Optimizing multicoloured LEDs for identifying pigments

The technical photography (TP) method has been using in identifying pigments in paintings and the results could be used in the early stage before confirming with other methods such as X-Ray Fluorescence and Raman spectroscopy. The TP method employs visible, UV and IR as light sources with different filters. Moreover, the multi-spectrum method, using a continuous light source and various filters, has been used in the same application. This research proposed a method of using narrow multi-colored LEDs as light sources instead of colored filters. It aimed to optimize the number of LED channels for identifying pigments in conservation and restoration applications. Twelve tunable single-wavelength LEDs having wavelengths of 400-700 nm and 2 white LEDs, were used as light sources for capturing images of 357 Kremer pigments, applied on white card paper, without an optical brightening agent, through a visible pass filter, located in front of the single lens reflex Canon 5D Mark II camera that was modified by removing the UV and IR blocks. The 357 pigments including 9 main colors: 50 red pigments, 37 orange pigments, 82 yellows, 38 greens, 38 blues, 17 purples, and a group of greys browns, blacks and whites were used as samples. Subsequently, the pigments were classified, based on the captured images obtained from the individual LED. The redundancy of the CIEL*C*h data of each LED was reduced using the principal component analysis and the number of LEDs was optimized. It was found that all pigments could be classified and some pigments could be identified using single wavelength LEDs. The PCA should be applied to the CIEL*C*h data of pigments having similar shade. The 598 nm 425 nm LEDs were optimized for identifying and classifying blue pigments. The 430 nm, 640 nm and 540 nm were for purple, 403 nm, 540 nm, 660 nm and 672 nm for red, 403 nm, 447 nm, 540 nm and 672 nm for orange, 503 nm, 540 nm and 598 nm for yellow, 503 nm, 598 nm and 640 nm for green and 403 nm, 503 nm, 540 nm and 660 nm for a group of white, grey, black and brown.