Statement about Ground Observer Eye Safety The laser beam from CALIPSO expands as it travels away from the satellite. The rate at which it expands is controlled and selected so that a person on the ground is safe if exposed to the laser beam. The maximum permissible exposure (MPE), or the level of exposure below which is found to cause no harm to people, is established by the American National Standards Institute (ANSI). The CALIPSO instrument ensures that a ground observer is exposed to no more than half the MPE. At 532 nm the energy density at the surface is 0.5 times the ANSI Z136.1-1993 standard for MPE (0.5 mJ cm-2), presuming an observer with 50-mm collection optics. At 1064-nm the exposure is only 0.05 times the MPE.