How to design a singlet lens, Part 2: Analysis

This series of three articles is intended as an introduction to new users on interfacing with OpticStudio Sequential Mode. Using a singlet lens as an example, the articles walks you through the basic process of designing a lens, including building the system (Part 1), analyzing its performance (Part 2), and optimizing it for the required prescription and design constraints (Part 3).

Authored By Dan Hill

Introduction

This is Part 2 of a series of three articles. It introduces the Layout, to show a visual representation of the system, and then focuses on the Spot Diagram, OPD Fan and Ray Fan analyses, to evaluate the system aberrations. It also explains how the Quick Focus tool can be used to better position the image plane location.

In Part 1, we discuss how to set up the system using the System Explorer and the Lens Data Editor. In Part 3, we discuss how to optimize the singlet to achieve a better performance within the design constraints.

Evaluating system performance prior to optimization

There are many different analysis features included in OpticStudio, each of which can be used to evaluate the performance of a design. In this exercise, we will use four of the more fundamental, commonly known types of analyses of system performance to evaluate the singlet prior to optimization:

Analysis Description
Layout A layout may be opened by navigating to Analyze...System Viewer...Cross-Section. The Cross-Section option plots a YZ cross section through the lens, and is only valid for rotationally symmetric, axial systems. A layout diagram is always a useful visual representation of the current optical system.
Spot Diagram A spot diagram may be accessed by navigating to Analyze...Spot Image Quality...Rays & Spots...Standard Spot Diagram. The spot diagram gives indication of the image of a point object. In the absence of aberrations, a point object will converge to a perfect image point. By default, OpticStudio plots the spot diagram for each field point.
OPD Fan The Optical Path Difference (OPD) fan can be opened by selecting Analyze...Image Quality...Aberrations...Optical Path. The OPD fan is a plot of the optical path difference as a function of pupil coordinate. In a perfect optical system, the optical path of the wavefront will be identical to that of an aberration-free spherical wavefront in the exit pupil.
Ray Fan The Ray Fan plot in OpticStudio may be opened by selecting Analyze...Image Quality...Aberrations...Rays Aberration. The Ray Fan plots ray aberrations as a function of pupil coordinate. Generally, a given ray which passes through the optical system an onto the image surface, its point of intersection falls on some small but nonzero distance away from the chief ray. Once again, in a perfect optical system, the ray aberrations should be zero across the pupil.

 

The Spot Diagram, OPD Fan, and Ray Fan Plot are some of the most important tools that a lens designer has available for determining the different types and magnitudes of aberrations present in an optical system. However, the process by which a designer can determine the aberrations present in their design from these analysis features goes beyond the scope of this exercise. 

Given the arrangement of the design, we have constructed so far, open each one of the above analysis features to review the current lens performance.

InitialLayout     InitialSpotDiagram    

InitialRayFan     InitialOPD    

From evaluation of the four plots above, it is obvious that the singlet design has a significant number of aberrations, including but not limited to spherical, coma, distortion, defocus, field curvature, and astigmatism. In addition, the geometrical and RMS radii (as is reported at the bottom of the Spot Diagram) at the maximum field are roughly 734.581 and 1774.42 μm, respectively:

SpotRadiusVals1

Using the Quick Focus tool

As could be seen from the four analysis features, the performance of the singlet at this point is certainly not optimal. A big factor in this fairly poor performance was the random selection of the location of the image plane. Even from the Layout above, it is obvious that the currently selected image plane is not at the "best focus."

Even prior to optimization, we can use a tool within OpticStudio to better position the current image plane location. The tool is known as the Quick Focus tool. Quick Focus is a feature in OpticStudio which adjusts the thickness of the surface prior to the image plane to minimize the RMS aberrations.

Open the Quick Focus dialog by selecting Optimize...Manual Adjustment...Quick Focus or by pressing <Shift + Ctrl + Q> on the keyboard. The targeted "best focus" will depend on the criterion selected. For the singlet design, we will use radial spot size with respect to the centroid. Select Spot Size Radial and place a check next to the Use Centroid box.

QuickFocus

Click OK to run and close the Quick Focus tool dialog.

Note the automatic change to the thickness of the surface prior to the image plane. The analysis windows will automatically update by default, but if they do not, you can update each of them individually by clicking Update in the upper-left hand corner the window. You should see fairly significant changes in performance simply due to the new selection of the image plane location. Most importantly, the RMS and Geometrical spot sizes at the maximum field decreased by nearly a factor of two!

LDEAfterQuickFocus

StopDiagramPostQuickFocus

Previous Article: How to design a singlet lens, Part 1: Setup,  to check how to use the System Explorer and the Lens Data Editor to correctly set up the singlet. 

Next Article: How to design a singlet lens, Part 3: Optimization, that explains how to set up and perform a system optimization as to achieve a better performance. 

 

KA-01822

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