Lens with coated sub-aperture
When an optical surface is coated, the specification for the coating defines a clear-aperture over which the specifications apply. Outside of this clear-aperture however, the surface may be bare (uncoated) or perhaps exhibit performance described by a different coating specification. In FRED, a single surface is assigned just one coating specification as seen on the Coating tab of the surface dialog below. So how do you define coatings for different parts of a surface?
There are two options to address this issue:
- Construct a user-defined script coating that implements spatially varying properties across the surface
- Define multiple versions of the surface, one for each of the required coating specifications, trim each surface so that it exists only in the spatial region of interest, and then assign each surface its corresponding coating property
The procedure to implement the second option above is described in this article. For this example, a Lens element will be customized so that the first surface of the lens has a coated clear-aperture and an uncoated region outside of the clear aperture. The clear aperture of the lens surface will be a racetrack curve in order to demonstrate the generality of the approach (any arbitrary closed curve can be used).
Step 1: Make a Lens
Construction of the model first starts with a Lens element, as seen in the image below. Surface 1 is the dark green surface shown in the 3D view and will be customized with two regions having different coatings.
Step 2: Convert the Lens to a Custom Element
Constructs, such as the lens, mirror, prism, element primitive, etc., enforce certain rules regarding their geometry construction in order to preserve the intent of the element (e.g., you cannot shift a surface of a lens element axially since the lens would no longer be a solid element). In this example, we are adding a new surface to the group of existing surfaces from which the lens element is built. In general, if you are going to be adding, removing, or repositioning the individual surfaces in one of FRED's native elements, it is good practice to convert the solid object to a custom element prior to performing the surface manipulations.
- Right mouse click on the lens node and select Convert to Custom Element from the menu.
The conversion process is irreversible, but FRED does present a confirmation dialog box to you before the custom element conversion. After confirming the conversion, the tree and 3D view looks like that shown below. Note that instead of the lens node on the tree, we now have a custom element. You are now free to manipulate the surfaces within the custom element as needed (FRED does not enforce any rules on the surfaces with regard to element construction).
Step 3: Duplicate the Surface
The surface of interest, Surface 1, should have two different coating specifications applied to it - one coating located within a custom clear-aperture and another located outside of the clear-aperture. However, as mentioned above, a single surface can have only a single coating. In this implementation, we are duplicating the surface of interest to create two distinct surfaces and then restricting each to lie either within or outside of a custom clear-aperture boundary.
- Select Surface 1 on the tree, and then select Edit > Copy from the menu. Select the Custom Lens node on the tree, and then select Edit > Paste.
Note that the above operation can be implemented using standard Windows Ctrl+C and Ctrl+V rather than using the Edit menu options.
- Select the new Surface 3 node in your tree and press F2 on your keyboard. Give the surface the name, "Surface 1 - Uncoated".
- Select the Surface 1 node in your tree and press F2 on your keyboard. Give the surface the name, "Surface 1 - Coated".
Your tree and 3D view should now look like the image below.
Step 4: Add Curves to Define the Coating Boundary
Next we define a custom coating boundary using FRED's curve construct. Note that the use of curves here is not required, though it may be common and is a general technique for trimming objects in FRED. The goal is to end up with two distinct regions for Surface 1 that can then be assigned their respective coatings. Surface or volume trimming operations may also be used to achieve this result.
- Right mouse click on the Custom Lens node in the tree and choose, "Create New Curve". Name the curve, "Racetrack Curve". Change the curve type to, "Racetrack curve" and then set the X and Y semi-apertures to 2.5 and set the corner radius to 0.3. Click OK to close the dialog box.
Your model should now show a racetrack curve floating in space in front of the lens element. This curve defines a closed path through space but otherwise has no significance to FRED. How should FRED interpret this curve? Is it a clear-aperture? Is it a hole? In order to assign specific physical meaning to an independent closed curve, we use a second curve type called the "Aperture curve collection".
- Right mouse click on the Custom Lens node in the tree and choose, "Create New Curve". Name the curve, "Clear Aperture Curve". Change the type to, "Aperture Curve Collection". Note that in row 0 of the dialog, the "Usage" entry is "Clear Aperture". In the "Curve Designation" column, select the Racetrack Curve node from above. Click OK to close the dialog box.
- Right mouse click on the Custom Lens node in the tree and choose, "Create New Curve". Name the curve, "Hole Curve". Change the type to, "Aperture Curve Collection". For the Usage entry in row 0, select "Hole". In the "Curve Designation" column, select the Racetrack Curve node from above. Click OK to close the dialog box.
The object tree, 3D view and each of the three curve dialogs should appear as shown in the graphic below.
Step 5: Trim the Surfaces
The next step is to restrict each of "Surface 1 - Coated" and "Surface 1 - Uncoated" to the regions inside and outside of the racetrack curve, respectively.
- Double click on Surface 1 - Coated to open its dialog box. Move to the Aperture tab. At the bottom of the dialog box, in the Surface Trimming Specification section, right mouse click on row 0 in the Operation List View and choose Select Entity. Choose the Clear Aperture Curve. Click OK to commit the change.
- Double click on Surface 1 - Uncoated to open its dialog box. Move to the Aperture tab. At the bottom of the dialog box, in the Surface Trimming Specification section, right mouse click on row 0 in the Operation List View and choose Select Entity. Choose the Hole Curve. Click OK to commit the change.
For visual clarity in the 3D view:
- Right mouse click on Surface 1 - Uncoated in the object tree and select, "Visualization Attributes". Toggle the "Reset Color" box and then choose "Blue" from the Predefined Color list. Press the Assign button and then click OK to close the dialog box.
The model should look like the following in the 3D view:
Step 6: Apply the Coatings
At this point, there are two versions of Surface 1 in the model and each is constrained to reside either within or outside of the racetrack boundary. The final step is to assign the appropriate surface properties to each surface. This can be accomplished by dragging and dropping the desired property onto the corresponding surface node in the object tree.
Associated FRED files: