Light metering consisted of full-aperture through-the-lens metering, using a six zone silicon photocell. Metering is powered by a ring photocell so no battery is required. It used a rather dated and slow CdS (cadmium sulfide) photocell for metering. Light passes through the crystal and dial before reaching the photocell. The first Nikon F Photomic viewfinder, delivered since 1962, had an independent photocell. It consists of a photocell and an optical system placed in a small tube. The presence or lack of light, detected by a photocell, represented a one or zero. It used photocell optics and sonar to navigate. A photocell is positioned behind the disk at the 12 o'clock position. Kurt Kramer introduced a selenium photocell to medical research. The photocell picked up both the direct and reflected flashes. The plan works, and the team is able to repair the photocell banks. The reason for the delay was the photocell problem.