Optical Systems Design with Zemax OpticStudio. Lecture 1

Optical Systems Design with Zemax OpticStudio Lecture 1

Why Optical Systems Design Optical system design is no longer a skill reserved for a few professionals. With readily available commercial optical design software, these tools are accessible to the general optical engineering community and rudimentary skills in optical design are now expected by a wide range of industries who utilize optics in their products. Optical Systems Design 2

Course Aims To introduce the design principles of lens and mirror optical systems and the evaluation of designs using modern computer techniques. The lectures will cover lens design, aberrations, optimization, tolerancing and image quality metrics. Optical Systems Design 3

ZEMAX Optics Studio The ZEMAX optical design program is a comprehensive software tool. It integrates all the features required to conceptualize, design, optimize, analyze, tolerance, and document virtually any optical system. It is widely used in the optics industry as a standard design tool. This course will introduce the basics of ZEMAX using the recently released (2014) OpticStudio interface. Optical Systems Design 4

Other Optical Design Software Code-V (Optical Research Associates) OSLO (Sinclair Optics) OpTaliX (Optenso Ltd) ASAP (Breault Research) TracePro (Lambda Research) FRED (Photon Engineering) Optical Systems Design 5

Local Experts Stephen Rolt Jurgen Schmoll Ariadna Calcines Colin Dunlop Tim Morris Undoubtedly others Optical Systems Design 6

Course Outline Lecture 1: Introduction Lecture 2: Sequential Systems Lecture 3: Optimization Lecture 4: Tolerancing Lecture 5: Non-sequential & other stuff Web page: http://astro.dur.ac.uk/~rsharp/opticaldesign.html Optical Systems Design 7

Objectives: Lecture 1 At the end of this lecture you should: 1. Be able to install a version of the Zemax optical design programme on a Windows PC 2. Understand the main tasks involved in optical systems design with Zemax 3. Be aware of Zemax notation for the 5 main Seidel aberrations 4. Know the relevance of the terms: optical axis, stop, pupil, chief ray, marginal ray, point spread function for Zemax 5. Use the Zemax lens data editor to enter the specifications of a simple lens Optical Systems Design 8

Getting started Download a current copy of OpticStudio from: http://www.zemax.com/support/downloads/ CfAI/Atmol members can use the shared license server on zemax.cfai.local. This requires a copy of the file sntlconfig.xml from the server Exchange/installers/ Zemax to be copied into the main OpticsStudio directory (C:\Program Files\Zemax OpticStudio) Five licences are available. See who is using them at: http://zemax.cfai.local:7002 Log out from Zemax if not actively using! Non-CfAI/Atmol members should download the OpticStudio demo Optical Systems Design 9

Recommended Texts OpticStudio User Manual and Getting Started Using OpticStudio (access from programme help) Introduction to Lens Design with Practical Zemax Examples, Joseph M Geary (Willmann-Bell Inc.) Optical Systems Design, Robert Fischer & Bijana Tadic(SPIE Press) Practical Computer-Aided Design, Gregory Hallock- Smith (Willmann-Bell Inc.) Astronomical Optics, Dan Schroeder (Academic Press; GoogleBooks) Optics, Jeff Hecht (Addison Wesley) Also the Zemax knowledge base: http://www.zemax.com/support/knowledgebase Optical Systems Design 10

Optical Systems Design Science or art of developing optical systems to image, direct, analyse or measure light. Includes camera lenses, telescopes, microscopes, scanners, photometers, spectrographs, interferometers, Systems should be as free from geometrical optical errors (aberrations) as possible. Correcting and controlling aberrations is one of the main tasks of the optical designer (includes performance evaluation and fabrication/tolerancing issues). Optical Systems Design 11

Historical Note Lens design has changed significantly since ~1960 with the introduction of digital computers and numerical optimisation. Equations describing aberrations of lens/mirror systems are very non-linear functions of system parameters (curvatures, spacings, refractive indices, dispersions, ) Only a few specialised systems can be derived analytically in exact closed-form solutions. Analytical design methods (Petzval, Seidel) were historically based on a mathematical treatment of geometrical imagery and primary aberrations still useful for initial designs. Numerical evaluation methods ray trace many light rays from object to image space. Optical Systems Design 12

Seidel (3 rd order) Aberrations 1. Spherical aberration 2. Coma 3. Astigmatism 4. Field curvature 5. Distortion 6. Longitudinal chromatic aberration 7. Lateral chromatic aberration Optical Systems Design 13

Numerical Evaluation Methods Assume only trigonometry, law of reflection and Snell s law n 1 sinθ 1 = n 2 sinθ 2 For each ray calculate new ray parameters at each surface Sequential ray-tracing assumes that light travels from surface to surface in a defined order. Non-sequential ray-tracing does not assume a predefined path for the rays, but when a ray hits a surface in its path, it may then reflect, refract, diffract, scatter or split into child rays (scattered light). Optical Systems Design 14

Numerical Optimisation Methods Given a starting configuration, the computer can be used to optimise a design by an iterative process. Final image quality is best that can be achieved under constraints of basic configuration, required focal length, f/ number, field of view, wavelength etc. Programs are still dumb. Designer must supply intelligence through selection of starting configuration, control of optimization parameters, understanding of underlying optical theory, etc. Optical Systems Design 15

