A method for measuring a concentration of a substance within a body, the method comprising the steps of: positioning the body with respect to an infrared detector (32, 62, 64), an optical system (29, 31), and a heating/cooling apparatus; activating the heating/cooling apparatus to change a body surface temperature from a first temperature to a second temperature; activating the heating/cooling apparatus to change the body surface temperature from the second temperature back to the first temperature at a predetermined rate or using air at an ambient temperature to change the body surface temperature from the second temperature back to the first temperature; measuring absorption/emission of infrared radiation over each of a first and a second wavelength band, an ambient temperature, and the body surface temperature at a plurality of predetermined time intervals during the change of the body surface temperature from the second temperature to the first temperature, the first wavelength band being a wavelength band or bands in which the substance emits and absorbs infrared radiation, and the second wavelength band is a wavelength band or bands in which the substance has no or negligible emission and absorption of infrared radiation; calculating a normalized ratio parameter as the average ratio or the average logarithm of the ratio of the two radiation measurements normalized against a black body at each time interval; correlating the normalized ratio parameter with the ambient temperature and the body surface temperature and using an empirically-derived lookup table correlating the normalized ratio parameter with the concentration of the substance in the body to calculate substance concentration; and displaying the substance concentration. positioning the body with respect to an infrared detector (32, 62, 64), an optical system (29, 31), and a heating/cooling apparatus; activating the heating/cooling apparatus to change a body surface temperature from a first temperature to a second temperature; activating the heating/cooling apparatus to change the body surface temperature from the second temperature back to the first temperature at a predetermined rate or using air at an ambient temperature to change the body surface temperature from the second temperature back to the first temperature; measuring absorption/emission of infrared radiation over each of a first and a second wavelength band, an ambient temperature, and the body surface temperature at a plurality of predetermined time intervals during the change of the body surface temperature from the second temperature to the first temperature, the first wavelength band being a wavelength band or bands in which the substance emits and absorbs infrared radiation, and the second wavelength band is a wavelength band or bands in which the substance has no or negligible emission and absorption of infrared radiation; calculating a normalized ratio parameter as the average ratio or the average logarithm of the ratio of the two radiation measurements normalized against a black body at each time interval; correlating the normalized ratio parameter with the ambient temperature and the body surface temperature and using an empirically-derived lookup table correlating the normalized ratio parameter with the concentration of the substance in the body to calculate substance concentration; and displaying the substance concentration.