The last major SPIE meeting on lens design was held about two years ago. The next international conference on lens design will be held in about a year and a half. It is appropriate to consider what has happened to lens design in the past two years and provide some speculation upon the technology that drives the near term future of the subject. Some things that were predicted to happen two years ago seem to have happened, but a number have not. Some may by the time of the next meeting.
In this talk I would like to review some of the things going on in lens design, and attempt a few predictions of things to be heard from soon. Obviously these thoughts are somewhat personal, and cannot be presumed to be all inclusive.
The field of lens design has continued to remain healthy. The number of inquiries from companies seeking to employ lens designers has not decreased over the past two years. Any of our students who can report that they have done well in the lens design course have no difficulties in finding employment somewhere in the United States, unless they have some additional problem (such as lack of permanent residency or citizenship).
Many of the questions that are being asked of lens designers seem to be much the same as those traditionally asked, leading to a concern whether the same problems are just being studied over and over instead of developing into a continuing production of useful hardware. There does seem to be wider interest (and some greater understanding) in design by all levels of engineers. There is significant, and growing, interest in the design of optical systems that use some of the novel technologies for fabrication of optical components, especially using aspheric or holographic surfaces.
This emulates the continuing development of new technology. The use of holographic optical components for laser systems, including optical data storage systems, appears to be increasing. Heavier use of aspherics and gradient index materials is probable in the future. The use of precision molded glass elements has increased, with some indication that the approach to design has changed as a result of the new options possible in fabricating non-spherical surfaces in quantity. New systems, such as aperture arrays, which add a novel twist to traditional optics continue to be of interest. New technologies, such as diode laser arrays, integrated optical systems and x-ray lithography will provide some new opportunities for inventive optical system designs.
Several different reasons are given as to why it is vital to have 5th-order aberration capabilities in a computer program in order to have a realistic chance at making substantial performance improvements over existing designs.
The domain of optical design is vast and complex due to the diversity of applications, the large number of lenstypes, as well as to the even larger variety of operating conditions. The use of standardized aberration coefficients is suggested to gain control over this domain. A method to this effect is outlined and some examples are given.
A striking similarity is shown between the shapes of focused coherent and non-coherent optical energy beams. This similarity is shown to be an exact analogy under broadly applicable conditions. It is proven that non-coherent beam focusing equations can be obtained from the well known Gaussian beam propagation equations simply by replacing the Rayleigh range with a new parameter, the focal range, equal to the input beam waist radius divided by the beam angular divergence. The adapted beam equations enable calculation of the first order location and radius of the focused waist minimum for non-coherent energy input beams of finite diameter and finite angular extent - a situation not previously amenable to simple calculation and analysis. A fictitious wavelength can be calculated from the input beam parameters, and this wavelength permits the use of standard Gaussian beam propagation programs to analyze non-coherent beam focusing. The effects of aberration on the waist location and radius calculated with the adapted beam equations is investigated with a new energy ray tracing technique that uses standard spot diagram data obtained under special input conditions and by ray tracing backwards through the lens system. Intensities computed with this technique are compared with actual measured values for an example system, and the agreement is found to be very good. The new ray tracing techniques can be used to analyze the focusing of energy from a wide variety of sources, from low divergence multi-mode lasers to the wide divergence beams from arc lamps with ellipsoidal reflectors. The results presented in this paper reveal that there is a continuity in the characteristics of focused energy beams that extends over the entire range of source coherence, from fundamental mode lasers to the ordinary light sources of everyday experience.
A computer program has been written which allows the user to draw a y-ӯ diagram on the computer screen. Based on the diagram data, the system prescription is calculated. The first order properties such as effective focal length, pupil size and pupil position are also determined. The program can modify a system by lens bending or thickening, and by conjugate and stop shifts. The software runs on IBM PCs and compatibles.
We have used the gaussian-integration techniques described by Forbes to build a default error function for our optical design program. To make the function suitable for practical design, we have added ad-hoc extensions that account for vignetting and allow for a variety of user-defined weights. We have now acquired about a year's experience in using the function to design several different systems. Our results have been generally good, and we have recently extended the function to provide additional control over astigmatic aberrations.
We have developed an interactive tolerancing capability for our optical design program, using the optimization error function as the performance criterion. The implementation was carried out using the program's built-in command language, and allows evaluation based on either the error function used during the actual design phase or a standard criterion (magnification, distortion, rms spotsize or wavefront error, etc.). This approach is more flexible than schemes that permit only pre-defined criteria.
