1967 Philips GR60D

When Philips released its first green fluorescent indicator glow lamps in 1965, they classified all nine models as high-brightness sources. This included the small GR60D shown here, which was designed for a direct operation on 220 V AC mains thanks to the ballasting resistor integrated in its EX10 end cap, limiting the discharge current to 1.5 mA. This corresponds to a power dissipation of a third of a Watt only, and the “high brightness” label was certainly added because of the lamp’s spectral emission peaking at 528 nm, near the eye’s maximum sensitivity at 555 nm. This is achieved by using a green-emitting zinc silicate phosphor activated with divalent manganese and excited by the deep ultraviolet emission from a glow discharge burning in a low-pressure neon-xenon atmosphere. Such gas mixture has several interesting characteristics, such as a low breakdown voltage due to Penning reactions between neon and xenon atoms, and a light spectral emission dominated by xenon due to its lower atomic energy levels. This results in a relatively efficient emission of shortwave radiation with a much weaker red light output than from classic neon(-argon) glow discharges. The latter is crucial to the realization of a saturated green light emission from the fluorescent lamp.