A system for measuring one or more characteristics of light of a photon energy Eph from a light source, that can be determined from measuring three-photon absorption events, the system comprising: a) a detector having a band gap material characterized by gap energy between 2.1 and 3 times Eph; b) an optical element configured to concentrate a beam of light from the light source on the detector; c) a signal amplifier that amplifies an output signal indicative of when three photons produced by the light source undergo a three-photon absorption event in the band gap material; and d) an analyzer that analyzes the output signal to count or measure a rate of the three-photon absorption events, and determines the one or more characteristics of the light from the light source.