General Electric PR-1 Exposure Meter c.1950s
Exposure meters such as the PR-1 relied on Selenium. This GE meter was known for its size and portability, the point and lock feature of the needle, and the new trident analyzer.
A trident analyzer—an indicator with three markers—allowed the photographer to align the F-stop with either the accurate light reading, one F-stop higher, or one F-stop lower. Since light meters are not completely accurate, making additional exposures that are one stop up and one stop down from the given reading compensate for possible miscalculation by the meter. This process is called bracketing.