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Masks and Apertures

Masks and Apertures

Imaging applications are an expansive topic in optical systems. Simulating image quality and imaging optical system performance in a correct theoretical way is important for understanding how the real system will behave. Aberrations, optical and mechanical tolerances, and physical limitations can make or break an imaging system if all aspects are not well understood. Improving image quality through limiting ray angles and pupil matching can help to drive better imaging performance as well. Masks and apertures are great tools to increase the usefulness of optical system analysis and performance.
In the 3DOptix cloud-based simulation tool these objects can easily be customized and inserted into any optical system. We will overview a few methods of using them and some ideas on how to determine performance metrics.
Our optical system will consist of a 4F system with the following components:
  • Edmund Optics 32-477, plano-convex
  • Edmund Optics 32-477, plano-convex
  • Dot Matrix Target
  • Line Target
  • US Air Force 1951 Resolution Target
  • Light Source
    1. Plane Wave
    2. Rectangular, 5×5 mm half dimensions
    3. 633 nm wavelength
    4. Power, 1 W
    5. Unpolarized
  • Detector: Image Plane
    1. Spot: Coherent Irradiance
    2. Analysis Rays: 1 million
    3. 500×500 pixels
You can see the image of our optical system below. The 3DOptix simulation file can be downloaded to see additional information about the optical system such as component spacing and analysis detectors. 

There are two ways to insert masks and apertures into a 3DOptix optical system. The first is to create a CAD object, custom or downloaded from a manufacturer’s website, or to upload an image in .png format. For this application, we will upload an image using the ADD USER-DEFINED OBJECT option from the drop-down in the 3D viewer. This is the simpler method.

From here we want to upload an image of the mask or aperture we want to include. We will choose a few different masks to include and the first will be the USAF 1951 target. We will change MAJOR DIMENSION HALF SIZE to 5mm to match the light source of the optical system. From Here we want to upload an image of the mask or aperture we want to include. We will choose a few different masks to include and the first will be the USAF 1951 target. We will change MAJOR DIMENSION HALF SIZE to 5mm to match the light source of the optical system.

We will complete the same procedure to add two more masks; a dot matrix and a variable line target.We will complete the same procedure to add two more masks; a dot matrix and a variable line target.
Now let’s look at the image of the mask on the detector with all objects at the optimal configuration.
As can be seen these are perfect representations of the input masks at the image plane of the 4F system. Note that the lens configuration flips the images about the horizontal axis.
We can compare the USAF 1951 resolution target image quality of the 4F system with an aspherical lens imaging the same target in Zemax. A visual inspection of the image quality appears to be in very close agreement between them.

To simulate a non-ideal (real world) optical system imaging the masks we want to include aberrations to the system. To do this we will make the curved surface of lens 2 a ZERNICKE FRINGE SAG surface. We will keep the NORMALIZED RADIUS at 12.5 mm and change TERM 4 which is defocus to 0.05 mm.

The USAF 1951 target is used to see the “blurring” of the image with this aberration. Note that the target was flipped to generate an upright image at the image plane. The image on the left is ideal and the right aberrated.

Next, we will insert the dot matrix target, reset all terms, and change the horizontal coma, TERM 5, to 0.1 mm.

Now we will look at the line target with some amount of spherical aberration. First resetting all terms, then changing TERM 12 to 0.1 mm to introduce the aberration.

Using these different types of masks and a custom surface to introduce various aberrations we can get a good estimate of the optical system imaging quality. Visual analysis of the ideal and the aberrated image can generate instant feedback for designers for quick performance analysis. The dynamic addition of masks and apertures, both custom and stock, are important tools for designing useful and high precision optical systems.
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Available on January 30th, 2023