Quantitative optical spectroscopy in the skin

The human eye is the primary optical diagnostic instrument used in dermatological practice. Despite the ease of a visual skin inspection, it is important to recognise that in many cases the eye or a simple imaging device is not well suited for distinguishing subtle differences in erythema, scaliness or melanin content between dermatoses. The subjectivity of visual analysis remains the primary challenge with the majority of clinical diagnoses. In the present article we describe the underlying interaction between light and the physio-anatomical structure of the skin, the nature and limitations of a visual skin assessment and identify the potential for more advanced approaches. Specifically we make the case for the use of fiber optic spectroscopy, utilising two or more optical fibers with different diameters. When combined with the use of theoretical models, which describe the way that light interacts with tissue, spectroscopy enables the quantification of the absorption, fluorescence and scattering properties of skin. We illustrate the potential benefit of quantitative reflectance and fluorescence spectroscopy in patients with actinic keratosis undergoing optical diagnosis and aminolevulinic acid photodynamic therapy and highlight how these approaches may be implemented in longitudinal population based studies.