The reservoir is the coolest part of the arc tube - excess sodium-mercury condenses and collects at this point.

Figure 9.17. Lucalox lamp operation.

The Lucalox operating principles are shown in Figure 9.17.

The electrodes are double layer tungsten coils with rare earth oxides like other HID lamps. Lucalox lamps have no starting electrode. The external control gear includes an ignitor that provides a high voltage pulse to start the arc. (This same principle is used for the European type metal halide lamps). The starting pulse for a 400W lamp is 2500-4500 volts and lasts for less than 1 microsecond. The short duration means little energy is used but is sufficient to ionise the xenon gas. Once the current flows through the ionised gas the heat from the arc evaporates the amalgam mixture. The warm-up period of high pressure sodium lamps is longer than for most metal halide lamps, around 5 minutes, but conversely the hot re-strike period is shorter, typically less than one minute.

As the sodium vaporises during warm-up, the colour shifts from blue to orange to golden white as the pressure rises. Colour changes very little throughout lamp life. Lucalox lamps offer several advantages over mercury and metal halide lamps. High pressure sodium lamps have good lumen maintenance, see Figure 9.18. With Lucalox XL the end-of-life lumen output is 90% of the initial lumens.

Figure 9.18. Lumen maintenance comparison.