Performance comparison of rectangular (4-point) and diagonal (2-point) dither

Staring array imagers can exhibit sampling artifacts. Dither is a mechanical means of raising the spatial sampling rate without increasing the number of detectors on the focal plane array. Diagonal (two-point or slant-path) dither is easier to implement mechanically than rectangular (four-point orbow-tie) dither. Also, diagonal dither generates half the data rate of rectangular dither. However, diagonal dither does not sample the image as effectively as rectangular dither. The cost and complexity advantages of diagonal dither must be traded against the expectation of reduced performance. This paper discusses analytical and empirical predictions of the performance difference between diagonal and rectangular dither. In order to compare diagonal and rectangular dither empirically, a target identification experiment was performed. The visual task involved discriminating among a set of twelve tactical vehicles. High-resolution (close-up) images of the targets were blurred and down-sampled to produce images representative of those seen with a 320 by 240 staring array sensor. The non-dithered imagery was poorly sampled, and the imagery with rectangular dither was adequately sampled. The experiment compared the target identification performance of non-dithered, diagonally dithered, and rectangularly dithered sensors. Only static imagery was used in the experiment. The analytical and empirical results are described in the paper. The paper also discusses the analytical problems that arise when predicting, and the technical difficulties when measuring, the performance of diagonally dithered sensors. The typical process of describing a sensor by its separate horizontal and vertical responses is not applicable to diagonally dithered imagers.