From Hertz’s Spark to Ultrafast Photoemission Patents, Outsiders and the Invention of Functions the Eye Never Had

Photoelectric detection did not merely replace the eye; it created measurement functions the eye has never possessed. This Perspective follows that transition from Hertz’s accidental observation in 1887 that ultraviolet light facilitates a spark discharge, through the quantitative work of Hallwachs and Stoletov, to German patent No. 66969, granted in 1892 to the schoolteachers Julius Elster and Hans Geitel for measuring light intensity with a photoelectric vacuum cell and allowed to lapse in 1895. This first patented photoemissive detector delivered an electric current proportional to the incident light, objective and recordable, five years before the electron was identified and thirteen years before Einstein, then a patent examiner in Bern, explained the effect. The article discusses the plausible but undocumented influence of the patent office on Einstein, follows the descendants of the Elster–Geitel cell into image sensors and photomultipliers, which made possible the tests of Bell’s inequalities with entangled photons recognised by the 2022 Nobel Prize, and shows that the pattern recurs in the laser era, where multiphoton, strong-field and attosecond photoemission resolve electron dynamics on timescales shorter than an optical cycle. The photoelectric streak camera and its attosecond counterpart, in which the optical field replaces the deflecting voltage, join detection and ultrafast photoemission in a single principle. Throughout this history, working technology has repeatedly preceded physical understanding.