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How does ZEMAX Model the Thermal Expansion of Optical Mounts?
- By Nam-Hyong Kim
- Published 19 March 2007
- Thermal Analysis
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The Thermal Expansion of Lens Mounts
The opto-mechanics or spacer separating 2 lenses (surface 2 in the example below) is represented by an infinitely thin cylinder tube that connects the two lenses. This tube has the same radial extent as the semi-diamater of the surface, so when performing thermal analysis, lens semi-diameters should be set equal to the radial height at which the mount contacts the lens. A user-defined aperture may be applied to the lens so that it draws at its real mechanical size: this is what is done in the drawings below.
The thermal coefficient of expansion (TCE) of the spacer is specified in the TCE column of the Lens Data Editor (LDE). In the example below, the length of the spacer is larger than the air center thickness, specified in the LDE, due to the lens curvatures and the non-zero radial contact point. The spacer expands or contracts with temperature; both radial and axially. (i.e. it expands in the X, Y and Z axis). The center thickness change is then the result of the change in spacer dimension and lens curvature.
At the nominal system temperature, the semi-diameter of the spacer is the same as the semi-diameter of the contacting glass surfaces (surfaces 2 and 3 in the below example). The semi-diameters of the spacer and lenses will change at different rates if their TCEs are different. The TCE of a glass is specified in the glass catalog. Since the semi-diameter represents the location at which the mount and the lens physically contact at nominal temperature, it should be set equal to the height at which the mount contacts the lens.
Consider the following system.
At the reference/nominal/system temperature, the front part of the spacer is contacting the front lens at semi-diameter of 80 lens unit and the rear part at 100. Conceptually, you can imagine the spacer being as shown below (colored as light brown in the shaded model below). To create the image below, additional dummy surface was added in order to change the semi-diamter of the mount before the right lens (see as step change in semi-dimater).
The following animation shows the spacer and the lenses expanding at a different rate (different TCE).
If the TCE of the spacer is set to zero, the length of the spacer does not change with temperature, as shown in the animation below. However, even with zero spacer TCE, the center thickness between the two glasses will change if the radius of any two contacting glass surfaces change with temperature. Observe how in the animation below the center thickness changes while the length of the spacer (highlighted in red) remains fixed.
The thermal coefficient of expansion (TCE) of the spacer is specified in the TCE column of the Lens Data Editor (LDE). In the example below, the length of the spacer is larger than the air center thickness, specified in the LDE, due to the lens curvatures and the non-zero radial contact point. The spacer expands or contracts with temperature; both radial and axially. (i.e. it expands in the X, Y and Z axis). The center thickness change is then the result of the change in spacer dimension and lens curvature.
At the nominal system temperature, the semi-diameter of the spacer is the same as the semi-diameter of the contacting glass surfaces (surfaces 2 and 3 in the below example). The semi-diameters of the spacer and lenses will change at different rates if their TCEs are different. The TCE of a glass is specified in the glass catalog. Since the semi-diameter represents the location at which the mount and the lens physically contact at nominal temperature, it should be set equal to the height at which the mount contacts the lens.
Consider the following system.
At the reference/nominal/system temperature, the front part of the spacer is contacting the front lens at semi-diameter of 80 lens unit and the rear part at 100. Conceptually, you can imagine the spacer being as shown below (colored as light brown in the shaded model below). To create the image below, additional dummy surface was added in order to change the semi-diamter of the mount before the right lens (see as step change in semi-dimater).
The following animation shows the spacer and the lenses expanding at a different rate (different TCE).
If the TCE of the spacer is set to zero, the length of the spacer does not change with temperature, as shown in the animation below. However, even with zero spacer TCE, the center thickness between the two glasses will change if the radius of any two contacting glass surfaces change with temperature. Observe how in the animation below the center thickness changes while the length of the spacer (highlighted in red) remains fixed.
1 Response to "How does ZEMAX Model the Thermal Expansion of Optical Mounts?" 
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said this on 29 Mar 2007 1:20:58 PM PDT
The animation makes the explanation very clear
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