A method for non-invasively measuring a concentration of a substance in a body, the method comprising the steps of: measuring a first amount of infrared radiation absorbed or emitted from the body in a first IR wavelength band in which the substance emits and absorbs IR radiation; measuring a second amount of IR radiation absorbed or emitted from the body in a second IR wavelength band different from the first IR wavelength band and which is either the entire spectrum in which the body emits and absorbs IR radiation in the far IR spectrum or is a wavelength band or bands in which the substance has no or negligible emission and absorption of IR radiation; measuring a temperature of a surface of the body; measuring an ambient temperature; operating a processor to (a) calculate a first normalized IR radiation measurement which is the first measured amount of IR radiation normalized against a first blackbody radiation in the first IR wavelength band, (b) calculate a second normalized IR radiation measurement which is the second measured amount of IR radiation normalized against a first blackbody radiation in the second IR wavelength band, and (c) calculate a normalized ratio parameter value as the ratio of the normalized first radiation measurement value to the normalized second radiation measurement value; storing in a memory data for a series of measurements across a statistically representative population at various concentrations of the substance taken according to a conventional invasive method and constructed into an empirically derived lookup table by taking, and storing for the measurements in said series body surface temperatures, ambient temperatures and normalized ratio parameters; and operating the processor to calculate the concentration of the substance in the body using the empirically derived lookup table by correlating the concentration with the normalized ratio parameter, the body surface temperature, and the ambient temperature. measuring a first amount of infrared radiation absorbed or emitted from the body in a first IR wavelength band in which the substance emits and absorbs IR radiation; measuring a second amount of IR radiation absorbed or emitted from the body in a second IR wavelength band different from the first IR wavelength band and which is either the entire spectrum in which the body emits and absorbs IR radiation in the far IR spectrum or is a wavelength band or bands in which the substance has no or negligible emission and absorption of IR radiation; measuring a temperature of a surface of the body; measuring an ambient temperature; operating a processor to (a) calculate a first normalized IR radiation measurement which is the first measured amount of IR radiation normalized against a first blackbody radiation in the first IR wavelength band, (b) calculate a second normalized IR radiation measurement which is the second measured amount of IR radiation normalized against a first blackbody radiation in the second IR wavelength band, and (c) calculate a normalized ratio parameter value as the ratio of the normalized first radiation measurement value to the normalized second radiation measurement value; storing in a memory data for a series of measurements across a statistically representative population at various concentrations of the substance taken according to a conventional invasive method and constructed into an empirically derived lookup table by taking, and storing for the measurements in said series body surface temperatures, ambient temperatures and normalized ratio parameters; and operating the processor to calculate the concentration of the substance in the body using the empirically derived lookup table by correlating the concentration with the normalized ratio parameter, the body surface temperature, and the ambient temperature.