The image sensor resolution often attracts primary attention during the initial design phase of an imaging hardware platform. While sensor pixel size is a key factor, to acquire a high-resolution image, the lens must able to project the required level of finer details onto the image plane. Consequently, resolution matching between the sensor and the lens is critical in optimizing acquisition hardware performance, rather than selection of the sensor pixel size alone. With pixel sizes converging toward 1 m, lens technology is progressing to increase line-pair resolution from the current typical machine vision figure of 30 to 50 line pairs to over 100 line pairs per mm. This increase in the field-of-view resolution requires reduced image distortions and larger lenses (lenses get bigger with increasing resolution). Since these larger lenses demand higher manufacturing precision and, hence, cost, lens requirements need to be set out in the context of overall system specifications.
The process of 'designing a lens' follows a well-established routine: estimate the field of view, compute the nearest focal length (in viewing angles for far objects, in size for near objects), select lens f-stop number for the required depth of field and ambient lighting levels, and select lens spatial resolution to match sensor pixel size and desired distortion limits on the image plane. While several computer-aided design suites are available, optimizing lenses to specific applications remains a skilled occupation, requiring appreciation of the limitations of image-geometry modeling as well as knowledge of lens material properties and manufacturing processes if cost and performance are to be matched. As background building, core concepts and definitions associated with lens data, lens-diaphragm geometries, resolving power, and the notion of a diffraction-limited lens are introduced in this chapter. These are followed up in Chapter 7 to develop image formation and modulation transfer function theories.
Although glass lenses offer superior optical characteristics and environmental stability, with improvements in processing technologies, plastic lenses have almost become the industry standard for high-volume, low-resolution devices. With the plastic lens glued to the image sensor, the sensor-lens combination is treated as a single component in the assembly of many portable, hand-held imaging systems. A short review of plastic lenses is given at the end of this chapter to provide a basis for comparison with glass lenses.