WO2009115841A2

WO2009115841A2 - Display system

Info

Publication number: WO2009115841A2
Application number: PCT/GB2009/050257
Authority: WO
Inventors: James Sirmon; Anthony Cole
Original assignee: Litelogic Ip Limited
Priority date: 2008-03-19
Filing date: 2009-03-19
Publication date: 2009-09-24
Also published as: WO2009115841A3; GB0805157D0

Abstract

A spacer (8a, 8b, 8c, 8d) for holding a first printed circuit board (PCB) (7a) in registration with a second PCB (7b) is disclosed along with a housing (10a, 10b, 10c, 10d) for a display assembly made up from a plurality of PCBs held in registration with each other using the clips, and a cover (30a, 30b, 30c, 30d) for LEDs mounted on the PCBS for improving the contrast ratio of the display assembly.

Description

DISPLAY SYSTEM

This invention relates to a display system, and in particular to improvements made in the registration of the light sources making up the display system, provision of power to the display system, and to the contrast ratio of the display system.

Display systems of the type in which a printed circuit board (PCB) is populated with light emitting diodes (LEDs) are well known. The LEDs are generally laid out on a constant pitch and can be individually controlled to emit light of a desired colour so as to create a desired static or moving image. Large display systems are generally created by providing a plurality of such PCBs adjacent to each other.

There are various problems with this type of display system. Firstly, the registration of the individual PCBs making up a large display is critical. It has been found that even a small error (e.g. 1 mm) in registration between adjacent PCBs can be detected by viewers and significantly affects the quality of the image created.

Secondly, when multiple PCBs are laid out to make a large display, it can be extremely complicated to couple power and video signals to each of the PCBs as separate cables must be run to each one. This is exacerbated when the display must be installed in a confined environment, and indeed in some circumstances may become impossible.

Another problem comes around when the display systems are used outside. In such applications, the incident light from the sun can easily overpower the emitted light from the LEDs, rendering the image created by the display system invisible. Furthermore, in outdoor environments it is quite usual for viewers to approach the display system from directions other than the normal intended viewing axis (i.e. the viewers may be a long way off-axis). There is therefore a need to enhance the viewing angle of this type of display system as much as possible. These two problems affect both display systems comprising single and multiple PCBs. In accordance with one aspect of the invention, there is provided a spacer for holding a first printed circuit board (PCB) in registration with a second PCB, the spacer comprising at least one attachment member, the or each attachment member being adapted for engagement in use with a corresponding through-hole provided at a predetermined location in the first PCB; and at least one spacer coupling member, the or each spacer coupling member being adapted for engagement in use with a corresponding hole in a second spacer, which is adapted for coupling to a second PCB in use, whereby the first and second PCBs may be coupled together in use and held in registration with each other.

The spacer provides the capability to couple PCBs together in a desired configuration to form a display system in which the registration of the PCBs with respect to each other is ensured. The problem of lack of alignment of adjacent PCBs described above is therefore overcome.

Normally, the spacer further comprises at least one spacer coupling hole for receiving the spacer coupling member from a third spacer in use.

In a preferred embodiment, the or each attachment member comprises a plurality of resilient mounting fingers provided in a spaced relationship and adapted to be introduced through the corresponding through-hole in the first PCB, and locking portions on the free ends of the resilient mounting fingers gripping behind the edge of the through-hole in the first PCB.

The resilient mounting fingers may be integral with a base section, which itself is integral with the spacer. Typically, this base section is cylindrical.

The resilient mounting fingers may be circular sector-shaped.

Typically, each locking portion comprises a radially outwardly projecting flange which tapers towards the free end of the resilient mounting finger.

The spaced relationship may be such that the resilient mounting fingers are circumferentially separated. Preferably, the or each spacer coupling member comprises a plurality of resilient mounting fingers provided in a spaced relationship and adapted to be introduced through the corresponding through-hole in the second spacer, and locking portions on the free ends of the resilient mounting fingers gripping behind the edge of the hole in the second spacer.

The resilient mounting fingers of the or each spacer coupling member may be integral with a base section, which itself is integral with the spacer. Typically, the base section of the or each spacer coupling member is cylindrical.

The resilient mounting fingers of the or each spacer coupling member may be circular sector-shaped.

Typically, each locking portion of the or each spacer coupling member comprises a radially outwardly projecting flange which tapers towards the free end of the resilient mounting finger.

