Penning discovered many years ago that a discharge whose characteristics depend on maintenance of a population of metastables can have its operation altered drastically when the metastables are removed by incident light. We have re‐examined the effect in the light of more modern techniques with a view toward developing a highly efficient, narrow‐band photodetector. A detector has been built which has sensitivity for the 20 581‐A line of He comparable with that of commercially available detectors, but an acceptance bandwidth of only 0.1 cm−1.