Diode lasers are different from other types of lasers in that they almost always incorporate a lens to create a beam that is somewhat collimated. Due to the physical size of the diode source, some of these laser systems qualify as extended source lasers according to the ANSI Z136.1 Standard for Safe Use of Lasers, and the IEC 60825-1.2 international laser safety standard. The maximum permissible exposure (MPE) for these systems may be increased several fold over solid state or gas lasers producing similar irradiances or radiant exposures on the cornea. The increase in MPE is based on the apparent visual angle of the source (or source size). Verifying the magnitude of the increased MPE has been the topic of much discussion in recent years, and no official standardized method for determining source size has been outlined in the various standards.
One of the difficulties with measuring source size is that it doesn’t remain constant, but depends on factors such as viewing distance and optical aids, and may even vary based on ambient illumination. Also, no one measurement technique may work for all extended laser sources. In addition, for diode lasers, the apparent source is a virtual image of the emitting diode and usually appears to be located several meters behind the laser exit port. Diode lasers that have an external beam waist add more complexity.