1. A method for measuring skin erythema comprising: a) acquiring a digital image of skin under orthogonal polarization conditions with a white calibration standard included in the field of view of the image; b) identifying median intensity values for a red, a green, and a blue channel for the white standard; c) translating skin reflectance values given by the red, green, and blue intensity values of each pixel of the image to absorbance values by taking the logarithm of a ratio of the white standard values to the skin reflectance values at each of the red green and blue channels separately, where the absorbance equals: 
 log( I white (red,green,blue)/ I skin (red,green,blue)) for all pixels representing skin; d) subtracting the red channel absorbance values from the green channel absorbance values for all pixels representing skin; e) subtracting the red channel absorbance values from the blue channel absorbance values for all pixels representing skin; f) calculating a ratio of the value calculated at step (d) to the value calculated at step (e); and g) building an intensity map of the ratio calculated at step (f).