Stray light analyses Power Point Presentation (Click here)

Return

Tolerancing Power Point Presentation (Click here)

Return

This know how has been presented at the International Conference of Space Optics (Toulouse 2000) and at the SPIE-EOS Optical Design Conference (St Etienne-2003 – SPIE Proceeding N° 5249). Ghost are due to multiple reflections between the optical surfaces of the lens and between the focal plane and the optical surfaces. The solution to make them less vible consist in modifying curvatures, thicknesses, etc..during the optimization, and this is difficult because other ghost may appear during this process.
In order to deal with this problem, we have developed a special software, and called it PARASIT. This software is running using the CODE V routines in the Active X environment.
This tool is now of common use in many applications where it is required to control the ghost during the design phases, i.e. during the optimizations using CODE V, without having the need to another software like LightTools, certainly more complex and time consuming to use in parallel.
Without having to modify the CODE  V file used for the optimization, PARASIT makes it possible to quantify in several minutes the stray light ratio due to the ghosts and in the presence of some source in the field of view.

In it's more complex recent version, it makes possible a complete quantitative prediction taking into account the coatings on the surfaces, and the polarization state.
The applications of PARASIT are numerous, especially :

            - In the  3-5 microns band, it is now routinely used during the design phase.                This is because the contrast is  especially  high in this spectral range, and the                ghosts must be attenuated up to a high level of stray light ratio's

            - In photolithography, it is used to predict the stray light on the whole wafer.               This point is especially important for the  machines of the "Steppers" type.

              In a similar manner, PARASIT has been used to predict the stray light in optical               system using diffractive elements, taking into account the several orders of               diffraction of the diffractive element.

In a similar philosophy, some special tools have been developed to help and make more systematic the analysis of the stray light in general, i.e. taking also the mechanics, etc…For this purpose, LightTools is used together with MATLAB and a special methodology has been defined and MATLAB macros developed.

 

In every optical study, the question regarding the tolerancing arises and takes a significant part of the time dedicated to the study.
For this purpose, CODE V offers several routines for tolerancing which can be used to treat also the more complex tasks. Nevertheless, when the optical system is complex, with many compensators and associated labels to simulate the adjustments, the problem takes another dimension. It is indeed not exceptional to use more than 500 tolerances, with four or five different compensators. In such a situation, the complexity is so high, that it is useful to use special tools to collect, sort out and present the result in the most synthetic way.

We have developed specials tools to this purpose. Written in EXCEL VBA and using EXCEL LINK, we have now at our disposal the possibilities to present the results on EXCEL sheets on which the tolerances are organized and classified. They are also labeled to correspond exactly to the surfaces on which they are attached, and not only their numbers. Most often, it appears that only a few tolerances are responsible for 90 % of the degradations. It is then much easier to identify them and make corresponding modifications. Anwering to questions like :

   - which tolerances are most responsible to performance degradation for a given field?

   - which tolerances have the larger contribution to the compensators ranges?

are now straightforward.

These tools also permit to trace the exact M.T.F. taking into account the tolerances, for comparison with the theoretical curves, and to construct the CODE V files which correspond to the worst case.

This new set of CODE V and EXCEL macros which we have called TOLTRI has been presented at the SPIE-EOS conference Optical Design and Engineering in Iéna in September 2005 (SPIE Proceeding N° 5962). It has been completed in 2006 in order to consider also the tolerancing using the TOLFDIF routine of CODE V. Comparing on EXCEL spreadsheets the results obtained with TOR and TOLFDIF is easy because both spreadsheets have the same format. This gives a new and extremely interesting horizon to the question of tolerancing.