Optical design elements

Although some optronic systems are limited to the detector alone, even the simplest applications generally benefit from interposing an optical system between the source and the detector. Optics can be used to collimate the emission of a small source to make a torch, to focus the image of a target on a detector, to efficiently transfer energy from a source to a detector or, in many cases, to produce fine imaging of a scene over several million resolution elements. In some systems, particularly in the infrared, it is necessary to scan the image across the detector. In many optronic systems that operate under strong ambient lighting or in the presence of parasitic sources, optics provide spatial or spectral discrimination between useful and parasitic signals. Optics can also be used in signal or image processing, for example for two-dimensional Fourier transforms.

There are many different approaches to implementing optical systems. Because non-recurring costs are very high in optics, the first question to ask is whether the desired function can be fulfilled by a system that already exists and is available on the market. For example, if a zoom lens is required for a camera equipped with a charge-coupled device (CCD), it is often preferable to use existing optics, even if they don't exactly meet the specifications. It only makes financial sense to design a complete optical system if a large number of identical copies are to be produced, in which case it's best to use the services of a professional optical designer or team of designers. If you can't find a suitable system, the alternative is to assemble components from stock, especially if you want to set up an experimental assembly quickly and inexpensively, using components that may be available at a university or engineering school. This procedure is not always optimal, however, as it escapes conventional optimization methods.

The design of an optical system is based first and foremost on the laws of geometrical optics, the branch of optics devoted to the propagation and path of rays, apart from the wave aspects of light. As the latter cannot be neglected if the performance of optronic systems is to be properly assessed, these aspects will also be covered.

First, we consider ideal components, such as thin lenses, used with beam diameters and angles of incidence sufficiently small (Gaussian conditions) that optical defects, or aberrations, can be neglected. This is rarely the case in practice, but the procedures and figures presented here are intended to illustrate principles rather than represent...