paper and silk substrates in museums. Four narrow-emitting light sources with different peak wavelengths, red (650 nm), amber (583 nm), green (510 nm), and blue (450 nm), which together constitute the spectrum of a white light emitting diode (LED), were used as experimental light sources. As experimental specimens, paper and silk substrates were irradiated by the four light sources. The Raman spectra of the specimens were collected before and after the illumination. By analyzing the variations in characteristic Raman peak intensities, the relative damage coefficients of the four light sources on microscopic molecular structures in the specimens were studied. Then, we were able to design the white LED spectral irradiance distribution of the two substrates according to the corresponding damage coefficients. In conclusion, a research method for studying the microscopic molecular structural changes in the substrates of traditional Chinese painting and calligraphy based on Raman spectroscopy has been proposed, finding a solution to the long-standing challenge of how to quantitatively evaluate illumination-induced mechanical damage of substrates. These results are instructive for designing the spectral ratios of white LEDs suitable for illumination of traditional Chinese painting and calligraphy in museums.

A correlated colour temperature tunable, four-component, LED system suitable for illuminating traditional Chinese paintings painted with inorganic pigments (iop-TCPs) has been designed. This light source can meet the requirements for protective illumination and colour quality in museum illumination. The damage to iop-TCPs caused by the individual 450 nm, 510 nm, 583 nm and 650 nm monochromatic LEDs used to construct the LED system were obtained through a long-term illumination experiment. Then, we calculated the damage resulting from the corresponding four-component white LEDs with various spectral power distributions, the intensities of which were iterated by a brute force algorithm. The constructed spectral power distributions were evaluated by the damage and colour quality formulae. The results showed that the better colour quality spectral power distributions with higher correlated colour temperatures cause less damage to iop-TCPs. The lowest damage spectral power distributions, satisfying colour quality requirements with correlated colour temperatures ranging from 2700 K to 4000 K, were obtained by calculating the corresponding parameters of the formed spectral power distributions and further selection. This paper not only provides a colour temperature tunable, four-component, white LED system suitable for illuminating iop-TCPs but also sets out a methodology that can be used to identify white LEDs suitable for other cultural relics based on damage limitation and visual requirements.