Imaging System Aperture Masks for Image Plane Exit Pupil Characterization

Imaging system exit pupil characteristics are typically difficult and expensive to measure directly. In most cases, the exit pupil in an optical system is a virtual image of the system’s aperture stop and requires additional powered optical elements, such as lenses and curved mirrors, in the optical path to facilitate its observation and characterization. In the past, measurements of a system’s exit pupil have typically not been performed due to this complexity. Recently, the use of image-based, wavefront sensing algorithms to measure and improve an imaging system’s performance has become more widespread. Wavefront sensing algorithms such as these require knowledge of an optical system’s exit pupil shape and illumination uniformity to develop accurate estimates of the wavefront generated by an optical system, forcing the use of expensive and complicated optical components for exit pupil characterization. To reduce or eliminate the need for the introduction of such hardware, an aperture mask architecture was developed that provides for the characterization of an imaging system’s exit pupil using image intensity variations at the system’s image plane. Such masks are inexpensive to manufacture, and generate farfield diffraction patterns that can be easily analyzed to determine both exit pupil distortion and illumination characteristics.

One of the key optical parameters of interest for an optical system is the system’s exit pupil. An optical system’s exit pupil is the image of the system’s aperture stop as viewed from its image plane. The size, shape, and illumination characteristics of the exit pupil affect and limit the spatial frequency content that can be detected at the image plane. It determines the image quality. If an aperture stop is placed in an optical system such that no other optical elements come between the aperture stop and the image plane, then the aperture stop and the exit pupil are one and the same, and its characterization is straightforward using mechanical measurement techniques. But for most imaging systems, aperture stops are located at intermediate locations within their optical paths. When intervening optical surfaces are located between an aperture stop and a system’s image plane, those optics affect the exit pupil characteristics and eliminate the possibility of directly measuring the exit pupil through mechanical means.

This technology is unique in that it can determine imaging system exit pupil characteristics from information recorded in the system’s PSF (point spread function). This provides a large advantage in terms of cost (savings of three orders of magnitude or more), mass (at least two to five times less), and complexity over other known methods of exit pupil characterization.

This work was done by Brent Bos of God dard Space Flight Center. GSC-15957-1

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