41—the apertures 46 being shown diagrammatically in FIG. 8. The apertures 46 lead to spaces or channels 54 defined between the display panels 2 and their respective glasses 27. The internal air (A) continues to flow downwardly until it passes through apertures 47 (represented diagrammatically) in the top of the lower box 42. From the lower box 42, the internal air (A) flows upwardly through further apertures 47 into the bottom of the tubes 34, through the tubes 34 and back into the upper fan box 41 via apertures 46. The internal air (A) continues to circulate in this way, flowing over the fronts of the display panels 2 to provide cooling of the display panels.

As indicated above, external air (B) is drawn continuously into the top of the second enclosure 32 by the fans 10, which cause the external air to flow over the tubes 34 and panels 35, 36 of the heat exchange unit 30, before the external air is then exhausted through the bottom ducts 33. Where the external air (B) is cooler than the internal air (A)—which is a typical situation—it serves to cool the internal air as the external air 38 flows over and around the heat exchange unit 30.

As may be seen in FIGS. 4 and 8, external air 38 not only passes over and around the heat exchange unit 30, it passes over the metal plates 22 that are in close contact with the backlights 21 at the back of the display panels 2. As the metal plates 22 are thermally conductive, the external air 38 serves to provide cooling of the backlights 21 and thus further cooling also of the display panels 2.

Whilst the display panels 2 are shown in FIG. 4 as being in contact with their respective backlights 21, it is alternatively possible for there to be a gap between each display panel 2 and backlight 21. However, in such a case, internal air is not forcibly circulated through such a gap, which need not be sealed. Cooling of the display panels 2 is principally by internal air flowing over the fronts of the display panels 2 and, as just described above, by external air 38 flowing over the metal plates 22.

It may be appreciated that the heat exchange unit 30 is of relatively simple construction and may therefore be produced at relatively low cost. Nevertheless, it may be very effective. Because the internal and external air flows are in opposite directions, excessive variations may be avoided in the temperature differences between the internal and external air. That is, when internal air enters the bottoms of the tubes 34, it is at its warmest, having passed over the display panels 2. When the internal air is at the top of the tubes 34, it is at its coolest, having been cooled by heat exchange with the external air 38 via the heat exchange unit 30. When the external air is at the top of the heat exchange unit 30, it is at its coolest, having just been drawn into the second enclosure 32 via upper ducts 33, from outside the display 1. When the external air 38 is at the bottom of the heat exchange unit 30, it is at its warmest, having exchanged heat with the internal air via the heat exchange unit 30. Thus, at the bottom of the heat exchange unit 30, both the internal air and the external air are at their warmest. At the top of the heat exchange unit 30, both the internal air and the external air are at their coolest. Hence, variations in the temperature difference between internal air and external air at any point are minimised. This reduces the likelihood of hot spots or cold spots on the heat exchange unit 30. Since the temperature difference across this counterflow heat exchanger may be substantially constant, then this maximises the heat transfer from internal to external air.

Another significant feature of the illustrated embodiment is that the overall dimensions of the heat exchange unit 30, or of the second enclosure 32 that contains the heat exchange unit 30, are comparable to the overall dimensions of the display panels 2. This means that a single heat exchange unit 30 is sufficient to provide cooling of the display panels 2.

For example, as may be seen in FIG. 4, the overall width of the heat exchange unit 30, from left to right as seen, is a little less than the overall width of the display panels 2, from left to right as seen. As may be seen in FIG. 8, the overall height of the heat exchange unit 30, from top to bottom as seen, is substantially the same as the overall height of the display panels 2, from top to bottom as seen.

In more general terms, the overall width of the heat exchange unit 30, from left to right as seen, may be between 80% and 100% of the overall width of the display panels 2, from left to right as seen. The overall height of the heat exchange unit 30, from top to bottom as seen, may be between 80% and 100% of the overall height of the display panels 2, from top to bottom as seen.

Alternatively, the two-dimensional area of the heat exchange unit 30 as seen in front elevation (e.g. FIG. 7) may be between 65% and 100% of the two-dimensional area of the display panels 2 as seen in front elevation.

The above comments as to the width of the heat exchange unit 30 may alternatively apply to the width of the heat exchanger that is made up of the second enclosure 32 and the heat exchange unit 30. That is, even if the heat exchange unit 30 were to be less wide, the width or area of the second enclosure 32 would still bear a relationship to the width or area of the display panels 2, as set out above.

As best seen in FIG. 7, the housing 3 defines side compartments 39 that are part of the first enclosure 31. Local electronic control components 56 are conveniently located in the side compartments 39. Whilst most supply and control components are provided in the separate ECA 60, which is described in further detail below, the local control components 56 are provided where they need to be close to the display panels 2 or associated components.

In order to provide cooling of the local control components 56, a small amount of internal air is allowed to pass through upper bleed holes 43 in the upper fan box 41 and flow downwardly through the side compartments 39 and through lower bleed holes 44 in the lower box 42, where it mixes with the main internal airflow that is returned upwardly through the tubes 34. Further cooling may be provided by direct thermal contact of hot components 56 with the side wall of the housing 3, which in turn is in contact with surrounding ambient air.

Whilst the illustrated embodiment is intended principally for cooling of the display panels 2, there can be occasions when the ambient temperature is rather low and it is desirable to warm up the electronic display 1 either before or during a switch on sequence. To this end, one or more heater 45 can be provided conveniently in the side compartments 39 such that, with the internal air circulating via fans 40, but with the external air fans 10 temporarily switched off, the internal air may be warmed up by the heaters 45.

