Effect of a camera on scene correlation length

Abstract

There has been much interest in scene correlation length as a tool for characterizing backgrounds. However, since a camera acts as a band-pass filter for spatial frequencies, with the high frequency cutoff determined by the resolution and the low frequency cutoff being determined by the field of view estimates of scene correlation length that have been calculated from digital images must be interpreted in terms of the pass-band of the camera used. The investigator must consider whether the pass band of the camera has significantly affected the spatial frequency spectrum and, as a result, the measured correlation length. In addition to filtering, the spatial frequency spectrum and the measured correlation length may be affected by aliasing. In general, high-pass filtering results in a reduction in the apparent correlation length while aliasing and low-pass filtering result in an increase in the apparent correlation length. Varying the position of the camera provides a means of detecting both filtering and aliasing, and the authors suggest criteria for determining whether these effects have significantly affected the spatial frequency spectrum and the resulting correlation length.