The focal plane is thus located outside of the eyepiece and is hence accessible as a location where a graticule, or micrometer crosshairs may be placed.
Traditional welding helmets use a sheet of smoked glass for the eyepiece, cutting down on the dangerous glare from the welding process itself, but also reducing overall visibility.
A retinal scan is performed by casting an unperceived beam of low-energy infrared light into a persons eye as they look through the scanner's eyepiece.
