This paper presents a method for automatic exposure time adjustment for multispectral and hyperspectral cameras. The method presented here is based upon a spectral power image. Here, we use the photopic response function due to its widespread usage in photography and psychophysics. Note that, however, the method presented here is quite general in nature and can employ a number of spectral sensitivity functions for the computation of the spectral power image. Making use of this spectral power image, the exposure time is then computed via iterative updates so as to minimize the squared error between a target image and the current spectral power yielded by the imager. This target image is recovered in a straightforward manner using histogram equalization and the Commission Internationale de l’Éclairage (CIE) photopic function. This, in turn, yields an automatic method devoid of calibration targets or additional inputs. We perform a stability and controllability analysis of our method using a state-space representation. We also show the applicability of the method for exposure time calculation on staring array and multicharge coupled device architecture cameras on real-world scenes.