Athermalization of infra-red lenses via optical phase mask with no performance degradation at nominal conditions
Infra-red (IR) imaging (in the LWIR and MWIR bands) is widely used for observation and surveillance applications. Most of IR optical materials are very sensitive to temperature changes. This sensitivity is reflected in a temperature dependent refractive index, that in turn it affects the focal length. In order to manufacture a stable IR lens, this problem needs to be solved (the solutions are referred as 'IR lens Athermalization').
Need and Advantages
There are some approaches and solutions to IR lens athermalization. The active approach solves the problem by refocusing the optical system using motors. If one desires not to use motors, there are also passive solutions, which use mixture of optical/mechanical materials, which in tandem create a temperature stable focal length. However, these solutions require the usage of exotic materials and also a complex opto-mechanical design. All of the known solutions are not suitable for low-cost applications where a simple solution is sought. Our proposed solution is a full-passive all-optical solution, based on incorporating a phase element (mask) along the optical path, which compensates for the thermal effects. It is inexpensive and does not require moving components, thus attractive for surveillance applications in remote locations. Moreover, unlike other masks that reduce the performance of the imaging system at the nominal temperature, our mask does not deteriorate the image quality at the nominal temperature and improves it very much at other temperatures (see Fig.).
The proposed solution can be applied to any IR imaging system, but it is particularly suitable for low-cost systems, where a simple and passive solution is needed. Ideal for IR cameras deployed unattended and without active control.
Stage of Development
This project has been analyzed theoretically as well as in computer simulations. Testing it in IR imaging systems is under consideration.