In recent years, more advanced UV-VIS detectors have been increasingly used, based on diode arrays and entire spectrum collection at any given moment of data collection.  The are called Diode Array detectors, and they collect entire UV spectra of every point the eluting peaks, while operating as a multi-wavelength UV-vis detector. This way they give additional information, which help understand more about the nature of the substances appearing in the chromatogram and allow their identification. Since the detector facilitates better peak identification it is the preferred detector for HPLC method development.
A schematic of the optical systems is shown in Figure 1. The variable UV-vis absorbance detector's optical bench is showing how the flow cell is positioned after the optical system, including the monochromator, which typically has a physical slit and a moving grating, so it is illuminated by a selected wavelength, reaching a photo-diode. The bench of the diode array detector, however, is configured so that the flow cell is positioned before the optical parts, so that the beam containing the entire spectrum is passing through it.  The optical parts consist also with a monochromator and a slit, but with a fixed grating, which disperses the light onto a diode array imaging element.