A method of operating a spectrometer to determine the wavelength of an optical signal, in particular for determining the resonant wavelength of an optical fiber Bragg grating. The spectrometer comprises an array of photosensitive pixels each of which generates an output signal in response to the intensity of light incident on the pixel, and a refractive element arranged to direct light to a particular position in the array depending on the wavelength of the light. The method involves selecting a first group of pixels in the array by reference to an expected wavelength distribution of the optical signal and monitoring the output signals from the first group of pixels. On the basis of the output signals from the first group of pixels a second group of pixels is selected and the wavelength of the optical signal is determined from the output signals of the second group of pixels. The method has the advantage that the wavelength of the optical signal can be determined without scanning every pixel in the array individually.