a field - of - operation illuminating device has at least one discharge lamp concentrically surrounded by several halide incandescent lamps at its bottom , which faces the field of operation . the discharge lamp is accommodated in a stationary reflector and illuminates the recesses of the field . the incandescent lamps are mounted in reflectors that can be adjusted so that their beams will overlap in the field . in the event of power outage or failure of the mains , power is supplied to the lamps from a battery - supported substitute source . the incandescent lamps , due to their thermal inertia , continue to burn almost uninterruptedly , whereas the discharge lamp remains off long enough to cool - down before being restarted . the advantages are less heat during normal operation and immediate availability of the incandescent lamps in an emergency .