This paper examines the use of customized spacers to compensate for symmetric errors in lens systems. Examples of errors amenable to this type of compensation include radius, thickness and index of refraction of lenses and lens-to-lens spacing in lens barrels. Spacers used to separate consecutive lenses are selected after the optics have been fabricated and measured. A method is described for specification of spacer thickness as a function of the errors to be compensated by the spacer. The use of binning to enhance the efficiency of spacer selection is briefly explored.
Fizeau type phase measuring interferometers are widely used in the optics industry for surface metrology. Measurement of spherical surfaces requires the use of transmission spheres which are commercially available in various F-numbers. A basic assumption of Fizeau interferometry is that the light reflected off the reference surface, called the reference beam, and the light reflected off the surface being tested, called the test beam, follow a common path back through the optics. For this to be strictly true, we would need the surface being tested to be perfectly spherical and positioned exactly concentric with the reference surface. Measurement inaccuracy that results from failure to meet this condition is referred to as retrace error. Retrace error has been largely ignored with regard to testing nominally spherical surfaces, yet it can be significant when high test accuracy is needed. In this paper, the author identifies two types of retrace error resulting from the test setup: Axial; induced spherical aberration resulting from defocus, and Transverse; induced coma as a result of tilt. The magnitude and exact form of retrace error is shown to be a function of the optical design of the transmission sphere. It is shown that, for the most part, measurement accuracy is independent of the transmitted wavefront error of the transmission sphere. It is shown that retrace error can be modeled in a lens design program with excellent agreement to measurement data. Specific design examples will be presented, including improvements to minimize retrace error. The significance of retrace error to the test accuracy of both spherical and aspheric surfaces will be discussed.
A key factor in the manufacturing difficulty of a precision optic, and therefore a key cost factor, is the surface quality
(scratch-dig) specification. On the other hand, surface quality usually has little or no significant impact on the
performance of the finished optical system, but rather is included in tolerancing for cosmetic purposes only. From a
manufacturing standpoint, center thickness, radius, irregularity, glass type and cosmetic quality are all inter-related.
When more importance is placed on any one of these, there is a tendency for the others to be negatively impacted. This
paper makes the case that when surface quality is over-emphasized, not only is it a cost driver but also the average "as
built" optical performance of the optical system will be lower because performance based tolerances are more likely to
be pushed to their limit.
The experienced lens designer is well aware of the potential advantages aspherics can afford. Within the last few years,
machines specifically designed for the CNC machining and polishing of glass aspheres have become commercially
available through several manufacturers. This has brought down manufacturing cost to the point that designs
incorporating aspheres can be used to reduce system cost compared to all spherical designs. (That is aspheres are no
longer used just to save space and weight at the expense of cost.) Not all aspheres are equally manufacturable, however.
Arbitrary choices at the beginning of a design can have major impact on manufacturing cost and limit final "as built"
performance. This paper considers factors in designing ground and polished (as opposed to molded) glass aspheres
which may not be obvious to even the experienced lens designer accustomed to using spherical surfaces or who has dealt
with diamond turned aspheres. Factors considered include the surface shape, how the shape is specified, how the surface
is to be tested and how it is toleranced. Emphasis will be placed on medium priced components where practical considerations are important.
Conference Committee Involvement (1)
Optical Manufacturing and Testing XI
9 August 2015 | San Diego, California, United States