Julie Bentley

Aberration theory: still the key to designing superior optical systems

Lenses used in everything from cameras and telescopes to optical data storage are sophisticated systems containing multiple lens elements. The design of these optical systems has changed significantly in the past few decades. The overall process— picking a starting layout and improving it using experience and numerical optimization software—remains largely the same. However, greater computing power and sophisticated commercial software can now rapidly generate multiple starting points.1 Lens design is a challenging problem that calls for costeffective solutions. It requires knowledge of first-order optics, wherein lenses are assumed to form perfect images, for system layout. At the same time, determining the best scheme demands an understanding of third-order aberration theory.2 This theory breaks down aberrations—unavoidable results of light refraction at spherical surfaces that lead to blurry or distorted images—into components that optical engineers can then correct or reduce. Before modern computing, when tracing single light rays required manual effort, aberration theory was the quintessential tool for gaining insight into a lens model. The method required cost and time, so a lens was typically developed and built as soon as a reasonable form was obtained. Beginning mid-century, numerical optimization gradually replaced many roles that the theory traditionally filled. However, aberration theory is still a useful tool for one critical design process step: the investigation of various starting layouts to choose viable candidates for further development. This is vital to avoid running multiple design efforts and choosing an inferior starting layout. It is indeed entirely possible to start with forms that cannot be improved due to higher-order aberrations.3 Figure 1. Different designs and third-order spherical aberration by surface for a high-numerical-aperture data-storage objective lens. The highlighted surfaces have noticeably large amounts of spherical aberration and are more sensitive to fabrication tolerances.

Thirty different views of a lens design solution space: a good example for teaching students how to design and not to design a lens

The design of high numerical aperture objective to be used as an optical data storage device was given out as a midterm exam in a lens design class consisting of twenty-nine students. The design space was restricted by the following constraints: the number of elements could not exceed three, only three material types were available, and the maximum overall length and the minimum working distance requirements were given. The students were allowed to pick their own starting point and were graded on both the maximum resolution they could achieve and the manufacturability of the design. The results were compiled into a design study. Although a few of the designs violated the specifications, the remainder could be grouped into three distinct design forms. The three different design forms were then analyzed for their ability to stretch the specifications (ex. working distance or resolution) while discussing the difference in tolerances between the design forms. The designs were also used to highlight the importance of controlling edge thicknesses and center thicknesses during the design process.

Optical design study of a VIS-SWIR 3X zoom lens

A design study is compiled for a VIS-SWIR dual band 3X zoom lens. The initial first order design study investigated zoom motion, power in each lens group, and aperture stop location. All designs were constrained to have both the first and last lens groups fixed, with two middle moving groups. The first order solutions were filtered based on zoom motion, performance, and size constraints, and were then modified to thick lens solutions for the SWIR spectrum. Successful solutions in the SWIR were next extended to the VIS-SWIR. The resulting nine solutions are all nearly diffraction limited using either PNNP or PNPZ (“Z” indicating the fourth group has a near-zero power) design forms with two moving groups. Solutions were found with the aperture stop in each of the four lens groups. Fixed f-number solutions exist when the aperture stop is located at the first and last lens groups, while varying f-number solutions occur when it is placed at either of the middle moving groups. Design exploration included trade-offs between parameters such as diameter, overall length, back focal length, number of elements, materials, and performance.

Design study for a 16x zoom lens system for visible surveillance camera

*avella@ur.rochester.edu Design study for a 16x zoom lens system for visible surveillance camera Anthony Vella*, Heng Li, Yang Zhao, Isaac Trumper, Gustavo A. Gandara-Montano, Di Xu, Daniel K. Nikolov, Changchen Chen, Nicolas S. Brown, Andres Guevara-Torres, Hae Won Jung, Jacob Reimers, Julie Bentley The Institute of Optics, University of Rochester, Wilmot Building, 275 Hutchison Rd, Rochester, NY, USA 14627-0186 ABSTRACT High zoom ratio zoom lenses have extensive applications in broadcasting, cinema, and surveillance. Here, we present a design study on a 16x zoom lens with 4 groups (including two internal moving groups), designed for, but not limited to, a visible spectrum surveillance camera. Fifteen different solutions were discovered with nearly diffraction limited performance, using PNPX or PNNP design forms with the stop located in either the third or fourth group. Some interesting patterns and trends in the summarized results include the following: (a) in designs with such a large zoom ratio, the potential of locating the aperture stop in the front half of the system is limited, with ray height variations through zoom necessitating a very large lens diameter; (b) in many cases, the lens zoom motion has significant freedom to vary due to near zero total power in the middle two groups; and (c) we discuss the trade-offs between zoom configuration, stop location, packaging factors, and zoom group aberration sensitivity.

