low-pressure mercury lamp (arc)

Resonance lamp that contains mercury vapour at pressures of about $0.1 Pa$ ( $0.75 × 10 −3 Torr$ ; $1 Torr = 133.33 Pa$ ). At $25 °C$ , such a lamp emits mainly at $253.7$ and $184.9 nm$ . They are also called germicidal lamps. There are cold- and hot-cathode as well as cooled electrode-less (excited by microwaves) low-pressure mercury lamps. The Wood lamp is a low-pressure mercury arc with an added fluorescent layer that emits in the UV-A spectral region ( $315$ – $400 nm$ ).