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optical systems design with zemax


Optical Systems Design with Zemax
Lecture 1

Why Optical Systems Design
Optical system design is no longer a skill reserved for a few professionals. With readily availabl

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

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Local Experts
??Robert Content ??Jurgen Schmoll ?? Stephen Rolt ??Colin Dunlop ??Tim Morris ??Andrew Kirby

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Course Outline
?? Lecture 1: Introduction ?? Lecture 2: Sequential Systems ?? Lecture 3: Optimization ?? Lecture 4: Tolerancing ?? Lecture 5: Non-sequential & other stuff

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Objectives: Lecture 1
At the end of this lecture you should:
1.? 2.? 3.? 4.?

5.?

Be able to install a version of the Zemax optical design programme on a Windows PC Understand the main tasks involved in optical systems design Be aware of the 5 main Seidel aberrations Know the definitions of the terms: optical axis, stop, pupil, chief ray, marginal ray, point spread function Use the Zemax lens data editor to enter the specifications of a simple lens

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Getting started
?? Download a copy of Zemax from

www.zemax.com/downloads ?? CfAI members can use the shared license server on elservidor. This requires a copy of the file sntlconfig.xml from the server Exchange/installers/Zemax to be copied into the main Zemax directory (C:\Program Files \Zemax) ?? Five licences are available. See who is using them at elservidor.phyaig.dur.ac.uk:7002 ?? Non-CfAI members should use the Zemax demo
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Recommended Texts
?? Zemax manual ?? 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 www.zemax.com/kb
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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).
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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. ?? Numerical evaluation methods ray trace many light rays from object to image space.
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Seidel (3rd order) Aberrations
1.? Spherical aberration 2.? Coma 3.? Astigmatism 4.? Field curvature 5.? Distortion 6.? Longitudinal chromatic aberration 7.? Lateral chromatic aberration
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Numerical Evaluation Methods
?? Assume only trigonometry, law of reflection and

Snell’s law

?? ?? 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 pre-defined 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).
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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.
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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 physicals 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)
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Objects, Light Rays & Wavefronts
Optical axis Wavefronts

Object Plane Ray bundles

Image Plane

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The Optical Axis
?? Most optical systems are collections of

rotationally symmetric surfaces whose centres of curvature are all located along a common optical 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.
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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 focal length (for object at ∞) to entrance pupil diameter (EPD)
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Stops & Pupils

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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).
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Marginal & Chief Rays

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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.
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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 initial moves in the direction of +Z. ??Co-ordinate breaks (rotations) are defined in a right-handed sense.
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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).
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Surface Parameters
?? Surface number ?? Radius of curvature ?? Thickness to the next surface ?? Glass type in the next medium ?? Aspheric data (if any) ?? Aperture size (semi-diameter) ?? Tilt and decenter data (if any)

One surface is designated the stop surface.
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Using the Lens Data Editor
??Gen button: define entrance aperture ??Fie button: define field angles (FoV) ??Wav button: define wavelengths ??Singlet lens prescription:
R1 = 100 mm, t1 = 10 mm, Glass = BK7, Semi-D1 = 25 mm R2 = -100 mm, t2 = Quick-focus, Air, Semi-D2 = 25 mm An aperture stop (entrance pupil) is placed at the 1st lens surface (D = 40 mm).

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ZEMAX Lens Data Editor

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First Order Properties

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Summary: Lecture 1
?? Optical design has changed radically

since the introduction of modern raytracing 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, ray-tracing an optical layout and varying parameters of the specification to improve performance
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Exercises: Lecture 1
??Install Zemax (or the Zemax demo)

on your PC ??Use the lens data editor to input the optical prescription of the biconvex singlet from the lectures ??Investigate how the focus depends on wavelength and lens curvatures ??Investigate how the image quality depends on the thickness of the lens
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