CFS Training aid 2013: p. 75 of 238 The reference blocks appear immediately prior to the subject test and calibration check. The reference blocks signal that 1. stability of the IR signal is being verified 2. zero reference point for the alcohol measurement that follows is being determined [I'm not sure if the latter is true because zero adjusts with every air blank] During the reference blocks, the amount of IR energy striking the 3.4 _m and 9.4 _m detectors is measured. Once it is established that the signal is stable (i.e., there is no positive or negative drift), the voltage from each of the detectors is taken as the initial intensity (Io) for the Lambert-Beer equation. [I'm not sure if that's true either.]