Editing Image intensifier (section)

===Generation 3: high sensitivity and improved frequency response===
[[Image:Gen3-Image-Tube.jpg|thumb|right| A third generation Image Intensifier tube with overlaid detail]]
While the third generation of tubes were fundamentally the same as the second generation, they possessed two significant differences. Firstly, they used a [[gallium(III) arsenide|GaAs]]—[[cesium oxide|CsO]]—[[aluminium gallium arsenide|AlGaAs]] photocathode, which is more sensitive in the 800&nbsp;nm-900&nbsp;nm range than second-generation photocathodes. Secondly, the photocathode exhibits negative [[electron affinity]] (NEA), which provides photoelectrons that are excited to the conduction [[band theory of solids|band]] a free ride to the vacuum band as the Cesium Oxide layer at the edge of the photocathode causes sufficient [[band theory of solids|band]]-bending. This makes the photocathode very efficient at creating photoelectrons from photons. The Achilles heel of third generation photocathodes, however, is that they are seriously degraded by positive ion poisoning. Due to the high electrostatic field stresses in the tube, and the operation of the MicroChannel Plate, this led to the failure of the photocathode within a short period - as little as 100 hours before photocathode sensitivity dropped below Gen2 levels. To protect the photocathode from positive ions and gases produced by the MCP, they introduced a thin film of [[sintered]] [[aluminium oxide]] attached to the MCP. The high sensitivity of this photocathode, greater than 900 μA/lm, allows more effective low light response, though this was offset by the thin film, which typically blocked up to 50% of electrons.