Passive athermalization is the process of designing an optical system, such as a lens assembly, to minimize its sensitivity to temperature changes without the need for active temperature control. This is particularly important in applications (like Drones) where the optical performance needs to remain consistent across a wide range of temperatures. Here are some considerations to keep in mind when designing a lens for passive athermalization:

  1. Material Selection: Choose materials with low thermal expansion coefficients. Materials that have similar coefficients of thermal expansion will change dimensions at a similar rate when the temperature changes, reducing the effect on the lens performance.
  2. Lens Design: Utilize lens designs that compensate for the changes in refractive index with temperature. Athermal lens designs often involve using multiple lens elements made of different materials to counteract the temperature-induced changes in optical properties.
  3. Material Combinations: Pair materials with different thermal properties to balance out their effects. For instance, combining a material with a high thermal expansion coefficient with a  material having a low thermal expansion coefficient can help cancel out the net change in size due to temperature variations.
  4. Lens Element Spacing: Adjust the spacing between lens elements to account for changes in their refractive properties. Changing the spacing can counteract shifts in focal length and aberrations caused by temperature fluctuations.
  5. Optical Coatings: Apply specialized coatings to lens surfaces to reduce temperature-related effects. Anti-reflective coatings and other coatings can be designed to help maintain optical performance over temperature changes.
  6. Thermal Isolation: Minimize the transfer of heat between the lens system and its environment. This can be achieved through thermal insulators or by ensuring that the lens is not in direct contact with components that experience significant temperature changes.
  7. Housing Design: Design the lens housing to minimize heat exchange with the surrounding environment. Proper insulation and heat sinking can help stabilize the lens temperature.
  8. Environmental Conditions: Consider the expected operating temperature range for your lens and design it to perform well within that range. If the application requires extreme temperature stability, you might need to make more substantial design adjustments.
  9. Modeling and Simulation: Use optical design software and thermal analysis tools to simulate the behavior of the lens under different temperature conditions. This will help optimize the design for athermalization.
  10. Testing and Validation: Perform thorough testing of your athermal lens design under various temperature conditions to ensure that the actual performance matches your design expectations.
  11. Manufacturing Tolerances: Take manufacturing tolerances into account. Even with an athermal design, there might be slight variations in dimensions and material properties during production that can affect performance.
  12. Cost Considerations: Athermal lens designs can be complex and involve multiple lens elements and materials. Consider the trade-off between performance and cost to ensure that your design is practical.

Designing a lens for passive athermalization requires a deep understanding of optical principles, materials science, and thermal dynamics. It's often a balance between achieving optimal performance and managing the complexity of the design. Collaboration between optical engineers, material scientists, and thermal experts is crucial for success.