In a previous paper, the use of a bistatic optical instrument for substance-on-surface chemical recognition was introduced. The apparatus proposed showed that the bistatic arrangement of multiwavelength emitter and sensor potentially allow certain unknown optical properties of the interrogated chemical (on a supporting surface) to be measured in real time. These optical variables have long plagued the results of monostatic infrared chemical recognisers with ambiguity, by virtue of being unknown and difficult to measure in real time outside of a controlled laboratory setting—(1) unknown optical properties of the supporting surface beneath the chemical layer, (2) unknown thickness and refractive index of the chemical film and (3) unknown angles of incidence and detection. It was previously shown that it is possible (and essential) to limit to a single pair, the number of narrow laser beams reaching the bistatic apparatus’s detector from its emitter, after the beams have impinged on and propagated through the interrogated substance. In this paper, a mechanism and feedbackcontrol method are discussed to accomplish two tasks: (1) limit the number of narrow beams reaching the detector from the emitter to two, and (2) determine the separation between the resulting pair of beams in real time. The beam separation is a real-time variable that is essential to determining the thickness of the unknown substance in the field, thereby removing one cause of false substance identification. The beams discussed are narrow Gaussian beams that are frequency-modulated. A generic movable variable aperture apparatus device is described that, when controlled via feedback, can position its aperture and set it to the appropriate sizes so as to exclude multiple reflections of the incident interrogating beam and thereby limit to two, the number of beams entering the detector. The feedback control system is also described with an appropriate set of state-space equations and a prototype for a robust feedback control methodology.