Vacuum-UV large-area avalanche photodiodes from Advanced Photonix Inc. use a spectral enhancement to optimize their sensitivity at 157 nm, where they deliver a gain of 250 and can sense wavelengths as short as 100 nm. Applications include photolithography and spectroscopic techniques. The all-solid-state devices, which offer advantages over PMTs, have quantum efficiencies up to 120% and are supplied windowless, with active-area diameters of 3, 5, 10 and 16 mm. They are compact and offer higher spatial uniformity than PMTs and immunity to stray magnetic fields. They also provide a greater dynamic range, no settling time and simple driving circuitry. The large-area APD is not overloaded or easily damaged by exposure to bright light.