Electron energy analyzers measure the number of ejected electrons as a function of the electron energies. The analyzers must be located in a high vacuum chamber and isolated from stray magnetic fields (including the earthís) that deflect electrons. Past Auger spectrometers used several types of electron energy analyzers, including spherical sector and cylindrical mirror analyzers. However, modern instruments nearly always incorporate cylindrical mirror analyzers because their high transmission efficiency leads to better signal-to-noise ratios. The schematic shows a cross section of a cylindrical mirror analyzer in red. The primary electron beam hits the sample surface at the source point of the analyzer. Auger electrons move outward in all directions and some pass through the grid covered aperture in the inner cylinder. A variable negative potential on the outer cylinder bends the Auger electrons back through a second aperture on the inner cylinder and then through an exit aperture on the analyzer axis. The energy of transmitted electrons is proportional to the voltage (-V) on the outer cylinder.