A wearable device includes a housing; a wrist band attached to the housing; first and second emitters positioned within the housing and configured to respectively emit, through a back of the housing, a first beam of electromagnetic radiation having a first infrared (IR) wavelength and a second beam of electromagnetic radiation having a second IR wavelength. The second IR wavelength is different from the first IR wavelength. The wearable device also includes a photodetector positioned within the housing and filtered to detect a set of electromagnetic radiation wavelengths including the first IR wavelength and the second IR wavelength; and a matter differentiation circuit configured to indicate, at least partly in response to signals indicating amounts of the first IR wavelength and the second IR wavelength received by the photodetector, whether the back of the housing is likely proximate to human tissue.