The Mark I, also known as the SLAC - LBL Magnetic Detector, was a particle detector that operated at the interaction point of the SPEAR collider from 1973 to 1977. It was the first 4π detector, i.e. the first detector to uniformly cover as much of the 4π steradians (units of solid angle) around the interaction point as possible with different types of component particle detectors arranged in layers. This design proved quite successful, and the detector was used in discoveries of the J/ψ particle and tau lepton, which both resulted in Nobel prizes (for Burton Richter in 1976 and Martin Lewis Perl in 1995). This basic design philosophy continues to be used in all modern collider detectors.