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) calculating 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 the ratio of the white standard values divided by 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 the ratio of the value calculated at step (d) divided by the value calculated at step (e); and g) building an intensity map of the ratio calculated at step (f).