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Научни трудове на Съюза на учените в България-Пловдив. Серия В. Техника и технологии, т. XV, ISSN 1311 -9419 (Print), ISSN 2534-9384 (On- line), 2017. Scientific Works of the Union of Scientists in Bulgaria-Plovdiv, series C. Technics and Technologies, Vol. XV., ISSN 1311 -9419 (Print), ISSN 2534-9384 (On- line), 2017.
METHODICAL ASPECTS OF OPTICAL SYSTEMS OPTIMIZATION VIAZEAOSXOCTICAL STUDIO SOFTWARE
Evdokia Belina(l), Todorka L. Dimitrova (1), Georgi Dyankov(2)
(1): University of Alovdiv „Aaisii Hilendarski", Lzar Assen Otr. 24, 4000 Plovdiv (2): Institute of Optical Materials and Technology, HAS , 109, Acad. G. BontchevStr.,m3Sofia
Abstract
She quality of optical imaging strongly depends on the optical system geometry and on the relative aberrations. Using professional software is extremely helpful for computer simulation and optimization of optical systems, as well for educational purposes. En this work we discuss some methodical aspects of such procedures by using Zemax OpticOtudio. for simulation and optimization of a two lenses astigmatic objective working in the middle infrared range.
Key worlds: optical systems design, optical systems optimization, two lenses astigmatic objective, infrared optics
1. Introduction
Optical system design requires a prototype elaboration, then measurement and verification of its given parameters [1,2]. This process is time and money consuming and, for this reason, theoretical evaluation and optimization of the optical system is extremely needed. Nowadays, software development led to creation of specific programs, allowing fast calculation and evaluation of a large number of parameters, such as geometrical characteristics and various kinds of aberrations.
Zemax OpticStudio is professional software performing fast and precise calculations, visualization of the designed optical system and light rays tracing, as well preview of the optical aberrations effects by analyzing each one separately. This is extremely useful for understanding the importance of each optical element and the disadvantages of the optical system. Then, the optimization process should start by reducing the aberrations, which makes worse the image quality.
In this paper we establish a basic methodology for software analysis and optimization of optical systems by Zemax OpticStudio, using as an example a two lenses astigmatic objective, working in the middle infrared range, which basic parameters are specified by the customer [2].
The work was supported financially by Optix JSC, Panagyurishte, Bulgaria, where legal professional software was used and the prototype was elaborated and tested.
2. Optical system optimization by Zemax Optical Studio
The optimization procedure consists of several consecutive steps. First of all, the given parameters such as number of elements, lenses radii, thicknesses, diameters, etc. are inserted in the program. Then Zemax calculates the value of each aberration and the global optimization value. Besides, the designer can visualize the effect on the system and on the the image spot sizes of any examined aberration. In order to reduce the aberrations and improve the image quality, several variable parameters, such as curvature, thickness, index of refraction, etc., may be modified by the program. An error function (measure of optical quality; zero typically implying "perfection"), and constraints (boundary values that restrict possible configurations), are also defined. Furthermore, the designer sets the limits in which variables can vary. After any optimization step follows image quality analysis. Then, if necessary, the optimization procedure can be repeated.
After all, the lenses with optimized parameters calculated by Zamax are elaborated and tested. Comparison of the calculated with the measured values is always required. In the end, follows the construction and the technical control (measurement) of the optical system.
2.1. Optical system simulation
A simulation by Zemax starts with inserting of all given data in the program. The physical parameters are set in the "Lens Data Editor" (LDE). Any optical element is presented by its diameters, radiuses, thickness and optical material starting from the first to the last surface. The distances between the elements are given too. These parameters should always be entered from left to the right, starting from the object as an optical source, going forward through all elements and ending with the image. Designer shouldn't lose track of its work. That because, the program offers an additional field for every element "Comment", where helpful information can be written.
