Embodiments of the present methods measure a concentration of a substance, such as glucose, in a body. The described embodiments change the temperature of the surface of a body from a first temperature to a second temperature, then change the temperature of the surface of the body from the second temperature back to the first temperature, and measure 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 at predetermined time intervals during the time period that the temperature of the surface of the body changes from the second temperature back to the first temperature. The described embodiments also measure a temperature at the surface of the body and an ambient temperature. A normalized ratio parameter is calculated from the four measurements, 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.
