System design

Infrared filters and coatings are employed on many remote sensing radiometer instruments to measure the thermal emission profiles and concentrations of chemical constituents present in planetary atmospheres. The development of Earth and planetary remote sensing instruments to provide simultaneous multi-channel atmospheric monitoring subsequently requires highly complex optical systems with many refractive elements to provide high resolution images of the scene under observation. Optical coatings comprising bandpass filters, rejection filters, dichroic beamsplitters and antireflection coatings all contribute to separate and isolate the primary wavelength regions of interest at the focal plane.

Spectral performance models, designed to simulate the system throughput for each channel are have therefore been developed by the Infrared Multilayer Laboratory to determine the end-to-end spectral throughput profile of the instrument. Results from these models can then be used to demonstrate that the instrument-level radiometric requirements will be achieved. The spectral design uses an integrated systems performance approach taking the instrument specification as a target. The system takes account the spectral characteristics of the transmissive optical materials, relative response of the detectors, thermal emission from the instrument and atmospheric model to calculate the predicted radiance profile for each channel.

Using a systems approach the spectral performance of each optical coating is tailored specifically to perform its own specialised function from which optimal design is achieved. This approach further provides advantages during the evolution of the instrument by including the capability to interactively assess performance requirement changes as they occur. The ability to assess alternative component designs and evaluate or compensate for effects of non-compliant performance has proved an invaluable tool for the optical designer.