Much of the change of the appearance of skin due to aging such as age spots (irregularities in melanin distribution), telangectasia (abnormal, small blood vessels) and collagen degradation (characterized by yellow-brown discolouration) primarily arises from photo-damage caused by chronic sun exposure.
Original research undertaken at the University of Birmingham, subsequently described in WO 98/122023 argued that the Kubelka-Munk theory is sufficient to model light transport within skin. If exact scattering and absorption coefficients can be specified, then the Kubelka-Munk theory can be applied at each wavelength in the visible range and corresponding remittance spectrum obtained. This predicted spectrum, which will determine the colour of the skin, will be dependent on the histological characteristics of the tissue. Three parameters capture most of the variation in remitted spectra from healthy skin. These three parameters are concentration of epidermal melanin, concentration of blood and thickness of the papillary dermal layer (collagen thickness).
Using the RGB response curves for a digital camera together with a model of the scattering and absorption characteristics of the skin, it is possible to calculate the set of image values which would be measured by a digital camera when skin with a known remittance spectrum S(λ) is illuminated with light of known spectral characteristics I(λ). This is done by calculating the convolution integral for each channel, given as,ired=∫I(λ)S(λ)R(λ)dλ, igreen=∫I(λ)S(λ)G(λ)dλ, iblue=∫I(λ)S(λ)B(λ)dλwhere R(λ), G(λ) and B(λ) are the response curves for the red, green and blue channels and ired, iblue and igreen are the corresponding values recorded by the camera at a given pixel.
By ranging through all potential combinations of melanin, blood and collagen, it is possible to generate all possible spectra and therefore all possible sets of image values which would be measured by a digital camera. Once this information has been obtained a link can be established between image values and histological parameter values. This link can be expressed as a mathematical function.
Determining measurements of epidermal melanin, blood, collagen and dermal melanin directly from measurements of remitted light S(λ) requires that an area of skin is illuminated with light of known spectral characteristics. However, as is discussed in detail in Astron Clinica's prior patent application WO 04/010862 determining measurements from ratios of image values enables measurements of blood and melanin concentration to be obtained without the need for strict lighting control and calibration.
Although obtaining measurements of blood and melanin concentrations provide a further means for classifying the appearance of skin, obtaining a reliable measure of the extent of photo-damage remains difficult as photo-damage causes a variety of different histological and physiological changes. Although clinical scoring systems exist, such existing scoring systems all require subjective user interpretation of pigmentary irregularities.