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Spotlight on Optics


  • September 2011

Optics InfoBase > Spotlight on Optics > Null test for a deeply aspheric lens

Null test for a deeply aspheric lens

Published in Applied Optics, Vol. 50 Issue 25, pp.4977-4981 (2011)
by Roderick Willstrop

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Spotlight summary: When visiting historic chapels, it is often easy to appreciate the architecture and artwork but not always so to recognize the illumination engineering that may have taken place to illuminate such wonderful structures. In his paper, Willstrop succinctly describes his efforts toward designing a practical and affordable null test for an aspheric element required in an illumination system in King’s College Chapel at the University of Cambridge. The testing method proposed involves placing the aspheric lens under water.

In the field of optical testing, highly aspheric surfaces typically require at least some custom optics in order to perform an adequate test, and in general, the higher the aspheric departure, the more difficult it is to design a reasonable test. In a null test, the aberrations expected to be caused by the test optic are designed to be completely cancelled out by the test optical system; in an interferometer, for example, deviations from a null fringe indicate imperfections in the test part. It is usually wise to consider how an optic under design will be tested before finalizing the design; otherwise, the system needed to test it can become unwieldy.

In this case, the illumination system that is actually being built did not require any aspheres, but an alterative illumination system did. The author was interested to see if an existing spherical mirror could be used at practical conjugates such that it could generate enough spherical aberration to test the asphere, and he found that it could not if the test element was measured in air. However, if the test optic were submerged in water, which reduces the optical power of the element by about a factor of 3 owing to the smaller change in refractive index at the surface interface, the existing spherical mirror could be used to perform a null test.

Placing an element in water can extend the dynamic range of the optical test, but this comes at the expense of sensitivity to small surface errors. Surface accuracy requirements in illumination systems are often looser than imaging systems, and so this trade-off provides a big advantage for this application.

Perhaps illumination designers should take note of this simple yet effective technique for extending the dynamic range of a null test. Considering an immersion test during the design phase could result in simpler, more cost-effective illumination systems. As a result, the world, and not just chapels, can be made that much brighter.

--Eric Goodwin

OCIS Codes: (080.2740) Geometric optics : Geometric optical design
(080.3620) Geometric optics : Lens system design
(120.4630) Instrumentation, measurement, and metrology : Optical inspection
(220.1000) Optical design and fabrication : Aberration compensation
(220.4840) Optical design and fabrication : Testing

Posted on September 02, 2011

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