Developments in detectors and scanning systems have resulted in an increased demand for optics operating in the thermal infrared waveband (8-12 microns). A continuing concern however for optical and systems designers is the effect of temperature change on the performance of the optical systems. Several optical parameters can be affected by temperature variation but the main problem is that of focus shift (or decollimation in the case of afocal optics) and the consequential degradation in image quality. The correction of this effect is usually referred to as athermalisation. A variety of techniques are available. This paper reviews the options with an emphasis on optical athermalisation methods.
This paper will discuss the design techniques to athermalize infrared optical systems. The discussion will focus on specific state-of-the-art tactical weapon applications for thermally compensated optical imaging systems. The main objective is to athermalize the system by choosing the correct mounting materials without the use of active mechanisms such as electronic motor or liquid bellows lens focus drives. Two specific designs with their advantages and disadvantages will be discussed -- a catadioptric imaging system and an all refractive imaging system.
The optical design presented here is capable of projecting the image of two different reticles, in two different spectral regions, through a single exit aperture. The reticle projections can then be viewed by a single instrument capable of operating in either spectral region. The system features coaxial reticles for boresight stability between the two projections, broadband transmissive optics which allow the sharing of optical paths in the two spectral regions, a single exit aperture, a double pass of the shorter waveband projector (which helps minimize the number of required optical components) and a dichroic lens coating which allows for independent control of the optical parameters in the two spectral regions. The spectral regions of the two reticle projections can be any two regions in which broadband transmissive optics and dichroic lens coatings are available. The application that brought forth this design required the projection of a visible reticle and an infrared reticle (3.0 to 5.0 gm) and much of the discussion will be based on these two spectral regions.
The optical design of WIDE-BAND CATADIOPTRIC TELESCOPE (WFCT) has been presented in this paper. The optical system of focal length 950 mm, F/5.6 covers field of 1.5 degree and can be used for imaging purposes in the wavelength range of 450 - 860 nanometers. The performance of WFCT has been characterized by Modulation Transfer Function (MTF) values. It is a diffraction limited system. A tolerance analysis for fabricating the system has been carried out to identify the critical parameters.
Many factors beyond the optimization of radii, thicknesses, and glasses are critical to the optical system design task if a practical system is to be produced. We review the common factors and some that may not have been so obvious. We relate some specific examples from our experience. The designer is encouraged to keep various factors in mind when designing for manufacture with the aim of making cost effective designs.
The changing economies of optical system design and manufacture are discussed and their implications relative to software tools for the optical designer are listed. In response to these needs an optimization routine using generalized simulated annealing is described. In addition to global minima capability this routine has the ability to automatically select off the shelf catalog elements, select from existing test plate lists from several vendors and to consider costs of materials during the optimization process. As a result the lens designer can obtain a family of designs with varying performance/cost ratios merely by varying the number of custom surfaces and materials limits upon setup of the optimization run.
In the realm of fabrication of optical elements, one often overlooks one of the largest sources of optical components; the ophthalmic optician. Within the tolerances which the ophthalmic optician can operate, he can provide finished elements very quickly and inexpensively. Additionally, the ophthalmic optician routinely makes toric surfaces, a service which more conventional optical fabrication facilities cannot provide.
The development of a new telescope type, the ZOBIG, is described, which is capable of complete stray light rejection, parfocalization of all wavelengths, and diffraction limited image quality, all without aspherics, even for a 2-meter aperture. The main purpose of this new design is as a solar coronagraph, although there are several other possible astronomical and military applications.
This paper deals with the calculation of intensity associated with rays in optical systems. A new set of equations is derived using optical field techniques which gives the electromagnetic fields, including intensity and polarization, around an arbitrary skew base ray. These equations are then used to calculate the flare and ghost light reflected from lens surfaces. An example is given showing flare light and its reduction in several patented lenses using this approach. The system of equations is derived using a parabasal approach in which one uses a small angle spherical or elliptical wave expansion about a base ray which is propagating at an arbitrary angle. These are manipulated to give field transfer equations. The field expressions are matched at lens surfaces to give field refraction equations. The result is a set of equations for transferring complete electromagnetic field information through an optical system. The equations thus derived are applied to the minimization of scattered light in optical systems. They are incorporated in an optimizing lens design program. An additional term is included in the merit function of the program so that the lens can be redesigned to minimize flare and ghost light at the image plane. The equations will be presented along with the application to flare light minimization.
