Exploring Sequential Mode in OpticStudio

This article introduces prospective and new OpticStudio users to the capabilities of Sequential Mode. The discussion includes demonstrations setting system properties, using layouts, and some basic analyses, extended source modeling, and modeling off-axis systems.

Authored By Akash Arora

Introduction

Geometrical optics involves ray tracing - a widely applicable technique for modeling the propagation of light through an optical system.

In sequential ray tracing, rays are traced through a pre-defined sequence of surfaces, hitting each surface only once, while traveling from the object to the image plane. Imaging systems are well described by sequential surfaces. Sequential ray tracing is computationally fast and is extremely useful for the design, optimization, tolerancing, and analysis of such systems.

Many conventional optical systems can be classified as imaging systems, including photographic objectives, telephoto lenses, microscopes, telescopes, relay lenses and spectrometers.

The OpticStudio graphical user interface

When you first open OpticStudio (either the demo or full, licensed versions), you will see the Ribbon Bar, toolbar, System Explorer, status bar, and Lens Data Editor (LDE).

All of the features that OpticStudio has to offer can be accessed through the various menus in the Ribbon Bar, the editor specific toolbars, or the system explorer. Shortcuts to most of these features are available for convenience in the toolbar above the Ribbon Bar. The features in the toolbar can be changed via: File...Project Preferences.

Beneath the Ribbon Bar is the Lens Data Editor. The window containing the editor can be docked or floated. If it is floating, it can be moved and resized to any convenient location. Default dock/float behavior can be set in the Project Preferences or in the Setup...Window Control group. The Lens Data Editor has columns for Comments, Radius, Thickness, Glass, and Semi-Diameter (radial clear aperture) and Conic constant. The latter five data items are used to define the majority of optical components. These columns are organized in a default order. You can easily reorder them by click on the column header and dragging to a new position.

Each row corresponds to an optical surface. Each surface has its own local coordinate system. The position of each surface along the optical axis is referenced to the previous surface. In other words, the Thickness column in the Lens Data Editor refers to the distance from the previous surface and not from a global reference point.

By default, there are three surfaces shown: the object, stop, and image. These are denoted as such in the first column. The first column also displays a surface type, the default of which is the Standard surface. There are many other surface types available.  Some surfaces require more defining parameters than the Standard surface. When those types are defined, additional columns will appear as necessary.

The Lens Data Editor, and all other editors, have similar functionality to Microsoft's Excel spreadsheet. You can right-click columns or rows to freeze, hide, insert or delete. Multiple cells can be selected and copied, or data can be pasted from the clipboard.

Layout windows

Make sure the Use Session Files option is checked under Project Preferences...General. When you open an OpticStudio file and Use Session Files is checked, OpticStudio will automatically re-open any analysis windows that were open when the lens file was last saved. The Use Session Files option can be un-checked to load lens files more quickly without analysis windows. In addition calculated data can be saved in the session file if Include Calculated Data in Session is checked.

From the File Ribbon select Open. You will see a screen appear that lists directories containing sample files. Select the file {Zemax}\Samples\Sequential\Objectives\Double Gauss 28 degree field.zmx. The file will load, and the Lens Data Editor will fill with data. Note there are 12 surfaces, with surface 6 being the system stop surface (STO). Each surface has a Radius, Thickness, Glass, and Semi-Diameter. This is a commonly used photographic objective lens.

If the Use Session Files setting is checked, a Layout window will be opened when the Double Gauss lens file is loaded. Layouts are used to see what the loaded lens looks like.

The Layout window can be resized and moved like any other window. To change the settings for this window, select Settings from the Layout window menu bar. A dialog box will expand that allows you to customize the settings for this Layout diagram. Change the Number of Rays option to 7. Next check the Fletch Rays box. By default settings are auto-applied, meaning they are implemented as soon as you click away from the edited setting. You can disable this for the current analysis by unchecking Auto-Apply, or you can disable it for all analyses in Project Preferences...General. Once applied, the Layout will be drawn with 7 rays per field, instead of 3. The rays will also be fletched with arrows indicating the direction of light propagation. This basic procedure for changing the default settings for any window is consistent throughout OpticStudio.

To view the contents of the Layout window more closely, the window may be resized. You can also zoom in on any portion of the image with your mouse by using the scroll wheel on your mouse, or by holding down the left mouse button and dragging the mouse over a portion of the image. The plot may be zoomed multiple times to show an incredible amount of detail. To unzoom the view, select the option, select Reset Zoom from the menu bar of the Layout window.

