This type of UV detector only "sees" one UV wavelength at a time. First, the light from the deuterium lamp strikes the diffraction grating. The grating then spreads the light into a spectrum. The filter between the grating and the sample cell determines which wavelength is allowed to strike the sample. Varying this filter will vary the wavelength of light that strikes the sample. The photomultiplier tube detects the light passing through the sample and responds accordingly. The response from the photomultiplier is recorded.