High accuracy dual lens transmittance measurements.

We show how to determine the transmittance of short focal length lenses (f approximately 19 mm and f approximately 2 5 mm, in this case) with a combined uncertainty of 3 parts in 10(4) or better by measuring transmittances of lens pairs of a set of three or more lenses with the same nominal focal length. Uncertainties are minimized by optimizing the radiometric design of the setup and the measurement procedure. The technique is particularly useful in systems where the detector acceptance angle limits the beam geometry to relatively collimated beams.