filters mentioned above fig2 shows optical density od log 1 i values of radiation received from each of 12 near ir sources i e 12 ireds each having a corresponding filter during initial calibration of the instrument with a calibration standard in the above equation i is interactance and equal to es er es energy received from sample er energy received from a reference these initial od values are obtained by transmitting optical energy from each of the 12 near ir sources through an empty holding chamber the calibration standard being air according to the present invention as illustrated in the flow diagram in fig5 these initial od values are stored in the computer 54 individually and also combined to yield an average initial optical density value which also is stored in the computer each initial od value is then compared to the initial average value to obtain a 34 t 34 true value for each ired filter combination which also is stored in the computer the average initial or normalized optical density value of the calibration standard for all twelve of the near ir sources as shown in fig2 is 0 50 in this example at the time of initial calibration the optical density value of the calibration standard for radiation emitted by near ir source no 8 is 0 3 as shown in fig2 thus the true t value for rear ir source no 8 is determined by comparing the od value of source no 8 to the average od value for all twelve of the.