1. An apparatus for measuring the spectral composition of an image comprising: (a) a diffractive element for dispersing spectral components of an image and directing said spectral components into a volume having a cross-sectional area A and a length L; (b) a photodetector array comprising a planar surface having a plurality of discrete light detectors deployed thereupon, said photodetector array having an area substantially equal to A; and (c) means for changing the distance between said diffractive element with respect to said photodetector array in the direction of L; (d) means for measuring and storing the distance between said diffractive element and said photodetector array; (e) means for measuring and storing the intensity of spectral components of the image which impinge upon said photodetector array; and wherein said spectral composition of said image is determined by computing the intensity of the spectral components impinging upon said photodetector array as a function of the distance between the diffractive element and the photodetector array.