A system to compare erythemal zones includes a multispectral camera to acquire a multispectral image of each compared zone and a computer processing device to retrieve the images acquired by the multispectral camera. The computer processing device includes a memory, a display, an input device and a microprocessor. The system further includes an image processing program configured to perform spectral de-mixing of the acquired images and isolating the erythemal component of the images, and a program to compare the images based on the intensity of the erythemal components of each image.