Ray Tracing and Estimation of Image Quality

Abstract

As a rule, the process of designing of optical systems includes a stage when rays are traced through the system and the real of an is estimated by considering the effect of all residual aberrations. Indeed, paraxial ray tracing and aberration theory (see Chaps. 4 and 5) can at the best help to find an initial design where the low-order aberrations (up to the fifth order, usually) are fully or partly compensated. In spite of this there are always residual aberrations defining the maximum possible aperture and useful field of the system. Moreover, in most cases the solution found by the aberration analysis is just the starting point for further numerical optimization of the system parameters which is carried out by ray tracing. These methods for conventional optical elements have been long developed and are widely used. However, the introduction of diffractive and GRIN elements in optical systems requires these methods to be further developed. GRIN elements have particularly essential differences in calculation algorithms since rays have to be traced through an inhomogeneous medium. As a rule, when parameters of an optical system are numerically optimized, the correction of low-order aberrations is upset even if it has been done at an earlier stage. The residual aberration of the system after optimization is usually a complex set of terms of various orders balanced so that their joint effect on the is the least. The term image quality can be treated in different ways. In the general sense the is understood as a set of parameters characterizing the of an extended object. In this case, besides system properties one should take into consideration such factors as the conditions of illumination, detection, and observation of the object and its structure. If it is necessary to describe the of the optical system in terms of aberration properties, one should examine the of a point object (response to a pulse). In this case, one also needs to take account of system service conditions, the fashion of the recording, for example. However, the influence of this and similar factors is not very strong and mostly reduces to selection of criteria most suitable for estimation of the quality.