Several other types of layouts can be opened from the Analyze...System Viewers Group in the Ribbon Bar. The type of layout plot selected will instantly appear in a new window. Shortcuts for launching analysis plots can also be defined on the main toolbar. Keyboard shortcuts for launching analyses are also customizable under Project Preferences...Shortcut Keys.

TIP:  You can close any floating window by clicking on the "X" in the upper righthand corner of the window. Docked windows have a similar button in the tab listing their title.

Spot diagram analysis

There are many other types of analysis windows that OpticStudio can generate.

If the Use Session Files option is checked, a Matrix Spot Diagram will also be opened when the Double Gauss lens file is opened. This type of analysis window shows the spots formed by each field and wavelength combination individually.

Several other types of spot diagrams are available. To open a Standard Spot Diagram, select Analyze...Rays & Spots...Standard Spot Diagram from the Ribbon Bar. The spot diagram for the loaded lens will then appear.

Now select Settings from the Spot Diagram window menu bar. Note that there is an option in this settings dialog box labeled Pattern which is set to Hexapolar. This setting denotes the distribution of rays used to launch rays to the entrance pupil from the object surface. Hexapolar is the default pattern for rays in the pupil. Click on the drop-down box and select Dithered instead. Now click on OK. The spot diagram will be redrawn, now with a pseudo-random set of rays instead of a hexapolar pattern.

TIP:  Right-clicking in a window and selecting Open Settings will also invoke the Settings dialog box for that window.

Ray Fan and OPD Fan analysis

One way to assess the geometric aberrations of an optical system in OpticStudio is to use the ray aberration and optical path difference fan analysis capabilities. These can be accessed in the Analyze...Aberrations menu from the main menu bar.

Open a Ray Aberration plot now using the menu option. This window plots the ray aberrations for tangential and sagittal fans for each field point and wavelength.

In addition to ray aberrations, OpticStudio can also generate fans which plot the wavefront aberrations. This type of analysis is known as an OPD fan (Optical Path Difference). Open an OPD fan now using the ribbon option Analyze...Aberrations...Optical Path. OpticStudio will plot the wavefront aberrations for each field and wavelength.

TIP:  Many of the analysis plots support the active cursor feature, which displays the coordinates of the mouse cursor in the upper left corner of the window as you move the mouse over the graphic. Try this now with the OPD Fan or Ray Fan windows.

MTF analysis

OpticStudio also supports comprehensive diffraction analysis capabilities.

Select File...Open from the Ribbon Bar and open the file {Zemax}\Samples\Sequential\Objectives\Cooke 40 degree field.zmx. This file contains data describing a simple three lens objective.

TIP:  You can also use the Open icon on the toolbar to open a file.

To look at the MTF for this lens, select Analyze...MTF...FFT MTF from the Ribbon Bar. The tangential and sagittal response for each field point will be plotted versus spatial frequency using FFT (Fast Fourier Transform) technique. MTF calculations based on Huygens integral calculations are also available.

To show the diffraction limited MTF for this design, click on Settings from the FFT MTF window menu bar, then check the box that says Show Diffraction Limit, then click OK. The diffraction limit will be added to the plot.

Extended scene analysis

While most analyses sample the object with a series of field points, OpticStudio also supports several analyses that simulate the image from an extended object scene. Geometric, diffraction, partially coherent, and measured source variants are all available. Here we will take a look at some of the more basic analyses, while also providing links to further reading on the more advanced ones.

The Geometric Image Analysis feature can be used to model extended scenes in OpticStudio. It can be used to analyze useful resolution, display distortion, and calculate geometric efficiency for planar extended sources centered at any field point on the object surface.

You can examine the image analysis capabilities of OpticStudio using the Cooke Triplet design. In the File Ribbon click the arrow below the Open icon and notice that recently opened files are listed. Open the Cooke Triplet file we looked at earlier. To open a Geometric Image Analysis window, select Analyze...Extended Scene Analysis...Geometric Image Analysis. The plot of a letter "F" (the default extended scene) will then appear as a spot diagram.

To show the image of a grid instead of the letter "F", click on Settings from the Geometric Image Analysis window menu bar. Change the File setting to GRID.IMA. Details of the IMA format used to generate extended sources can be found in the Help System. You can also change the Show setting to False Color. This will simulate the display of the image of the extended scene on a detector with a finite number of pixels. Click OK to re-generate the analysis with these new settings.

