Method for measuring skin erythema

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(Iwhite(red,green,blue)/Iskin(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).