The influence of CCD pixel binning option to its modulation transfer function

In recent years CCD manufacturers have been supplying their devices with multi-purpose abilities to manipulate the CCD's readout pattern, where one of these versatile options is a flexible pixel binning option. The pixel binning is a process of combining multiple pixel charges in horizontal, vertical or in both directions simultaneously, into a single charge. The binning process positively influences to the signal-to-noise ratio, sensitivity and frame rate at the cost of decreasing spatial resolution, which, in its turn, negatively influences to the spatial frequency response of the imaging system (i.e. to the output image quality). The modulation transfer function (MTF) is an essential measure for characterizing the spatial-frequency response of the array imaging system. In this work we have performed a theoretical and experimental investigation of the MTF of CCD array in the context of the pixel binning option. We have derived a generalized equation of the geometrical MTF for the v x h binning mode, where v and h denote the numbers of binned pixels in vertical and horizontal directions, respectively. The MTF measurements were performed using a method, based on the generation of laser speckle and utilizing the high resolution (1360x1024) monochrome CCD array. The MTF of normal mode, 2x1-horizontal, 1x2 -vertical, and 2x2 quadratic binning modes were measured by employing single-slit aperture method. CCD binning is widely used in spectroscopy, astronomy, in many image processing applications, such as autofocus, object tracking, etc. The results of this work can be useful for designing optical systems, involving CCD pixel binning option.