chapter 1, Optical Design Concepts

Excerpt

1.1 A Value Proposition

A comprehensive optical design should yield both consistent manufacture and maximum performance. Such features are critical to biological applications, where the cost of consumables can be far greater than the cost of the optical hardware. Maximum performance can reduce the magnitude of consumables on a daily basis. Furthermore, a false negative can delay proper treatment of a critical condition. Consistent manufacture builds credibility in a clinical test. Sections 1.2 through 1.9 provide a review of essential optical design concepts. Expertise within these concepts is developed through education and practice. Perseverance is not a substitute for relevant expertise in optical design.

1.2 Specimen Model

The structure of a specimen determines the limits of optical performance. A small deviation from the nominal thickness can introduce a significant wavefront error that is not corrected by a lens. The light collected by the margin of the lens is not focused within a diffraction-limited spot. The marginal light is spread over many pixels, which lowers image contrast. Consequently, an optical design must accommodate the tolerances of the specimen. In particular, the cover strata of biological specimens present significant challenges in optical design.

1.3 Detector Parameters

Every detector has fundamental limitations, which may be expressed as equivalent incident photons. A dark current is converted to photons per time per area. Both full-well capacity and read noise are converted to photons per area. The dependence on area is very important because image contrast is frequently determined by image incidence (photons per second per area). Thus, an equivalent incidence of the detector must be established.