OpticStudio also includes a Geometric Bitmap Image Analysis which is similar to the Geometric Image Analysis feature except that it can use BMP, JPG, or PNG files as the object scene. Red, green, and blue wavelengths are traced to detector pixels on the image surface or any other surface to create RGB images. Any three wavelengths may be traced for analysis in other parts of the spectrum. This feature may be used to analyze photo-realistic images, or custom targets created and saved in a supported format. Large numbers of rays typically need to be traced to adequately sample the image.

Open a Geometric Bitmap Image Analysis window and change the settings as follows. Set the Input to ALEX200.BMP. Increase the number of pixels at the image surface by setting both X-Pixels and Y-Pixels to 100. Decrease the size of each pixel by setting both X Pixel Size and Y Pixel Size to 0.5. Lastly, increase the number of rays that are traced by setting Rays x 1000 to 100 and then click OK.

Both Geometric Image Analysis and Geometric Bitmap Image Analysis are ray-based analyses. A more comprehensive capability is supported with Image Simulation. It can perform geometric computations or diffraction computations based upon a PSF convolution method. Partially Coherent Image Analysis allows you to simulate partially coherent object scenes. Both capabilities are described in more detail in the following articles.

Off-axis systems

OpticStudio can also model off-axis optical systems such as systems with fold mirrors, tilted components and off-axis conical reflectors.

Open the file {Zemax}\Samples\Sequential\Tilted systems & prisms\Tilted mirror.zmx. This file demonstrates the modeling of a fold mirror in OpticStudio. As you can see from the 3D Layout that opens with this file, the mirror surface (surface 3) is tilted, creating a folded beam path.

The mirror in this system is tilted using the Coordinate Break surface type. Click anywhere on the row corresponding to surface 2 in the Lens Data Editor. Now, scroll to the right using the right arrow key on your keyboard. Scroll past the Conic constant column. You will see columns for decentering and tilting. Notice that the Tilt About X parameter for the Coordinate Break has been set to 10. This indicates that the mirror is rotated by 10 degrees about the x-axis.

Now, open the file {Zemax}\Samples\Sequential\Telescopes\Unobscured Gregorian.zmx. This is a telescope consisting of two conical mirrors. Observe from the Shaded Model layout that opens with this file that the primary and secondary mirror are tilted using a Coordinate Break. This moves the secondary mirror (surface 5) out of the initial beam path and eliminates the possibility of obscuration.

TIP:  Selecting a surface in the Lens Data Editor (by clicking on it) will highlight that surface in all layout windows. In the Shaded Model layout above, the primary mirror (surface 4) is highlighted.

The primary mirror in this telescope is an off-axis conical section. The decentered aperture on this surface aligns the mirror with the incoming beam. To see the aperture settings for the surface, select surface 4 and then click Surface Properties in the top-left corner of the Lens Data Editor. Alternatively, double-click where it says Standard for surface 4 in the Lens Data Editor. This opens the Surface Properties dialog for surface 4. Now, click on the Aperture Tab. Observe the settings for the decentered Circular Aperture on this surface.

TIP:  Users of the full, licensed version of OpticStudio can explore off-axis optical systems further using the Knowledgebase article, "How to tilt and decenter a sequential optical component"

System aperture, field and wavelength data

Every optical system has a system aperture specification, such as F/#, entrance pupil diameter, numerical aperture, or cone angle. This sets the width of the on-axis beam that the optical system will collect in object space. In OpticStudio, this data is easily specified in the Aperture section of the System Explorer. Simply click on the arrow to the left of Aperture to expand that group of settings.

The Field data is used to specify the points on the object surface from which rays are launched. This data can be accessed from the System Explorer as well. If you want to see all field settings instead of the quick view in the System Explorer, expand the Fields section and click Open Field Data Editor to launch the Field Data dialog.

The wavelengths of rays that are traced are set in the Wavelength data section. This is also in the System Explorer and a dedicated dialog can be opened by double-clicking the Wavelength heading.

Further exploration

Feel free to continue to explore Sequential mode in OpticStudio by opening the other sample files available in the Sequential folder, opening an existing custom file, or creating your own system from scratch. You can also try out the other analysis options available from the Ribbon Bar after opening any of these sample files.

The OpticStudio Help System is an excellent source of information that you can use while you continue to explore the capabilities of the software. It can be accessed from the Help tab in the Ribbon Bar.

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