Why would the chemical fingerprint of a sample, which is related to its absorbance (how much light is absorbed over a given distance) help to eliminate the blood vessel size effect? How might the measured absorbance depend on the absorption ability of oxygenated or oxygen-free hemoglobin (at a given wavelength), on their respective concentrations, and on blood vessel size? How could measurements at two wavelengths of light (red, infrared) help eliminate the size effect? Can you think of other technology examples where more than one measurement helps to eliminate such extrinsic effects?