Other Stuff

Other Stuff
Lecture 5
Objectives: Lecture 5
At the end of this lecture you should:
1.  Be aware of the Zemax capability to
approximate a lens design with catalogue
components
2.  Be familiar with the use of co-ordinate breaks in
Zemax to model off-axis systems
3.  Understand the use of non-sequential ray-tracing
to model scattered light
4.  Appreciate the capabilities of Zemax to model
physical optics wave propagation
5.  Be able to use Zemax to model the performance
of imaging systems using realistic images
March 17, 2015
Optical Systems Design
2
COTS Lens Substitution
• 
• 
• 
• 
• 
• 
• 
Zemax can take a custom design and substitute real
lenses
As an example start from paraxial lens model (DOUBLETELECENTRIC-PARAXIAL-LECT5.ZMX)
Select Libraries -> Lens Catalogue
Use Vendor(s) drop-down menu to search standard
manufacturers catalogues
Search on lens type, EFL, pupil size
Select best match and Insert (delete paraxial surface)
(DOUBLE-TELECENTRIC-EDMUNDOPTICS-LECT5.ZMX)
May need to reverse some lens elements to improve
performance, since convex surface of doublets always
optimised for ∞ conjugate (there is a convenient icon
above the lens data streadsheet to do this)
17/03/2015
Optical Systems Design
3
Co-ordinate Breaks
•  Non-axially symmetric systems where surfaces are
tilted or decentered require the use of co-ordinate
breaks
•  Rotate/shift local co-ordinate frame
•  Positive rotation (in ZEMAX) is clockwise as viewed
along +ve axis direction
•  Subsequent co-ordinate breaks refer to the newly
defined axis orientations
•  If a co-ordinate break is placed immediately before
an optical surface, it can be useful to put another
one with opposite sign immediately after, thus
undoing the tilt etc
•  There are now simple tools in the Lens Data icon bar
to tilt/decentre surfaces and add fold mirrors
March 17, 2015
Optical Systems Design
4
Nasmyth Field Derotator
FIELDROTATOR-LECT5.ZMX
March 17, 2015
Optical Systems Design
5
Non-Sequential Systems
•  No predefined sequence of surfaces
•  Objects encountered determined solely by physical
positions of surfaces and directions of rays
•  Co-ordinate system is global
•  Can deal with Total Internal Reflection (TIR), stray
light and illumination systems
•  Required for prisms, beamsplitters, light pipes,
faceted (array) objects etc
•  In some cases need mixed sequential/nonsequential ray tracing
March 17, 2015
Optical Systems Design
6
Non-Sequential Systems
Sequential
Non-Sequential
[PRISM-SEQ-LECT5.ZMX]
[PRISM-NONSEQ-LECT5.ZMX]
March 17, 2015
Optical Systems Design
7
Non-Sequential Systems
PENTAPRISM-NONSEQ-LECT5.ZMX
Can convert from sequential design using Tools -> Miscellaneous -> Convert to
NSC Group (need to first move STOP to front surface)
March 17, 2015
Optical Systems Design
8
Physical Optics Propagation
•  Geometrical ray tracing is an incomplete
description of light propagation
•  POP uses diffraction calculations to propagate a
light modelled as a wavefront through an optical
system
•  Wavefront is modeled by an array of complex
amplitudes which is user-definable in terms of its
dimension, sampling and aspect ratio
•  Applications include fibre coupling, diffraction by
apertures and beam irradiance calculations
March 17, 2015
Optical Systems Design
9
Gibbs Phenomenon
March 17, 2015
GIBBS-LECT5.ZMX
Optical Systems Design
10
Fibre Coupling
March 17, 2015
FIBRE-LECT5.ZMX
Optical Systems Design
11
Array Elements
•  Rectangular array
of spherical lenses
•  Modelled as a userdefined surface
(DLL)
•  LENSLET-LECT5.ZMX
March 17, 2015
Optical Systems Design
12
Image Simulation
•  For an optical designer the lens
performance is specified in terms of spot
diagrams, ray-fan plots, vignetting, field
curvature, astigmatism etc
•  In some cases its much more effective to
demonstrate what images will look like when
viewed through the lens
•  Zemax now has a nice feature called Image
Simulation to demonstrate this on an input
image
March 17, 2015
Optical Systems Design
13
Image Simulation
•  Object scene is represented by a source bitmap
(.BMP or .JPG)
•  Rays traced using the defined object through
the lens to the image plane
•  At detection surface place a pixellated detector
which receives the rays and builds up an image
of the source bitmap as seen through the lens
March 17, 2015
Optical Systems Design
14
Design for Fabrication
•  Primary considerations: optical
material, component size, shape, and
manufacturing tolerances
•  Minimize cost and delivery time by
using COTS items whenever possible
•  Minimize risk through prototyping and
pre-production models
March 17, 2015
Optical Systems Design
15
Optical Materials
•  Over 100 optical glasses available worldwide
•  Each manufacturer has a list of “preferred” glasses that are
most frequently melted and usually available from stock
•  Generally can substitute similar glasses from different
manufacturers (and re-optimise)
•  Material quality defined by tolerances on spectral transmission,
index of refraction, dispersion, striae grades (AA/A/B),
homogeneity (H1-H4), and birefringence (NSK/NSSK)
•  Tighter than standard optical tolerances require additional
cost and time
•  May be more economical to add a lens to the design in order
to avoid expensive glasses
•  Some glasses (e.g. SF-59) made much less frequently than
others (e.g. BK-7)
March 17, 2015
Optical Systems Design
16
Fabrication
•  Mechanical properties: hardness &
abrasion resistance (manufacture)
•  Chemical properties: resistance to
humidity, acids, alkalis
•  Thermal properties: expansion
coefficients from 4 -16 x 10-6/°K.
March 17, 2015
Optical Systems Design
17
Some Other Zemax Examples
ZMX/SES files on course website:
•  Cassegrain Telescope (WHT)
•  Ritchey Chretien Telescope (AAT)
•  Off-axis parabola
•  Melles Griot ball lens
•  Shack-Hartmann wavefront sensor
•  Palomar triple spectrographs
March 17, 2015
Optical Systems Design
18
Summary: Lecture 5
•  Co-ordinate breaks allow Zemax to model
arbitrarily complex off-axis systems in a local coordinate system
•  Need care in use to avoid over-complication
•  Non-sequential mode allows complex objects to
be defined using a global co-ordinate system
•  Can also be used to model scattered light and
illumination systems
•  Physical optics propagation in Zemax includes the
effects of diffraction
•  Fabrication issues need to be thought about early
in the instrument design phase
March 17, 2015
Optical Systems Design
19
Exercises: Lecture 5
•  Work your way through some
of the example Zemax files,
evaluating their performance
and making sure that you
understand the prescription
data.
March 17, 2015
Optical Systems Design
20
Homework Problem
•  Design a very simple telephoto lens with the following
first-order properties:
Effec&ve focal length = 120 mm Back focal distance = 50 mm Image space f/# = 10 Lens separa&on = 40 mm Field angles = 0°, 2°, 3° Wavelength = 0.587 µm •  Design goal: maintain all first-order properties and
achieve rms spot sizes ≤ 20 µm. Start from two paraxial
lenses with focal lengths 75mm and -75mm.
•  Final solutions should include a layout diagram, spot
diagram and system prescription data (also email the
Zemax file).
•  Hand in the solutions to my pigeon hole by Friday 17th
April.
17/03/2015
Optical Systems Design
21