Is the use of radar detectors legal in California?

A radar detector is an electronic device used by motorists to determine if their speed is being monitored. The term originates from early speed-detection technologies, in which police bounced a radio wave off a moving vehicle with a device called a radar gun that determined the vehicle's speed by the Doppler-effect-moderated change in the wave's frequency. Most of today's radar detectors detect signals across a variety of wavelength bands - usually X, K, and Ka bands (and Ku, in Europe, also recently approved for use in the U.S).