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The reflectance of light from in vivo tissue is described for wavelengths in the range from 420 to 940 nm, based on photon diffusion theory and on experimental results from studies of 17 subjects. The results show a minimum reflectance and a peak sensitivity to the blood pulsations in the wavelength range from 510 to 590 nm. Skin pigmentation is seen to attenuate reflectance rather than altering the character of the modulation spectra. Based on the model presented, the dependence of modulation spectra on mean blood fractional volume as well as wavelength is described theoretically and corroborated by further experimental data at 570 and 630 nm. At these latter wavelengths, the signal-to-noise ratio is calculated for the blood volume pulsation signal in the presence of physiological noise. The median for calculated ratios of reflectance modulation by blood pulsation and ratios of signal to noise between the two wavelengths are 13.1 and 7.5, respectively, for 93 sites in nine subjects. These results are seen to be consistent with the theory.