How to reverse an optical system

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This article explains how to use the Reverse Elements tool to reverse an entire Sequential optical system. It describes how to prepare the system before using the Reverse Elements tool. It also shows how to adjust object and image thickness, and field definition so the reversed system works properly. 

Authored By Andrew Locke

download example (.zip)

Introduction

At times, it may be easier or more convenient to reverse-engineer an optical system based on analysis data. OpticStudio makes this possible with the Reverse Elements tool. While its intended use is to reverse a single element or series of elements in an optical system, it may be repurposed to reverse an entire sequential system.

This article will describe how to reverse an entire optical system through the use of the Reverse Elements tool. While it was not created for this application, by taking a few simple extra steps, it certainly can be adapted for this purpose.

Preparing to reverse the system

To demonstrate how to reverse an entire optical system in OpticStudio, please download the attached sequential file. The inital layout is shown:

This is a typical Cooke triplet photographic objective system at infinite conjugates.

The first step you should take in reversing a system is to consider what system aperture definition to use in the reversed system. If possible, change the system Aperture Type to Float By Stop Size. This definition is preferred as the same system aperture definition can then be used for the original and reversed systems. If this is not possible, you will need to consider how to translate your system aperture definition from image space to object space (i.e. swapping the entrance pupil diameter with the exit pupil diameter). In the case of this example, Float By Stop Size will work fine. This can be changed in the System Explorer...Aperture:

Next, it is a good idea to turn on Paraxial Ray Aiming. Even if the original system does not have pupil aberration, this may not be the case for the reversed system. Ray aiming can be turned on at System Explorer...Ray Aiming. For this example, set Ray Aiming: Paraxial. 

More information about using Ray Aiming in OpticStudio can be found in the Knowledgebase article, "How to use Ray Aiming."

In order to ensure that the sizes of the optics do not change when we reverse the system, we will fix the Semi-Diameters of each surface. This is easily done by selecting Convert Semi-Diameters to Circular Apertures, found in Lens Data Editor toolbar.

Before continuing, we also need to give consideration to how we will define the field points in the reversed system. In order to do this, you will need either the position or angle of incidence for the chief ray from each field point on the image surface in the original system. If the reversed system will be focal in object space, either the position or angle of incidence data could then be used to specify field points using either Angle or Object Height definitions. On the other hand, if the reversed system will be afocal in object space, the fields will need to be specified in terms of Angle using the chief ray angle of incidence data. The position and angle of incidence data can be determined from the Ray Trace calculation (Analyze...Rays & Spots...Single Ray Trace). The position data can also be determined using the Spot Diagram analysis (using a chief ray reference).

In this example, the reversed system will be focal in object space so we can use the position of the chief ray for each field point on the image surface. Click on Analyze...Rays & Spots...Standard Spot Diagram to open a Spot Diagram window. Then, make a note of the image (IMA) coordinates for each field point.

In this case, the image coordinates are 0, 12.419 and 18.137 mm for Fields 1, 2, and 3, respectively. These will be used in the Object Height field point definition in the reversed system.

Reversing the system

Next, reverse all of the optical surfaces in the system by selecting Reverse Elements in Lens Data Editor toolbar.

When specifying the range of surfaces to reverse, include all surfaces except the Object surface and the Image surface.

What needs to be done in the next step depends upon the nature of the original system:

In this example, the original system was afocal in object space and focal in image space. To properly define the new object an image thicknesses, copy the Thickness from the surface before the Image surface to the Object surface.

We now have:

Next, we may need to change the Afocal Image Space setting in the Aperture tab of the System Explorer. If the original system was afocal in image space, you should uncheck this box. On the other hand, if the original system was focal in image space (as is the case in this example), you should check the Afocal Image Space setting.

Likewise, we may need to change the Telecentric Object Space setting in the Aperture tab of the System Explorer. If the original system was telecentric in image space, you should check this box. On the other hand, if the original system was not telecentric in image space (as is the case in this example), you should make sure this setting is unchecked:

Lastly, we will need to convert our field definition as mentioned earlier. Since our original system was afocal in object space and focal in image space, we will use the original chief ray image coordinates to set the field points in the reversed system as Object Height. We can use the Convert To: tool but must then flip the negatives on the resulting heights to obtain the desired field values.  

If you now open a new 3D Layout window, you can see that the optical system has been successfully reversed.

If you have trouble viewing the entire system, as shown above, you may need to alter the First Surface setting.

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