Due to the acceleration experienced in the bending magnets or insertion devices, the electrons emit a narrow fan of intense white (polychromatic) radiation ranging from soft UV to hard X-rays over a very tightly defined angle tangential to the ring. The radiation is 'tunable' by cutting out fine bands (few eV or 10-5 Å wide) of wavelengths appropriate for particular experiments with monochromator crystals that selectively pass the wavelength of choice.