Parameters, such as apertures values, wavelength range and field of view are next to be indicated in the program. A primary wavelength (WL) can be chosen. The field of view can be entered either in angles or in millimeters, referring to the image high. These options are usually found on the left side next to the LDE. In the top menu bar, there are 2D and 3D imaging sections. After updating the system with the "update button", a picture of the optical system appears visualizing the results of the entered data.
Since our work is methodical, the given numerical parameters of the two-lens objective used as example are not reported here. In Fig. 1 is presented the first its design by Zemax. The calculated Effective Focal Length is shown on the right bottom corner as EFFL value. Text document containing all calculated values and named "System info", or "System data", can be open from the top menu. Optionally, the document content can be changed and save as a txt file.
Fig. 1. Visualizing window: a) 2D; b) 3D
2.2. Optical system optimization
Usually there is a basic idea, or, in other words, first-order requirements to be satisfied. In this work these are the EFFL (f') with a value of 90 +/- 1mm and the Total Optical Track (TOTR -L). The optical materials for all Lens 1, Lens 2 and the Optical Window should transmit in the range (3,4 -^5,1)«m. Their selection from software catalog is performed by entering of the characteristics of different combination of materials and lenses in "Lens Data Editor" searching for combination which gives EFFL value, equal or close to 90mm. In our case, we use to change the distances between the elements for image quality optimization. The software recognizes the variable parameters, when they are marked by "V". This is done with the optimization menu, called the Merit Function Editor (MFE). In "Type" column are inserted the parameters under optimization. The values with zero aberrations are entered in column "Target". EFFL is fixed to 90mm and Zemax do not change during the optimization.
The main optimization criteria is the different kind of aberrations. Usually, their effect on the image quality can be realized, after the system is elaborated and measured. Zemax offers an easy way to evaluate in advance which aberration has most powerful influence, by calculating the total optimization value. In principle, the first task is decreasing of spherical aberration, coma and astigmatism. All calculated values after optimization are presented in a table. Merit Function value presents the total rate of optimization for sagittal and tangential plane with respect to a specific parameter. Designers can compare this value reducing different aberrations one by one.
In our case, simulation is performed for four different lenses combinations. The Merit Function analysis led to the conclusion that the spherical aberration needs to be decreased first.
Scattering spots in the range (3,4 ^5,1)«m referred to image high of 0mm (the axes ray), 1mm and 3mm obtained after optimization of the optical system are presented in Fig. 2.
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Spot Diagram
8/8/2016 Units are jim. Field : 1 2 3 Zemax OpticStudio 14.2 SP3
GEO radius : 42.377 50.021 52.624
Fig.2. Scattering spots referred to image high of 0mm (the axes ray), 1mm and 3mm
The smaller optical blur for all wavelengths is formed for the axes rays. Off-axes rays have different optical paths in the sagittal and tangential planes. Every off-axes optical blur in the image plane is more scattered due to the spherical aberration.
The MTF presents the resolution as a function of image spatial frequency, and depends from its brightness. Smoother the transfer function is, better is the resolution. In Fig. 3 and Fig. 4 is presented the MTF before and after the optimizations. Next steps consist of decreasing spherical aberration, coma, astigmatism, filed of view (see Fig. 5a), distortion (see Fig. 5b,c) etc.
After the optimization is completed, the optical system is elaborated and tested. The comparison between the measured and the calculated required parameters and the image quality analysis can suggest an additional optimization before starting real production of the optical device.
Fig. 3. MTF before the optical system optimization
Spatial Frsquency in cycles per mm
Fig. 4. MTF after the optical system optimization
Figure 5. a) Field curvature; b) Distortion; c) Distortion grid.
Conclusion
We have discussed some methodical aspects on optical system optimization by Zemax Optical Studio software on the base of two-lens astigmatic objective designed for the near infrared range. Some experimental results are presented and analyzed.
Literature
1. http://www.zemax.com/os/opticstudio
2. Customer's specification documents # 34
e-mails: evdokiyabelina@yahoo.de; tldimitrova@abv.bg ; e-mail: gdyankov@iomt.bas.bg
