An image sensor frame rate can be increased by “interlaced” mode operation whereby only half the number of lines (alternating between odd and even lines) of an image is transported to the readout circuitry. This halves the integration time but also halves the resolution of the sensor. The reduction is tolerable for motion characterization as long as sufficient image resolution remains. Accordingly, in one embodiment, an image sensor operated in an interlaced fashion is first exposed to a scene under a first illumination form (e.g., narrowband illumination), and a first set of alternating (horizontal or vertical) lines constituting half of the pixels is read out of the array; the sensor is then exposed to the same scene under a second illumination form (e.g., existing ambient illumination with the illumination source turned off), and a second set of alternating lines, representing the other half of the pixel array, is read out.