Apparatus and associated methods relate to determining the wavelength of a narrow-band light beam. Two portions of the narrow-band light beam are projected onto two dissimilar photodetectors, respectively. The two dissimilar photodetectors have dissimilar spectral responses over a domain of wavelengths that includes the wavelength of the narrow-band light beam. Each of the two dissimilar photodetectors generates an output signal indicative of a photocurrent induced by the projection of the portion of the narrow-band light beam thereon. A ratio of the differences between the photocurrents to the sum of the photocurrents of the two dissimilar photodetectors is determined. The determined ratio is a monotonic function of wavelength over the domain wavelengths including the wavelength of the narrow-band light beam. The determined ratio is thereby indicative of the wavelength of the narrow-band light beam.