The spaced relationship of the resilient mounting fingers of the or each spacer coupling member may be such that the resilient mounting fingers are circumferentially separated.

In one embodiment, the spacer comprises at least one integral standoff for holding the first PCB at a predefined height above a supporting substrate.

Preferably, the spacer comprises a pair of L-shaped arms, the first ends of the L- shaped arms being joined by a first link and the second ends of the L-shaped arms being joined by a second link.

In this case, each L-shaped arm may comprise two PCB attachment members projecting from the L-shaped arms in a first direction.

The first link may comprise two spacer attachment members projecting from the first link in a second direction which is opposed to the first direction, and the second link may comprise two spacer attachment holes. Typically, the two L-shaped arms and the second link lie in a first plane and the first link lies in a second plane disposed form the first plane in the first direction.

In a second aspect of the invention, a display assembly comprises a plurality of display PCBs, each of which has a plurality of light sources and is attached to a corresponding spacer according to any of the preceding claims, the spacers on the PCBs being coupled together such that the display PCBs are held in registration with each other and in a desired arrangement.

The light sources may be disposed such that each light source in the assembly lies at an identical predetermined distance from adjacent light sources.

In accordance with a third aspect of the invention, there is provided a housing for a display comprising a frame having a pair of apertures, each of which receives a respective end of a bus bar for coupling power to display PCBs disposed within the frame in use.

This housing allows for easy connection of power and/or video signals to the display PCBs disposed within the housing. It removes the need for each PCB to be individually wired up and provided with a separate cable as one or more bus bars can be routed behind all the PCBs to couple power and/or video signals into each.

In a preferred embodiment, the frame is assembled from a plurality of side pieces, each side piece being joined to neighbouring side pieces in the frame by a connecting piece.

Typically, each of the pair of apertures is provided in a respective one of two side pieces, which oppose each other in the frame.

The two opposing side pieces typically comprise a second aperture through which a coupling terminal can be passed, in use, to engage and electrically connect with the bus bar. In a typical embodiment, each of the two opposing side pieces comprises four apertures, each aperture receiving, in use, a respective end of a respective bus bar.

Each of the side pieces is normally a metal or plastic extrusion.

The or each bus bar may be insulated from the frame by way of insulating sleeves inserted in the apertures.

Typically, each connecting piece comprises two projecting portions, each projecting portion engaging with a corresponding recess in the pair of adjacent side pieces joined by the connecting piece.

Preferably, each of the side pieces has a rebate such that the frame provides a recess for receiving a back panel.

Normally, a transparent cover is fitted to the frame, the transparent cover being coated with an infrared reflective coating.

In a fourth aspect of the invention, there is a kit of side pieces and connecting pieces as defined above suitable for constructing a housing according to the third aspect.

In a fifth aspect of the invention, a display assembly comprises a housing according to the third aspect, at least one display PCB bearing a plurality of light sources disposed within the housing, and a pair of coupling terminals for the or each bus bar.

In a preferred embodiment of this assembly, the or each display PCB bears a first set of light sources adapted to emit light of a first colour and a second set of light sources adapted to emit light of a second colour, the first set of light sources being electrically coupled to a first one of the bus bars which is supplied with power at a first voltage, and the second set of light source being electrically coupled to a second one of the bus bars which is supplied with power at a second voltage.

As discussed below, this arrangement leads to a marked increase in operating efficiency and reduction in component count. Furthermore, the increase in operating efficiency results in a reduction in operating temperature which prolongs the life of the light sources, especially when LEDs are used.

The first set of light sources is typically one element (e.g. the red element) of a tricolour LED whilst the second set of light sources is one of the other elements (e.g. the green or blue element).

The first and second set of light sources are normally coupled to the bus bars through driver circuitry which can selectively operate the first and second set of light sources.

In accordance with a sixth aspect of the invention, there is provided a cover for a surface mount light emitting diode (LED), the cover comprising at least one side wall and an end wall which together define a recess for receiving and engaging the LED in use, the cover further comprising an aperture within the end wall and a registration feature adapted to ensure that the aperture aligns with a light emitting portion of the LED in use, wherein the surface characteristics of the cover are such that substantially all incident visible light impinging on the cover is absorbed thereby providing an enhanced contrast ratio with respect to light emitted by the LED in use.

