The first problem to be solved in most optical designs with respect to stray light is that of internal reflections on the several surfaces of individual lenses and mirrors, and on the detector itself. The level of stray light ratio can be considerably reduced by taking into account the stray light during the optimization to determine solutions in which the irradiance due to these ghosts is kept to the minimum possible value.
Unhappily, the routines available in most optical design software’s, for example CODE V, do not permit all alone to make exact quantitative calculations of the stray light due to these ghosts. Therefore, the engineer in charge of the optical design is confronted to the problem of using two different software’s, one for the design and optimization, for example CODE V, one for stray light analysis, for example ASAP. This makes a complete optimization very complex .
Nevertheless, using special techniques and combinations of the routines available in CODE V, it is possible to have at its disposal a software macro tool to do such an analysis quickly and accurately, including Monte-Carlo ray tracing, or taking into account diffraction effects. This analysis can be done in a few minutes, to be compared to hours with other software’s.
To analyze ghost effects in optical systems including decentred coatings with thickness variations, polarized light and interactions
with object and image planes is a difficult task. Thanks to a battery of tools especially developed in the last ten years and applied to
many different situations, it is possible to do such jobs in a quick and elegant manner. As an example, we shall consider optical relay
lenses used for photolithography. The object plane is the reticle; The image plane is the wafer.
Tolerancing can be done with three different methods: using differential equations, using exact calculations and using the Monte-
Carlo technique. Thanks to the possibilities offered by modern computations, what is called integrated engineering using the
Microsoft Windows Component Object Model (COM), it is now possible to use different softwares to make the calculations,
collect the results and produce synthetic tables and figures which are quickly and clearly interpretable. This is possible with optical
design softwares implementing the Application Programming Interface (API).
Lenses for cameraphones are recent examples in which tolerancing is difficult. In order to take right decisions in choosing the
tolerances, we have been using the three methods for tolerancing. We are using this example to compare the results obtained.
After having summarized the different methods in use to tolerance optical systems, with advantages and drawbacks, we present a new software, called TOLTRI, to post process the data's of tolerancing obtained with the TOR routine of CODE V and also introduce new features which are not possible in another way. This software accelerate the interpretation and discussions between the different disciplines involved, by presenting and sorting the results on separate EXCEL sheets and Matlab figures.
Quantitative analysis of stray light in optical system is often a burden for optical designers. Ghost images due to multiple reflections on the surfaces of lenses and mirrors must be minimized during the design and optimization phases. Stray light in general must be controlled also during the optomechanical drawings. Already presented at the ICSO 2000 conference (Ref. 1), the CODE V macro software PARASIT makes it possible to have a complete quantitative analysis of the ghost in a few minutes. The possibilities of PARASIT are summarized with emphasis given to recent developments and comparison are given with LightTools. To make a full evaluation of the stray light in general, the possibilities of Light Tools, MATLAB and EXCEL interacting together using the Microsoft Active X protocol are unbeatable, as long as methods and corresponding macros written in EXCEL VBA and MATLAB .m are at the disposal of the optical designer. A short overview of these possibilities are demonstrated in this paper.
We are interested in calculating precisely the PSR far from its maximum, where the maxima of the irradiance are falling to 1E-06, or less. The first and most used method consist in calculating the Fourier transform of the wavefront using the Fast Fourier Transform algorithm (FFT). Another method is using the beam superposition technique (BST) to decompose the wavefront in Gaussian beams, propagate those beams, and recompose to obtain the result. The third method is to apply the exact equations derived at the end of last century and described in reference books like Born and Wolf or Marechel and Francon. We shall compare the result obtained with the three methods, FFT, BST, and exact calculation in the case of a F/3 system working at 4000 nm, in focus and in presence of small defocusing.