Background: Quantification of erythema and pigmentation is useful for analysis of skin tests and management of skin diseases. However, reflectance instruments for this purpose suffer from many technical and financial disadvantages. The aim of this study was to establish a method for evaluation of the amounts of haemoglobin and melanin using ordinary digital cameras and Image J freeware.
Methods: Based on the theories on the absorbance of a multilayered skin model, the erythema index (EI) and melanin index (MI) images were derived by image processing from digital colour images of the skin, which were obtained with four kinds of digital cameras. The specificity of these indices and the linearity between index values and the amounts of haemoglobin and melanin were determined by using images of various concentrations of haemoglobin and melanin solutions. The accuracy of both types of index images was also examined by comparing the index values of UV-induced erythema and pigmentation of various intensities with those measured with a reflectance spectrophotometer.
Results: The specificity of EI and MI images was good as little interference was noted with each other. The linearity between EI and haemoglobin concentration, as well as that between MI and melanin concentration was excellent. For UV-induced erythema and pigmentation, good linear correlation was confirmed between two types of EI obtained by the two methods, as well as between two types of MI. However, the index values derived from digital images depended on the camera used as well as on the circumstantial conditions, such as the distance from objects and illumination.
Conclusion: EI and MI images are reliable and useful for quantifying erythema and pigmentation, if obtained under constant and consistent conditions. Apart from financial benefits, this method has many advantages and greater clinical utility in comparison with reflectance instruments.