Abstract: In a radiation-utilizing measurement system including a rotatable reflector, a point source, and a point detector, there being a plane both at right angles to the rotational axis and including the source and the detector, the plane also including the optic axis of the system extending equidistant between the source and the detector, and there being a plane including the rotational axis and the optic axis, there is provided a refracting element with zero power in the first plane, and a finite positive power in the second plane; or, at least the equivalent of, two refracting elements, the second refracting element having zero power in the second plane, and a finite positive power in the first plane. Thereby, respectively, the effects caused by reflector fluctuation; and also the effects caused by reflector rotation, when the source provides a conically divergent beam of radiation; are obviated.