Highly accurate measurement of lens surface distances within optical assemblies for quality testing

In the assembly of multi-component optical systems the precise positioning of every single lens element respectively lens surface is key to reach an optical performance that corresponds to the optical design. Here, in addition to lateral decentering of single elements – such as shift and tilt – the accurate positioning of the optical surfaces along the optical axis is an essential requirement. In this contribution we present the highly accurate determination of lens center thicknesses and air gaps of optical assemblies in a non-contact manner. The measurement technique is based on time-domain low coherent interferometry (also known as A-scan optical coherence tomography). Here, a low coherent interferometer signal is recorded in a Michelson-type setup as a function of a variable optical delay in the reference arm. Whenever the variable optical path length matches the path length to a lens surface, a coherence peak occurs. Thus, relative surface distances can be derived from the optical delay between two peaks. For a highly-accurate measurement a precise determination of the optical delay length with minimized Abbe-errors is required. Here, a precise long-coherent reference interferometer is superimposed to the short-coherent signal in the optical delay line. Both signals are recorded simultaneously; subsequently, the data is transferred to a PC system for the analysis. With the presented technique lens thicknesses and air spacings of up to 800 mm can be measured with a resulting accuracy significantly below 0.5 μm. The obtained results can be used for the compensation of potential deviations/errors in the manufacturing process or for quality control using a pass/fail-evaluation.