Suspended lighting fixtures have well known applications in commercial, industrial and instituional buildings.  Many of these fixtures provide both primary lamps which are normally illuminated and secondary lamps which provide lighting in the event power is interrupted to the primary lamp.  See, for example, the industrial luminaires shown in Holophane Industrial Lighting Products Digest, HL-693.  These luminaires include an integrally mounted ballast having a stand-by lighting circuit to provide incandescent light in the event power is interrupted to the primary high intensity discharge lamp.  This circuit has been incorporated by applicant in these luminare designs for the last several years. 
See also, U.S. Pat.  No.  4,451,762 to Forte et al.,  which discloses a control circuit and system for providing emergency lighting from a single electrical power source.  As disclosed by Forte, the circuit is used only for special circumstances where both an HID lighting fixture and an incandescent stand-by light source--either in the HID lighting fixture or in a separate adjacent fixture--are powered from the same single wiring circuit which is being fed through a battery back-up power supply system which produces a square wave output voltage.  The purpose of the circuit is to detect whether the input voltage has a sinusoidal or a square wave shape.  Because the HID lamp will not operate properly with a square wave input, the circuit designed to switch power from the HID fixture to the incandescent stand-by source.  When external power is restored to the battery back-up power supply system, a sinusoidal voltage wave will again be supplied which will be detected by the circuit to switch power back from the incandescent stand-by source to the HID lighting fixture. 
Although functional, the fixtures disclosed above have proven architecturally unattractive and thus inappropriate for many indoor applications such as offices, classrooms and retail stores.  Fixture designers have thus turned their attention toward the development of luminaires which may be suspended from drop ceilings such that all but the suspended optical assemblies are hidden from view within the recessed plenums.  See, for example, U.S. Pat.  No.  3,860,829 to Fabbri, which discloses a fluorescent lighting fixture having an auxiliary bulb which operates as an emergency light.  Similarly, U.S. Pat.  No.  4,890,200 to Mandy discloses a down lighting system for elevators which includes standard and emergency lights.  For aesthetic purposes, these fixtures have typically been designed for support within their corresponding recessed plenum.  Thus, in drop ceiling applications, prior art fixtures have been designed for support by conventional networks of intersecting cross-members. 
In applications where HID-type lamps are used as the primary lighting source, those skilled in the art have recognized a need for ballasts to provide the proper voltage and wave-shape.  Typically, such ballasts have been wired directly to electrical distribution systems distant from the suspended fixture.  See, however, U.S. Pat.  No.  4,361,992 to Rapp which discloses a support base for carrying a ballast and an electrical junction box above a ceiling tile and for further supporting a side-mounted bracket for suspending a luminare.  Significantly, however, Rapp does not make any provision for a secondary lamp or a stand-by lighting circuit.