A sensing device includes an aperture structure having an aperture and imaging optics configured to direct polychromatic light toward the aperture. The imaging optics separates the light according to spectral range longitudinally along a first axis The aperture substantially transmits a spectral range of the light and substantially blocks other spectral ranges of the light. An optical detector is arranged to receive the spectral range of the light that is transmitted through the aperture. The optical detector generate an electrical output that corresponds to a centroid of the spectral range of the light.