A lock‐in amplifier uses demodulation techniques to extract the component of an electronic signal which has the same phase and frequency as an oscillating reference signal. We have used a digital image processor to perform this demodulation simultaneously on all pixels in a sequence of video‐taped images. To demonstate the use of this technique, we extract, from a recording of a hydrodynamic flow pattern exhibiting complicated spatial and temporal behavior, an image of a spatially localized mode which oscillates at a discrete temporal frequency. Demodulation at synthesized mixing frequencies can reveal the spatial dependence of the nonlinear interactions between different localized modes.
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