Optical Bandpass filters (BP Filters) are used in spectral photometry, clinical diagnostics, chemical analysis, colourimetry, astronomy, and numerous applications where spectral isolation is required. A Band-pass Filter transmits light only within a defined spectral band ranging from less than one to many nanometers wide and can be made by combining a Long pass and a Short Pass filter. Examples of bandpass filters are the Lyot filter and the Fabry-Pérot interferometer. The Dutch Open Telescope and Swedish Solar Telescope are examples where Lyot and Fabry-Pérot filters are being used.