Although it may be convenient to locate heaters 45 in the side compartments 39, one or more heater 45 may be located in another part of the internal airflow—for example, in the upper fan box 41 and/or the lower box 42.

In an alternative configuration, instead of using internal air, external air may be bled from the main flow to pass through side compartments such as 39 and then be exhausted back into the ambient atmosphere, either with the main flow of external air or directly.

Whilst plates 35, 36 are shown as connecting and projecting from the sides of the tubes 34, connecting elements of alternative configurations may be employed. For example, in order to increase surface area, the plates 35, 36 may be of zigzag or undulating shape. The plates 35 and/or 36 may be replaced by further tubes for the passage of external air. The plates 35 and/or 36 may be omitted, for a simpler configuration.

Whilst the illustrated embodiment and above description refer to internal air passing through tubes 34 and external air passing over and around the tubes 34, an alternative configuration is for external air to pass through tubes such as 34 and internal air to pass over and around those tubes.

A particular advantage of the illustrated embodiment is that the electronic display 1 may be made relatively thin. Here, the counterflow configuration of the heat exchange unit 30 allows a much thinner construction than may be required with a crossflow heat exchanger which, by its very nature, tends to have a more three-dimensional construction. Also, as indicated above, as the overall outline of the heat exchange unit 30 and the second enclosure 32 is comparable to the overall outline of the display panels 2, just a single heat exchange unit 30 can provide adequate cooling for the display panels 2 and any local, associated components. Moreover, it is of note that, in contrast to previous proposals, there is no forced air circulation between the display panels 2 and their respective backplates 21—further contributing to a relatively thin construction.

Whilst a counterflow configuration of the heat exchange unit 30 may be particularly advantageous, to obtain effective temperature differentials between internal and external air as mentioned above, alternative embodiments of the invention may adopt a parallel flow configuration, in which both internal and external air pass through and around the heat exchange unit 30 in the same direction. Such a configuration may contribute to making the electronic display 1 relatively thin.

Even with the counterflow heat exchange unit 30, a parallel flow configuration may be selectively adapted for use in cool environments. To this end, the external fans 10 are turned off, as has been mentioned above. As the external air 38 is heated by the heat exchange unit 30, it is caused to rise by convection and is outlet via the upper ducts 33. In this case, both the internal air and the external air flow in parallel through the heat exchange unit 30, from the bottom to the top of the display. External air enters through the lower ducts 33. With reference to FIG. 7, the direction of flow indicated by the arrows B is reversed.

This option allows for reduced cooling and so may be suitable for use in cool environments.

In yet cooler conditions, it may be desired to conserve heat within the electronic display 1 and/or to provide a certain amount of heating. To this end, the upper ducts 33 are provided with closable vents 52, to control the flow of external air 38.

With the external fans 10 turned off and the vents 52 closed, the flow of external air through the heat exchange unit 30 is substantially reduced or stopped. Heat generated within the electronic display 1—for example, by the backlight 21—is retained within the display. One or more heater 45 may be activated, according to needs. One or more additional or alternative heater 45′ may be provided in or on the heat exchange unit 30—for example, on a tube 34 or plate 35, as indicated in FIG. 7. Other optional locations for heaters such as 45 include the upper and lower boxes 41,42. Internal air fans 40 may be activated or not, depending upon heating or cooling requirements.

The closable vents 52 may be controlled and adjusted to intermediate positions between fully open and fully closed, to adjust flow, whether or not the external fans 10 are activated. Closable vents such as 52 may additionally or alternatively be provided in the lower ducts 33.

Activation of the closable vents 52 and/or the external and internal fans 10, 40 may be in response to temperature sensors within or on the electronic display 1. An example of a temperature sensor 55 is shown in FIG. 7. This may sense temperature both within the display 1 and around it. Other locations for temperature sensors may include the upper and lower boxes 41,42. A control element in the ECA 60 may control the closable vents 52 and/or the external and internal fans 10, 40 in response to temperature sensors.

Although the embodiment of FIGS. 1 to 8 is for a dual display, other embodiments of the invention may apply to electronic displays with a single display panel. An example of this is shown in FIGS. 9 and 10 where, as compared to the preceding embodiment, like reference numerals indicate like or corresponding parts and thus the reader will readily understand the mode of operation of the embodiment of the single display unit of FIGS. 9 and 10.

In FIG. 9, in the absence of a second display panel 2, a solid metal plate 48 closes the second enclosure for external air 38 and another solid metal plate 50 closes the first enclosure for internal air, to define an internal airflow channel 49 between the solid metal plates 48 and 50. Thus, internal air (A) in the channel 49 gets cooled by heat exchange with both of the metal plates 48 and 50, the latter plate 50 being in contact with the ambient atmosphere.

In another configuration, in a dual display, one of the display panels 2 may be smaller than the other. For example, behind a main, large display panel 2, a rear display panel may be of 46-inch (117 cm) diagonal size provided as, say, a ‘wayfinder’ touch screen, where a user can find directions in the local vicinity.

FIG. 10 shows a variant in which plates 48 and 50 of FIG. 9 are replaced with a single, solid metal plate 51, so that the channel 49 of FIG. 9 is dispensed with and all of the internal air is directed through the channel 54, over the front of the display panel