A method for testing the spectral responsivity of infrared cameras using tunable lasers

Abstract

We are developing a technique for accurately measuring spectral responsivity functions of infrared cameras using tunable lasers. We present preliminary results for uniform scenes where tunable infrared lasers illuminate an integrating sphere, diffusing the light to fill the imaging system optics. A commercial camera based on a liquid nitrogen-cooled InSb focal plane array was tested in the 1.4 micrometer to 4.7 micrometer spectral range using a continuously-tunable periodically-poled lithium niobate (PPLN) optical parametric oscillator. Another commercial camera based on an uncooled microbolometer array was tested using a discrete-tunable CO 2 laser in the 9 micrometer to 11 micrometer spectral range. Results from these tests show that signal-to-noise ratio, uniformity, stability, and other characteristics are favorable for use of this technique in the characterization of infrared imaging systems. We also propose a generalization of this technique, to include scenes with arbitrary, controlled spatial content such as bar patterns or even pictures, by illuminating a commercially-available digital micromirror device (DMD). Dependence on irradiance level, exposure time, and polarization can also be measured. This technique has an inherent advantage over thermal-emitter based methods in that it measures absolute spectral responsivity directly without requiring knowledge of the spectral emissivity or temperature of the source.