Compact fluorescent lamps (CFL) came on the market in 1981. Dutch lamp-maker Philips introduced two different versions with features intended to appeal to different markets. Ultimately both succeeded and are still produced today.
This unit, a model PL-7/9, dates from about 1983 and was the second of the two types. The key development seen in this lamp is the small glass connection between the two parallel tubes called a bridge weld. Making the bridge weld proved difficult but not as difficult as the glasswork in some other proposed CFLs. This glasswork combined with a new, more durable type of phosphor made the PL lamps practical. The PL reportedly stood for Pi Lamp since the two connected tubes resembled the Greek letter.
Philips intended this lamp for commercial use and included several design features not used on their consumer-grade SL lamps. First, the unit is modular: the ballast and tube assembly can each be separately replaced reducing the cost of replacing a failed lamp. Also, the tube assembly has a special plug-in socket, not a standard screw-in socket. This prevented theft of the new, expensive lamps, since only commercial-grade fixtures had the plug-in sockets. And it cut down on the rate of "snap-back," or replacement of a failed compact fluorescent with an old-fashioned, inefficient incandescent lamp.
Lamp characteristics: A modular unit with three separable components—an adapter, a twin-tube lamp, and a ballast. Adapter: nickle-plated brass medium-screw base-shell with a brass retaining ring. A plastic insulator is part of the adapter housing. The base-shell rotates in one direction to prevent over-tightening of lamp when it is inserted into the socket. Top of the adapter has a socket for a G23-based lamp, and the back of the adapter is slotted so the ballast module can slide on. Two small holes near the top are ports for the ballast pin-connectors. Lamp: G23 base with brass pins, plastic housing, and aluminum skirt. Two arc tubes connected by a glass bridge. Internal coating of rare-earth/alumina phosphors. Ballast: Magnetic coil-core ballast in a plastic housing. Two pins are on front to connect to adapter, and a yellow potting material is evident on bottom.
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