In this paper, the theoretical foundation and practical implementation of a controllable anti-aliasing filter for digital film
cameras is presented.
A prototype of an optical anti-aliasing filter that is based on moving a parallel optical window was designed and built to
demonstrate the ability to control the spatial frequency response of an acquisition system. During the image exposure,
four spring-preloaded voice coils rapidly change the pitch and yaw of the parallel window, resulting in a displacement of
the image content that is projected onto the sensor. The image content displacement during the exposure results in
altering the frequency response of the scene that is captured by the sensor.
Specifically, during the exposure time, a carefully controlled movement of the parallel optical window results in a
circular trajectory of the image content that is projected onto the sensor. By increasing or decreasing the radius of the
circular trajectory, the spatial cut-off frequency of the system is dynamically modified. In addition to the circular path,
this paper shows theoretical justification and demonstrates the use of more complex trajectories, such as double circular,
elliptical, and one-dimensional rectangular trajectories. These trajectories improve the suppression of aliased components
in the acquired image.
Most contemporary still and video cameras employ various optically birefringent materials as optical low-pass filters
(OLPF) in order to minimize alias artifacts in the image. Due to the slope characteristics of these filters, camera
designers are faced with the choice of either under correcting to maintain image resolution, allowing some aliasing, or
eliminating aliases with a more aggressive design that will also compromise the image modulation transfer function
(MTF). Furthermore, most OLPFs are designed as optical elements with a frequency response that does not change even
if the frequency responses of other elements of the capturing systems are altered.
In this paper, we demonstrate the use of a parallel optical window or, alternatively, a rigid mirror positioned between a
lens and an imager as an OLPF. Controlled x- and y-axis rotations of the window and the mirror result in a manipulation
of the point spread function (PSF) of the system and frequency content of imaged continuous scenes. We evaluate the
system MTF when various window functions are used to shape PSF, such as rectangle, triangle, Blackman-Harris etc.
We also present results of our experiments when the dynamic OLPF's support and shape are altered to accommodate the
optical performance of lenses and imager characteristics.