WHY DO VEINS APPEAR BLUE - A NEW LOOK AT AN OLD QUESTION

We investigate why vessels that contain blood, which has a red or a da rk, red color, may look bluish in human tissue. A CCD camera was used to make images of diffusely reflected light at different wavelengths. Measurements of reflectance that are due to model blood vessels in sca ttering media and of human skin containing a prominent vein are presen ted. Monte Carlo simulations were used to calculate the spatially reso lved diffuse reflectance for both situations. We show that the color o f blood vessels is determined by the following factors: (i) the scatte ring and absorption characteristics of skin at different wavelengths, (ii) the oxygenation state of blood, which affects its absorption prop erties, (iii) the diameter and the depth of the vessels, and (iv) the visual perception process. (C) 1996 Optical Society of America