Mobile imaging: the big challenge of the small pixel

As the number of image sensor pixels in camera phones increases, users expect camera phone image quality to be comparable to digital still cameras. The mobile imaging industry is aware, however, that simply packing more pixels into the very limited camera module size need not improve image quality. When the size of a pixel array is fixed, increasing the number of image sensor pixels decreases pixel size and thus photon count. Attempts to compensate for the reduction in light sensitivity by increasing exposure durations increase the amount of handheld camera motion blur which effectively reduces spatial resolution. Perversely, what started as an attempt to increase spatial resolution by increasing the number of pixels, may result in a reduction of effective spatial resolution. In this paper, we evaluate how the performance of mobile imaging systems changes with shrinking pixel size, and we propose to replace the widely misused “physical pixel count” with a new metric that we refer to as the “effective pixel count” (EPC). We use this new metric to analyze design tradeoffs for four different pixel sizes (2.8µm, 2.2µm, 1.75µm and 1.4µm) and two different imaging arrays (1/3.2 and 1/8 inch). We show that optical diffraction and camera motion make 1.4 µm pixels perceptually less effective than larger pixels and that this problem is exacerbated by the introduction of zoom optics. Image stabilization optics can increase the effective pixel count and is, therefore, an important feature to include in a mobile imaging system.