Abstract: A noninvasive infrared spectrometer which includes an infrared detector system for measuring the intensity, wavelength, and time varying nature of infrared energy emanating from deep layers within a body. Before detection, the energy emanating from deep within the body passes through layers of that body in the presence of a natural or induced thermal gradient. The measured infrared energy is processed into an absorption spectra and then into a concentration of at least one constituent of the body which concentration may be strongly dependent on the depth into the body. In one embodiment the temperature gradient is induced by chilling the surface of the body to provide a clearer indication of the infrared absorption levels of the deeper constituents. Other embodiments describe the sequential or simultaneous heating and cooling of the heterogenous body to induce and capture the transient infrared absorption spectral information.