Objective: In this study, optical data on color of tongue tips were collected by using a visible reflection spectrum system. Quantitative method of tongue inspection and classification of tongue states including pale, pale red, red and crimson manifestations were investigated.
Methods: Chromaticity coordinates of the tongue tips from 98 subjects were calculated on the basis of the visible reflection spectrum. The tongue color was quantified and classified by the visible reflection spectrum in the range from 590 nm to 780 nm, based on the higher red-band energy distribution on the tongue tip color.
Results: It was found that comparison and analysis of tongue tip color from different samples would be well-suited after spectrum vector normalization, and the difference of tongue tip color between different samples could be observed easily when the tongue tip optical data were sited in CIE 1931 chromaticity diagram. The tongue tip colors were analyzed quantitatively and classified by the spectral energy intensity at the wavelength of 670 nm and (or) spectral energy ratio of red bank from 590 nm to 780 nm. The results of classification showed that there was sound corresponding accuracy with the clinical diagnosis of pale tongue, pale red tongue, red tongue and crimson tongue.
Conclusion: In this study, based on the collection of the information data on tongue tip characteristics with the visible spectrum from 380 nm to 780 nm, the quantitative analysis of tongue inspection for pale tongue, pale red tongue, red tongue and crimson tongue was established. The quantitative value for tongue tip color is an effective method for classification of the condition of the tongue tip, which provides a useful tool for the modernization of tongue inspection in traditional Chinese medicine.

Key words: tongue inspection, tongue proper, tongue color, visible reflection spectrum, objective tongue inspection, quantization, classification, chromaticity coordinates