Richard N. Youngworth

An overview of power spectral density (PSD) calculations

Specifications for optical surfaces have traditionally been given in terms of low frequency and high frequency components, often with a separate classification for surface slope. Low spatial frequency components are commonly referred to as figure errors and can be described by the standard 37-term Zernike polynomial set. High spatial frequency errors are commonly referred to as finish and are quantified using rms roughness. Specification with the qualitative scratch and dig classification is done usually for cosmetic or aesthetic purposes. Mid-spatial frequency errors such as waviness, ripple, and quilting can be important and are not explicitly covered by such traditional figure and finish specifications. In order to bridge the gap to cover mid-spatial frequencies, in terms of quantifying surface characteristics, Power Spectral Density (PSD) can be utilized. For such usage, it is important for the greater optics community to understand the metric, how to calculate it, and how to use it. The purpose of this paper is to provide an overview of PSD, its application in optics, and an outline of calculations needed to effectively apply it to specify optical surfaces.

Fundamental considerations for zoom lens design

Zoom lens design requires a very strong understanding of geometrical optics and how it directly relates to an optical system. Understanding both first-order optics and pupil conjugation is absolutely essential to ensure that a lens zooms correctly and avoids discontinuities. This tutorial paper explains these first-order considerations in detail and illustrates how to derive a starting configuration. The tutorial also shows how to proceed toward a final lens optimization.

Alignment analysis of optical systems using derivative information

A key facet in taking an optical design from concept to an as-built system (or set of systems) is proper planning for alignment. Performing detailed analysis and investigating options for alignment are both imperative, especially in light of the role cost and performance typically have on success. In this paper we specifically discuss using derivative information in engineering an optical system for alignment. An example illustrates the flexible and powerful nature of such calculations in solving practical problems encountered in optical system design, development, and manufacturing.

Implementing ISO standard-compliant freeform component drawings

Abstract. Successful fabrication of aspheres requires all parts of the process chain including design, production, and measurements. Aspheres now are well-established and accepted as an equal optical element, when done properly. Research and industry have now started to focus efforts to develop the next element that propels the field forward in capability, namely the optical freeform surface. An essential factor enabling wide use of freeforms is communicating requirements. This paper discusses form description and tolerancing additions to ISO 10110 to accommodate freeform surfaces. Information stating how ISO 10110 and related standards documents such as ISO 14999-4 are being continually developed to meet the requirements for specifying freeform surfaces is also provided. This paper further provides an example monolithic freeform element using the recently updated relevant parts of ISO 10110. The first manufacturing of this component has been successful, and this paper shows the role the ISO standard has played in success. Definitions for toleranced parameters, such as surface registration (centration) and form deviation (irregularity, slope, Zernike, PV, and PVr), are also indicated. The monolithic example also shows how to use the defined data and definitions for metrology and data handling. Metrology results for the freeform surface are given.

Statistical truths of tolerance assignment in optical design

The process of assigning tolerances to optical designs is intrinsically statistical, regardless of volume. Papers covering the statistics of tolerancing, however, have been infrequent. This paper will discuss the statistical nature of tolerancing including ramifications that all optical designers and engineers should understand.

In the era of global optimization, the understanding of aberrations remains the key to designing superior optical systems

Historically, a thorough grounding in aberration theory was the only path to successful lens design, both for developing starting layouts and for design improvement. Modern global optimizers, however, allow the lens designer to easily generate multiple solutions to a single design problem without understanding the crucial importance of aberrations and how they determine the full design potential. Compared to pure numerical optimization, aberration theory applied during the lens design process gives the designer a much firmer grasp of the overall design limitations and possibilities. Among other benefits, aberrations provide excellent insight into tolerance sensitivity and manufacturability of the underlying design form. We explore multiple examples of how applying aberration theory to lens design can improve the entire lens design process. Example systems include simple UV, visible, and IR refractive lenses; much more complicated refractive systems requiring field curvature balance; and broadband zoom lenses.

The Optical Telescope Element Simulator for the James Webb Space Telescope

The James Webb Space Telescope Observatory will consist of three flight elements: (1) the Optical Telescope Element (OTE), (2) the Integrated Science Instrument Module Element (ISIM), and (3) the Spacecraft Element. The ISIM element consists of a composite bench structure that uses kinematic mounts to interface to each of the optical benches of the three science instruments and the guider. The ISIM is also kinematically mounted to the telescope primary mirror structure. An enclosure surrounds the ISIM structure, isolates the ISIM region thermally from the other thermal regions of the Observatory, and serves as a radiator for the science instruments and guider. Cryogenic optical testing of the ISIM Structure and the Science Instruments will be conducted at Goddard Space Flight Center using an optical telescope simulator that is being developed by a team from Ball Aerospace and Goddard Space Flight Center, and other local contractors. This simulator will be used to verify the performance of the ISIM element before delivery to the Northup Grumman team for integration with the OTE. In this paper, we describe the O OTE Sim TE Simulator (OSIM) and provide a brief overview of the optical test program. ulator

Efficient Assessment of Lens Manufacturability in Optical Design

One of the key challenges confronting optical engineers is efficient design form comparison, specifically evaluating cost-effective manufacturability. Traditional methods involve aberration balancing and assessing ray bending to determine the most relaxed design form. Such methods can be effective for experts. However, they only indirectly assess cost, are difficult to explain to non-optical engineers, do not directly relate to tolerances, and do not make any connection to the inherent challenges of holding a set of tolerances. The most desirable means of assessing manufacturability, especially during the early design phase should be efficient, simple to use and understand, and provide capability to directly assess error impact and relative cost. There are a number of ways to approach this challenge. Quite notably, this paper shows that a tolerance grade mapping system is particularly useful due to the balance it brings between its ease of use, flexibility, and detailed relation to cost. Two lens design examples are included that illustrate the method and its ease of use.

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.

Freeform capability enabled by ISO 10110

In the last 10 years aspheres have readily gone from new products and specialized components to wide acceptance in the market. Successful fabrication of aspheres requires all parts of the process chain including design, production, and measurements. Aspheres now are well-established and accepted as an equal optical element, when done properly. This segment has been the fastest growing market of all optical elements. Research and industry have now started to focus efforts to develop the next new element that propels the field forward in capability, namely the optical freeform surface. An essential factor enabling wide use of freeforms is communicating requirements. This manuscript provides an example monolithic freeform element using the recently updated relevant parts of ISO 10110. The first manufacturing of this component has been successful, and this manuscript shows the role the ISO standard has played in success. Specifically the description of the complex freeform element, as well as definitions for toleranced parameters such as surface registration (centration) and form deviation (irregularity, slope, Zernike, pv, and pvr), are indicated. The provided example also shows how to use the defined datums and definitions for metrology and data handling.