Athermalized birefringent filter apparatus and method

A multispectral imaging system, comprising:

• an imaging photodetector which images filtered input light, and
• a tunable filter which filters input light, the tunable filter further comprising a variable retarder element with an adjustable first retardance and a fixed retarder element with a second retardance, wherein the second retardance exhibits a thermally dependent characteristic such that the second retardance varies with temperature;
• and
• a control element in communication with the variable retarder element which adjusts the first retardance,
• wherein the control element selects a target retardance for the variable retarder element based on a selected temperature range and the thermally dependent characteristic of the second retardance.

A method of operating a multispectral imaging system which comprises a tunable filter and an imaging detector, the tunable filter having a variable retardance, the method comprising the steps:

• determining a temperature range over which the tunable filter operates;
• commanding the tunable filter to a first configuration wherein it transmits light of a first selected wavelength range, said step of commanding comprising adjusting the variable retardance to a target retardance, wherein the target retardance is determined based on the determined operating temperature range and a thermally dependent characteristic of the tunable filter;
• and
• taking an image with the imaging detector in that first filter configuration.