The invention provides a method and apparatus for determining the wavelength of a sample source of light, the apparatus having a reference light source of known wavelength, a collimator for collimating light from the sample source and from the reference source, a dispersing means for receiving and spatially dispersing collimated light from the collimator according to wavelength, focusing means for focusing dispersed light from the dispersing means, and a photodetector located in the focal plane of the focusing means and having an aperture for spatially selectively admitting light from the focusing means, and operable to provide a temporally calibratable output signal indicative of the wavelength of the selectively admitted light, wherein the apparatus is operable to scan the focused spatially dispersed beam across the aperture, and the photodetector output includes resolvable features corresponding to light from the reference source and sample source, whereby a time difference between the features is indicative of a wavelength difference between the light from the reference source and the sample source.