1. A method of determining when an image capture device with a rolling shutter is in an environment having a flickering illuminant, comprising: (a) storing rows of pixels from a first captured digital image in memory; (b) using the rows of pixels to provide a first column vector; (c) providing a first order difference vector from the first column vector; and clipping the elements in the first order difference vector and reconstructing those clipped elements into a first modified column vector; (d) storing rows of pixels from a second captured digital image in memory; (e) using the rows of pixels to provide a second column vector; (f) providing a first order difference vector from the second column vector; and clipping the elements in the first order difference vector and reconstructing those clipped elements into a second modified column vector; (g) subtracting the first modified column vector from the second modified column vector to provide a refined difference vector; and (h) computing autocorrelations of the refined difference vector at specified lag(s) and using the autocorrelations to determine when an image capture device with a rolling shutter is in an environment having a flickering illuminant.