Likewise a switch for the exposure meter was also dropped. The Canon FP without exposure meter was added later the same year. Historically, a photometer was separate from the camera and known as an exposure meter. Batteries were only needed for the exposure meter and LCD. Exposure is often determined using a reflected-light exposure meter. Exposure meter calibration is discussed in detail in the Light meter article. Retina IIIc was introduced in summer of 1957 with a single range exposure meter. For most cameras, this means using an on-board TTL exposure meter. The exposure meter was always on, even with the lens inserted. The exposure meter is a TTL open aperture centre-weighted type. The non-coupled selenium cell exposure meter reads out in exposure values (EVs) only. The exposure meter uses a PX625 mercury battery, which is now discontinued. Its TTL exposure meter requires batteries, but batteries are not otherwise needed. In each case, the actual exposure level is still determined by the camera's exposure meter. It also has a through-the-lens exposure meter controlling a needle visible in the viewfinder.