Background to the Invention Fluorescent lamps are preferred for use in road vehicles such as goods trailer vehicles because of their higher efficiency, resulting in a much lower drain on the vehicle's battery for a given light output. However, for temperature-controlled or refrigerated vehicles, fluorescent lamps have generally not been usable because, at best, their light output is substantially reduced at lower temperatures, and a long period of warming up of the tube is required before an adequate light output is achieved. At worst, the tube fails to start at low temperatures. The use of fluorescent lamps in such vehicles would be desirable not only because of the lower drain on the battery, but also because substantially less heat is emitted by such lamps as compared with tungsten lamps, so that the use of fluorescent lamps would have a smaller adverse effect on the temperature control of the vehicle's interior.

It has been proposed in US 5 768 898 to provide an auxiliary heater in the lamp enclosure to pre-heat the tube before starting, for refrigerator use. This leads to a significant delay before the lamp starts and light is provided, and would not be ideal for a battery-powered system in a vehicle, since the heater would add to the drain on the battery without adding initially to the light output.

US 5 903 105 discloses a fluorescent lamp for refrigerator use which includes a light-up detector to determine whether the tube has started and to repeat the start-up, including the pre-heating step, until satisfactory operation is achieved.

Summary of the Invention According to the invention, there is provided a fluorescent lighting unit for low- temperature use, comprising an enclosure with at least one light-transmitting face and containing a fluorescent lamp and an incandescent lamp, each lamp being connected to a power supply connector, and means for opening the circuit between the incandescent lamp and the power supply connector when the temperature in the enclosure exceeds the minimum operating temperature for the fluorescent lamp.

The minimum operating temperature of the fluorescent lamp is the temperature below which reliable operation with acceptable light output cannot be achieved. This will typically be at least 0 degrees C for a small tube such as is used in a low voltage DC unit.

Preferably, the enclosure also contains a control circuit for controlling the operation of the fluorescent lamp. For low voltage DC operation, this will include a transistor inverter.

Where the vehicle's battery power supply is also subject to additional loads which affect the voltage delivered to the lighting unit, for example an electrically-driven tail lift, the lighting unit will also comprise a voltage regulator. This is preferably also contained within the enclosure. It is desirable to include all the electrical components within the enclosure so that all thermal losses from the electrical components contribute to raising the temperature of the fluorescent lamp, thereby increasing energy efficiency. For the same reason, it is preferred to mount the fluorescent lamp as close as possible to the control circuit.

A thermal fuse may also be incorporated into the circuit supplying power to the incandescent lamp to ensure that, if the thermal switch fails to operate, and the temperature continues to increase within the enclosure, the incandescent lamp will be cut out.

The incandescent lamp is preferably a tungsten halogen lamp, since these give a high light output on a low voltage supply, and are relatively compact, thereby taking up only a small amount of room in the enclosure. More than one incandescent lamp may be included in the enclosure if required to increase initial light output and the rate of warming of the fluorescent lamp.

The fluorescent lighting unit of the invention is particularly useful in temperature- controlled vehicles, where light is required instantly for the purposes of unloading, but where the efficiency of fluorescent lighting units is particularly advantageous to provide high illumination with low battery drain.

Brief Description of the Drawings In the drawings, which illustrate exemplary embodiments of the invention : Figure 1 is a perspective view of a lighting unit with its cover removed; and Figure 2 is a diagram illustrating the electrical components of a modified version of the lighting unit shown in Figure 1.

Detailed Description of the Illustrated Embodiment Referring first to Figure 1, the lighting unit comprises a cylindrical enclosure 1 having at the open end the circumferential flange 2 whereby the unit may be secured in a ceiling panel of a vehicle, for example, with the enclosure 1 located in a circular cut out in the ceiling. A translucent cover plate (not shown) is secured over the flange with fixing screws engaging in internally threaded posts 3. A fluid tight seal on the cover plate ensures that moisture is excluded from within the enclosure 1. Mounted within the enclosure is a miniature fluorescent lamp 4 of the type comprising two U-tubes joined together at one end, mounted on top of control circuit components 5 which include a transistor inverter and a relay circuit which delays application of a voltage across the lamp for a short time to allow the heater filaments to reach their working temperature before striking the arc, in conventional manner.

A tungsten halogen lamp 6 is mounted in a socket 6a adjacent to the fluorescent lamp 4. The tungsten halogen lamp 6 is connected in series with a thermal switch 7 and a thermal fuse 8, the resultant circuit being connected in parallel with the fluorescent lamp and its control circuit components 5 to a DC supply lead 9 which enters me enclosure through a fluid tight grommet. The supply lead 9 is connected to the vehicle's 24 volt battery supply outside the enclosure 1. It will be understood that, while 24 volts is typical for a goods vehicle's battery supply, the lighting unit of the invention may readily be adapted to operate at 12 volts, for example, or at voltages higher than 24 volts.

The thermal switch 7 is adapted to open when the ambient temperature within the enclosure is at the minimum operating temperature of the fluorescent lamp 4, typically. In practice, this will depend on acceptable light output from the fluorescent lamp. Typically, lamps of the type with which the invention is concerned will give out only about 2-5% of maximum output at-20 degrees C; at-10 degrees this rises to about 25%, while at 0 degrees C the output is about 50% of maximum. Full output is

achieved at around 25-30 degrees. It has been found that an acceptable compromise between light output and electrical current consumption is achieved by setting the thermal switch 7 to open at above 0 degrees, for example in the range 0-10 degrees. It will be understood that exact light outputs at different temperatures will vary from one type of fluorescent lamp to another, but switching off the incandescent lamp (s) when the fluorescent lamp's output exceeds about 50% of maximum will generally be satisfactory.

In the event of failure of the thermal switch 7, the thermal fuse 8 prevents overheating of the enclosure and the components therein by opening the circuit to the tungsten halogen lamp 6 at a predetermined higher temperature.

In use, when electrical power is supplied to the unit the tungsten halogen lamp immediately light is to supply illumination as a high level, and to heat the fluorescent lamp 4. When the temperature in the enclosure reaches a level at which satisfactory operation of the fluorescent lamp is ensured, the thermal switch 7 opens, switching off the tungsten halogen lamp 6. At this stage, the heat generated by the fluorescent lamp and its associated components is sufficient to ensure its continued satisfactory operation.

Figure 2 illustrates diagrammatically the layout of the electrical components of the unit illustrated in Figure 1, but modified to include a voltage regulator to improve performance of the unit when used in environments where the power supply is subject to voltage fluctuations, for example through the use of other electrically-powered equipment associated with a trailer vehicle, for example a tail lift. The DC supply leads from the vehicle's battery supply are connected to the voltage regulator board 10, from where parallel circuits supply the tungsten halogen lamp 6 and the fluorescent lamp 4. In Figure 2, only the connector end of the fluorescent lamp 4 is shown for the sake of clarity. This is connected into a terminal block 11.