Objects, Light Rays & Wavefronts Objects composed of self-luminous (radiant) points of light Trajectories of photons from each of these points define the light rays Neglecting diffraction, these physical rays become geometrical rays (ray bundles) Wavefronts are surfaces normal to rays Light travel times along all rays to the wavefront from an object point are the same (for a fixed wavelength) Neglecting diffraction, physical wavefronts become geometrical wavefronts (good approximation except near boundaries or edges) Optical Systems Design 16

Objects, Light Rays & Wavefronts Optical axis Wavefronts Object Plane Ray bundles Image Plane Optical Systems Design 17

The Optical Axis Most optical systems are collections of rotationally symmetric surfaces whose centres of curvature are all located along a common axis (Optical Axis) Plane surfaces have infinite radius of curvature Intersection of the optical axis and a surface is at the surface vertex Longitudinal cross-section defines a meridional plane (all equivalent) Ray in this plane are meridional rays. Rays out of plane are skew rays. Optical Systems Design 18

Stops & Pupils Every optical system contains one physical aperture that limits the extent of the wavefront for the ray bundle which is transmitted through the system to the on-axis image point (aperture stop or stop) If optics are large enough then this will also be true for off-axis image points In many cases this is not true leading to mechanical vignetting of off-axis image points Size and location of the aperture stop can have important impact on system performance through its effects on geometrical aberrations Image of the stop in object space is the entrance pupil. Image of the stop in image space is the exit pupil. Focal ratio (e.g. f/5.6) is ratio of effective focal length (EFL) to entrance pupil diameter (EPD) Optical Systems Design 19

Stops & Pupils Optical Design (S13) Joseph A. Shaw Montana State University Demonstration of Pupils with a Camera Lens View a camera lens from the front and from the back to see the entrance and exit pupils. You are seeing the same iris from both sides, but it appears to be of different diameter because of the intervening optics. Entrance pupil Exit pupil Optical Systems Design 4 20

Marginal & Chief Rays Marginal ray originates at the object point on axis and goes to the edge of the stop of the system. Chief ray (principal ray) originates at the object point at the edge of the field of view and passes through the centre of the stop of the system. Axial height (transverse distance away from the optical axis) of the marginal ray is zero at the object and all images of the object. At these locations the axial height of the chief ray determines the size (semi-diameter) of the object and its images (magnification). These roles are reversed when considering the aperture stop and its images (pupils). Optical Systems Design 21

Marginal & Chief Rays Optical Systems Design 22

Point Spread Function (PSF) Impossible to image a point object as a perfect point image. PSF gives the physically correct light distribution in the image plane including the effects of aberrations and diffraction. Errors are introduced by design (geometrical aberrations), optical and mechanical fabrication & alignment. Optical Systems Design 23

Co-ordinate Systems and Sign Conventions No standardization between different codes! Zemax uses a right-handed cartesian co-ordinate system, where the Z-axis is the optical axis and light initially moves in the direction of +Z. Co-ordinate breaks (rotations) are defined in a right-handed sense. Optical Systems Design 24

Optical Prescriptions An optical design is described by a set of surfaces through which the light passes sequentially. Surfaces are tabulated in the lens data editor and are numbered sequentially from the object surface (surface 0) and ending with the image surface. A minimum of 3 surfaces is required (object, stop, image). Optical Systems Design 25

Surface Parameters Surface number Radius of curvature (R) Thickness to the next surface (t) Glass type in the next medium (or Air if blank) Aspheric data (if any) Aperture size (semi-diameter D) Tilt and decenter data (if any) One surface is designated the stop surface. Optical Systems Design 26

Using the Lens Data Editor Setup tab -> System Explorer: Aperture: define entrance pupil diameter (50mm) Fields: define field angle(s) (FoV) (0 deg) Wavelengths: define wavelength(s) of rays (632.8nm) Singlet lens prescription: R1 = 100 mm, t1 = 10 mm, Glass = BK7, Semi- D1 = 25 mm R2 = - 100 mm, t2 = Op9mize- >Quick- focus, Air, Semi- D2 = 25 mm The aperture stop (entrance pupil) is placed at the first lens surface (Diam = 50 mm). May also some*mes use dummy surfaces to help with plots. Optical Systems Design 27

ZEMAX Lens Data Editor Optical Systems Design 28

ZEMAX System Viewers Optical Systems Design 29

System Properties Analyze - > Reports: Optical Systems Design 30

Summary: Lecture 1 Optical design has changed radically since the introduction of modern ray-tracing software packages ZEMAX is a comprehensive software tool which integrates all the features required to design an optical system The optical design process involves developing a conceptual optial design, raytracing an optical layout and varying parameters of the specification to improve performance Optical Systems Design 31

Exercises: Lecture 1 Install Zemax Optic Studio(or the OpticStudio demo) on your PC Use the lens data editor to input the optical prescription of the biconvex singlet from the lecture Investigate how the focus depends on wavelength and lens curvatures Investigate how the image quality depends on the thickness of the lens Optical Systems Design 32