Polarization aberrations are variations of the instrumental polarization associated with different ray paths through an optical system. The polarization aberrations of an overhead projector are easily observed with two large sheets of Polaroid. The form of these polarization aberrations are described as functions of object and pupil coordinate and are related to the polarizing properties of the lenses and mirrors. These polarization aberrations do not adversely affect the performance of an overhead projector, but they do provide an easy and effective demonstration of the concepts of instrumental polarization.
Subject terms: polarization; polarization aberration; mirrors; retardance; diattenuation; Jones calculus.
The effect of aberrations on off-axis beam steering for multi-aperture systems was qualitatively investigated. A new expression analogous to Goodman's Thin Lens is proposed which shows the beam degradation that occurrs at angles (with respect to the optic axis) much greater than five degrees for a thin lens, and two degrees for a thick lens. This expression was then applied to three multi-aperture configurations as a way to correctly predict the field-of-view for various optical systems. Analytical predictions supported by computational and experimental results are included. Although the beam degradation as the field-of-view or field angle is increased has a dramatic effect on the image quality, the power loss as the field angle increases will be the major factor in the design of a system of this type.
Santa Barbara Research Center has been exploring technology related to the design, tolerance, and alignment of wide-field, all-reflective sensors for multispectral earth observation. The goals of this study are to design an optical system with reduced fabrication risks, to develop a detailed tolerance budget and to demonstrate our ability to align the system to a tolerance of 0.05 waves rms at 0.6328 microns. The final optical system is a three-mirror unobscured telescope. It is telecentric and flat field over 15 degrees at F/4.5, and achieves diffraction-limited imagery at visible wavelengths.
This paper describes the results of the design effort, the tolerance exercise, and a metrology approach for the optics fabrication. The alignment results are discussed in a separate paper.1 Three conclusions are made: 1) design of unobscured optical systems is still best approached using fundamental optical design principles, 2) time spent in careful modeling of fabrication, testing, and alignment interactions will result in more relaxed tolerances and a higher probability of success for the assembled system, 3) precise metrology of optical surfaces can be achieved with fairly simple techniques.
Santa Barbara Research Center has been exploring the technology related to the design, tolerance, and alignment of wide-field, all-reflecting sensors for multispectral earth observation. The goals of this study are to design an optical system with reduced fabrication risks, to develop a detailed tolerance budget, and to demonstrate our ability to align the system to a tolerance of 0.05 waves rms, at 0.6328 microns. The optical system is a three-mirror, unobscured telescope. It is telecentric and flat field over 15 degrees at F/4.5, and achieves diffraction-limited imagery at visible wavelengths. A separate paper 1 describes the design and error budget approach for the telescope. This paper reports on the effort that led to the alignment of a scaled, prototype optical system. The approach used interferometric measurements of the wavefront at multiple field points and a computer alignment algorithm to define the rigid-body adjustments of the mirrors to achieve the alignment goal.
A microlithographic projection lens is expected to give constant results from one lens to another, anywhere within the field, over the full Rayleigh depth of focus, and with a finite variation of exposure dose. In particular, the requirements for image dimension and position control are an order of magnitude smaller than the features being printed. These demand that the paper design have wavefront correction significantly better than the Marechal criterion, and almost zero distortion. Furthermore, impressive theoretical performance is of no use if the manufactured lens cannot always approach the ideal as closely as possible. Lens designers traditionally bridge the gap between customer requirements and production realities. Compensators -- parameters that are adjusted to compensate for construction imperfections -- are a well-known way of relaxing tolerances. For a lens on the edge of possibility, they come to the aid of the designer caught between two vociferous groups of people with contradictory needs -- the users and the manufacturers. A lens will be shown that is used in a Wafer Stepper capable of printing features in the 0.7- to 0.6-micron range, and the rationale given for an extensive scheme of compensation, both before and after assembly. A selected number of elements are adjustable after assembly, while the lens is tested both interferometrically and lithographically, after which they are fixed in place.
Iterative optimization of multielement lens performance using interferograms and CODE V(TM) will be described, with an emphasis on fixing power-related problems rather than alignment problems. The principle will be reviewed and an example optimization of a real system that exhibits performance problems will be presented.