In general, the performance of a lens or lens system in a specific circumstance should be determined by an
exact trigonometric ray trace. Melles Griot applications engineers can supply ray-trace data for particular
lenses and systems of catalog components on request. However, for certain situations, some simple guidelines
can be used for lens selection. The optimum working conditions for some of the lenses in the Melles Griot catalog have
already been presented. The following tables give some quantitative results for a variety of simple and
compound lens systems that can be constructed from standard catalog optics. The values are based on specific
Melles Griot lenses. Other lenses may perform differently.
In interpreting these tables, remember that these theoretical values obtained from computer ray tracing
consider only the effects of ideal geometric optics. Effects of manufacturing tolerances have not been
considered. Furthermore, remember that using more than one element provides a higher degree of correction
but makes alignment more difficult. When actually choosing a lens or a lens system, it is important to note
the tolerances and specifications clearly described for each Melles Griot lens in the product listings.
The tables give spot size for a variety of lenses used at several different f-numbers. All the tables are for
on-axis, uniformly illuminated, collimated input light at 632.8 nm. They assume that the lens is facing in
the direction that produces a minimum spot size. When the spot size caused by aberrations is smaller or equal
to the diffraction-limited spot size, the notation "DL'' appears next to the entry. The shorter focal-length
lenses produce smaller spot sizes because aberrations increase linearly as a lens is scaled up.
The effect on spot size caused by spherical aberration is strongly dependent on f-number. For a plano-convex
singlet, spherical aberration is inversely dependent on the cube of the f-number. For doublets, this
relationship can be even higher. On the other hand, the spot size caused by diffraction increases linearly with f-number. Thus, for some lens types, spot size at first decreases and then increases with f-number, meaning that there is some optimum performance point where both aberrations and diffraction combine to form a minimum.
Unfortunately, these results cannot be generalized to situations where the lenses are used off axis.
This is particularly true of the achromat/aplanatic meniscus lens combinations because their performance
degrades rapidly off axis.
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