Simple Passive Athermalization - NTE Spacers for Optics

NTE Spacers for Optic Athermalization

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Passive Athermalization

Maintaining performance and structural integrity of optical systems over a wide temperature range (passive athermalization) has been a practical and technical struggle for decades. Eliminate unreliable optical performance by simply replacing traditional athermalization solutions with the only negative CTE alloy in the world, ALLVAR Alloy 30. This negative CTE innovation renders traditional design paradigms inefficient and enables smaller, lighter optical system designs that were previously impossible.

Features

ALLVAR Alloy 30 offers a new material solution for athermalized optical assembles. This novel material:

Has a axial Coefficient of Thermal Expansion of -30.0ppm/°C at room temperature.
Has a positive CTE in the radial direction (similar in magnitude to Aluminum), allowing for compliant mounting of optical elements.
Exhibits mechanical and material properties similar to other Titanium alloys.
Alloy 30 can be machined like many other metals, tolerances of ±0.001” are easily achievable.
Offered in both round bar and tube formats; it is a natural fit for the design and fabrication of lens stack spacers, lens barrels, and lens to focal plane stand-offs.

Value Propositions - Athermalization

Reduce infrared optical assembly length up to 40%.
Reduce weight by decreasing optical element count.
Reduce manufacturing costs and lead time by decreasing part count and design complexity.
Increase yield by relaxing tight mechanical and optical element tolerances necessary for athermalization.
Close the 5%-10% gap in performance at temperature between design and manufacturing.

Specifically targeting fixed-focus objectives/cameras, the addition of ALLVAR Alloy 30 can eliminate nested barrel structures and complex compensation mechanisms, providing passive mechanical athermalization for maintaining optimum image quality from -140°C to +80°C and beyond.

Smaller, lighter, athermalization of optical systems

The plot to the left demonstrates how ALLVAR Alloys can simultaneously open the achievable series system design window and decrease the length of commonly used Aluminum and Delrin parallel systems. The term 𝐿2/𝑓 is used to quantify the increase in total thermal compensator length of the nested barrel compared to the series system while the term δ𝑓 is the thermal defocus coefficient or the change in focal length per degree Celsius of a given optic design. The plotted lines for different materials represent the increase in thermal compensator length required to athermalize an optic with a given thermal defocus. Comparing commonly used Aluminum and Delrin to ALLVAR Alloy 30 and Delrin, the savings are immediately apparent. Not only does the current theoretical parallel system window extend down to -30×10^{-6 K-1}, but the ALLVAR-Delrin system can reduce compensator length up to 40%.

Looking at the image above, several materials were used as spacers between a set of lenses that ultimately display a spot from a laser in an image. Between ALLVAR Alloy 30, Aluminum 6061, Invar 36 and Ti-64, Alloy 30 maintains the highest peak intensity – and best focus – as the temperature increases from 22° to 65°C. Click the button above to see a full video of the demonstration.

For those that prefer more technical detail, shown below are the Modulation Transfer Function (MTF) values versus temperature for each material’s image spot. The Aluminum and Ti6Al4V (Ti64) show very sharp peaks associated with rapid drop offs in performance above and below an optimal operating temperature. The Invar shows excellent stability between 20°C and 50°C, but MTF drops off well below 60% above and below this optimal operating temperature. The ALLVAR Alloy 30 material displays a much smoother response and excellent stability throughout with a small decrease from 85% MTF at -10 °C to 60% MTF at 70°C.

Products Available

ALLVAR can suit your project or program needs – either by providing semi-finished material in round bar or tube form, or fully machined parts to print. Maximum sizes are 2.25” round bar and 3.00” outer diameter tube.

ALLVAR Alloy 30 can be machined into custom products or components best suited for your specific product or project.
Not sure if ALLVAR is a good fit for your project? Please contact us. We offer a wide range of design services and machining, and we would be happy to discuss negative thermal expansion Alloy 30’s applicability for your application.