A method to measure a concentration of a substance, such as glucose, in a body comprising changing the temperature of the surface of a body from a first temperature to a second temperature, then changing the temperature of the surface of the body from the second temperature back to the first temperature, and measuring a first amount of infrared ("IR") radiation absorbed or emitted from the surface of the body in a first wavelength band, and a second amount of IR radiation absorbed or emitted from the surface of the body in a second wavelength band during the periods of changing temperatures. The method also measures a temperature at the surface of the body and an ambient temperature. A normalized ratio parameter is calculated and the concentration of the substance in the body is determined by correlating the normalized ration parameter with the body surface temperature and the ambient temperature.