Using in-flight images to model the centroid distortion in the LOng-Range Reconnaissance Imager (LORRI)

The purpose of this investigation is to understand the causes and implications of centroid distortion in the LORRI instrument on the New Horizons spacecraft. First, an introduction to the New Horizons space program is provided, and the design specifications of the LORRI telescope are discussed. Next, a general theory of perfect imaging is presented, with emphasis on the paraxial equations for the transfer of the chief ray, and the shape of the diffraction-limited point spread function (PSF). Centroid distortion is then defined with respect to these quantities, and methods for quantifying centroid distortion are explained. The nominal LORRI design is analyzed, and the centroid distortion predicted by the nominal design is shown. Astrometric reduction is then performed on a set of twenty in-flight LORRI images, and the actual centroid distortion of the telescope is modeled. Finally, Monte Carlo tolerancing techniques are used to attribute the differences between the predicted centroid distortion and the in-flight centroid distortion to a set of specific manufacturing tolerances.

Design Forms and Pupil Mangement for a High Resolution, Long Working Distance Zooming Microscope

A high resolution, long working distance zooming microscope of two design forms is compared: 1. a zoomed afocal system is paired with a fixed focal length objective and a fixed focal length tube lens and 2. an objective is zoomed and paired with a fixed focal length tube lens.

Optimal power distribution for minimizing pupil walk in a 7.5X afocal zoom lens

An extensive design study was conducted to find the optimal power distribution and stop location for a 7.5x afocal zoom that controls the pupil walk through zoom such that the lens can be coupled to a high-resolution microscope objective and tube lens with minimal vignetting and performance loss.

Chromatic correction for a VIS-SWIR zoom lens using optical glasses

With the advancement in sensors, hyperspectral imaging in short wave infrared (SWIR 0.9 μm to 1.7 μm) now has wide applications, including night vision, haze-penetrating imaging, etc. Most conventional optical glasses can be material candidates for designing in the SWIR as they transmit up to 2.2 μm. However, since SWIR is in the middle of the glasses’ major absorption wavebands in UV and IR, the flint glasses in SWIR are less dispersive than in the visible spectrum. As a result, the glass map in the SWIR is highly compressed, with crowns and flints all clustering together. Thus correcting for chromatic aberration is more challenging in the SWIR, since the Abbé number ratio of the same glass combination is reduced. Conventionally, fluorides, such as CaF2 and BaF2, are widely used in designing SWIR system due to their unique dispersion properties, even though they are notorious for poor manufacturability or even high toxicity. For lens elements in a zoom system, the ray bundle samples different sections of the each lens aperture as the lens zooms. This creates extra uncertainty in correcting chromatic aberrations. This paper focuses on using only commercially available optical glasses to color-correct a 3X dual-band zoom lens system in the VIS-SWIR. The design tools and techniques are detailed in terms of material selections to minimize the chromatic aberrations in such a large spectrum band and all zoom positions. Examples are discussed for designs with different aperture stop locations, which considerably affect the material choices.

New tools for finding first-order zoom lens solutions and the analysis of zoom lenses during the design process

We developed software design tools in MATLAB that are compatible with Code V for supporting the process of designing zoom lenses. These tools simplify the process of finding paraxial solutions and evaluating intermediary design steps. Paraxial solutions are found through a partially random search for four group zoom systems with moving second and third groups. It requires several user-specified system parameters and then randomly assigns powers to each group. This process of randomly assigning powers is done a set number of times and only the valid solutions where no lenses crash are considered for further use. The valid designs are plotted over different design criteria and can then be selected to retrieve the first order design parameters. For the intermediate design process, the software displays lens specifications and diagnostic results across zoom for the entire lens as well as the individual groups. Systematic evaluation of the intermediate design steps is useful in determining how to proceed and improve the design. The design process is described for two different zoom lenses to show the efficiency and utility of these tools. The two zoom lenses are a 16x surveillance camera zoom lens working in the visible and a 3X zoom lens working in the visible and short wave infrared. The design procedure for these lenses covers finding the paraxial solutions to evaluating the lens for further improvement.

Extreme retrofocus zoom lens for single-shot single-lens HDR photography and video

Traditional high dynamic range (HDR) photography is performed by capturing multiple images of the same scene with different exposure times, which are then digitally combined to produce an image with great detail in both its light and dark areas. However, this method is not viable for moving subjects since the multiple exposures are not captured simultaneously. Recently an alternative method has been developed in which beamsplitters are utilized to simultaneously record the same image on three identical sensors at different illumination levels. This process enables single-shot HDR photography as well as continuous HDR video. This paper describes the design of a 2.5x zoom lens for use in this application. The design satisfies the challenging working distance and ray angle constraints imposed by the placement of two beamsplitters between the lens and the image plane. The particular importance of first-order layout when designing a retrofocus zoom lens is also discussed.