An accessory used with an optical instrument or a detector of electromagnetic radiation to narrow down the wavelength band passing into the instrument. Colour filters are used as an aid to improving contrast and bringing out particular features in the visual observation of planets, and also in multicolour photometry. A very narrow-band filter may be used to isolate, for example, the hydrogen alpha spectral line, particularly for making observations of the Sun. Filters are also used to help eliminate unwanted light pollution in certain types of observation.