Variable wavelength detector: 

It is usually a wide band pass uv-visible spectrophotometer coupled to a chromatographic system. It offers a wide selection of uv and visible wavelength range. A versatile detection system is based on a spectrophotometer fitted with a grating monochromator and continuum source e.g. deuterium lamp for the uv region and and a tungsten- halogen lamp for the visible region as shown in Figure. These have double beam optics, stable low noise electronics and are microprocessor controlled. Some can be programmed to select a sequence of optimum monitoring wavelengths during chromatographic runs and recording of a complete uv spectrum.

Figure: A schematic diagram of a variable wavelength detector