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Chapter 11:
Cover Strata

11.1 Importance of Specimen Tolerance

A specimen tolerance budget is essential for consistent performance of an optical instrument. The cover strata should have a specification for nominal thickness and tolerance. Exceeding the specimen tolerance might create enormous spherical aberration. Spherical aberration can reduce contrast without an apparent growth in spot size. Other important issues for the specimen are intrinsic fluorescence, surface-form error, surface defects, and surface texture.

11.2 Perfect 10X for Air

A perfect 10X objective within air is defined by Prescription 11.1 in the Appendix. Equation (2.4) defined the focal length of a 10X objective as 20 mm in combination with a 200-mm tube lens. A typical sensor field is defined by a 12-mm diagonal sensor. Thus, the object field of a 10X objective is 1.2 mm in diameter or 0.6 mm in height.

A virtual object is located at the back focal point. A spherical principal plane defines the lens stop. The object distance from the lens stop is −20 mm in the prescription. The radius of the lens stop equals the back focal length at 20 mm. Rays traveling leftward from the object define paths of perfection without aberration. The rightward-traveling rays after the lens stop should overlay the paths of perfection, unless there is deviation from the cover strata of the virtual object.

The object-space NA is defined within the perfect part of the lens prescription. The lens-stop diameter at 10 mm is defined by the object-space NA. The image-space NA may vary due to aberrations in the cover strata of the image side. A planar lens stop is an acceptable alternative to a spherical lens stop. However, a planar lens-stop diameter requires an inflated diameter for the same object NA. The lens stop and focal length define the f/# as 2.0. A spherical lens stop provides a more accurate definition of f/#. The distance from the lens stop is variable, as indicated by a box around the thickness of the lens stop.

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