Without the cover, incident light is reflected by the body of the LED, which is typically white plastic or metallic. However, the cover obscures a large portion of the LED from view. Since the material from which the cover is made absorbs substantially all of the incident visible light falling on it, the light emitting portion of the LED, which is still visible through the aperture, is operating in conditions close to perfect to black body radiation. This dramatically enhances the contrast ratio.

Typically, the cover is black in colour. In a preferred embodiment, the surface area of the aperture is smaller than the surface area of the surface of the LED containing the light emitting portion.

In one embodiment, the aperture is at the centre of a depressed portion in the cover. The depressed portion may be bevelled.

The registration feature typically comprises a set of ribs within the recess which interdigitate with mounting legs on the LED when the cover is placed over the LED in use.

Preferably, the cover comprises two opposing walls spaced apart such that they make an interference fit with the LED when the cover is placed over the LED in use.

In a seventh aspect of the invention, a display assembly comprises a plurality of surface mount LEDs, each of which is fitted with a cover according to the sixth aspect.

In an eighth aspect of the invention, a display assembly comprises a plurality of surface mount LEDs, wherein the LEDs do not have lenses.

In this specification, the term "lens" is used to refer to a transparent element associated with the LEDs, which may be either integral with or separate from the LEDs, which causes redirection of the light emitted by the LEDs. Such redirection may be caused by any one of a number of physical processes, including refraction, diffraction, collimation and diffusion.

The lenses used in LEDs come in many variants. Clear lenses have no tint and provide no diffusion of the light. They produce the most intense light output and narrowest viewing angle by refactively causing convergence of the beam. They are designed for applications that require very high intensity but need to appear colourless when the LED is off. Tinted lenses are similar but are used to indicate what the LED colour will be when turned on. Diffusing lenses have tiny glass particles embedded in the lens. This spreads the light to a viewing angle of approximately +/-35 degrees from the central axis of the LED. These LEDs are often used for applications in which the LED protrudes through a hole in the front panel of electronic equipment.

Non-diffusing lenses that do not have glass particles in the lens produce a narrow viewing angle of +/-12 degrees from the central axis of the LED. They are often used in backlighting applications in which the LED is focused on a translucent window in the front of a panel.

It has been found that the lenses used in LEDs have the undesirable effect of reducing the viewing angle of the display systems dramatically. They are always fitted to LEDs because it is generally perceived that it is better to provide a narrower, brighter beam from the LED to improve its contrast.

However, when the display systems are used outdoor this is not necessarily the case as viewers can approach the display from all angles. Most display systems based on LEDs are deployed in situations where the audience is in a known viewing position. The display system described herein is adapted specifically for use in environments where the audience may be in any position relative to the display system. For example, the display system may be mounted on the side of a vehicle such as a bus and the audience may be standing on a pavement alongside the bus but beneath the display or they may look at the display from a building at a level which may be above, below or the same as the level of the display. Furthermore, people may view the display as the vehicle approaches or moves away from them.

Furthermore, the use of LEDs without lenses allows tricolour surface mount LEDs to be used instead of discrete red, green and blue LEDs. In a tricolour LED, the red, green and blue elements are each provided slightly off the central axis of the LED. If they are placed behind lens then the resultant red, green and blue images do not converge properly to form a combined full-colour image but instead all remain slightly off axis. Therefore, discrete red, green and blue LEDs had to be used in the past. This solution therefore cuts the number of LEDs that need to be placed on a PCB by a factor of three.

Each of the LEDs is preferably fitted with a cover according to the sixth aspect.

This has the added benefit of improving the contrast ratio as well as the viewing angle and it has been found that the resulting display assembly may be viewed with excellent contrast characteristics over almost an entire hemisphere.

The display assembly according to the seventh or eighth aspects may further comprise a plurality of display PCBs on which the plurality of surface mount LEDs is mounted, each PCB being attached to a corresponding spacer according to the first aspect, the spacers on the PCBs being coupled together such that the display PCBs are held in registration with each other and in a desired arrangement.

The LEDs may be disposed such that each LED in the assembly lies at an identical predetermined distance from adjacent light sources.

Typically, the plurality of surface mount LEDs is mounted on front surfaces of the display PCBs, the front surfaces being black in colour.

The display assembly according may further comprise a housing according to the third aspect, within which the plurality of display PCBs are disposed, and a pair of coupling terminals for the or each bus bar, whereby the coupling terminals are electrically connected to the display PCBs.

An embodiment of the invention will now be described with reference to the accompanying figures in which:

Figures 1 a and 1 b show a spacer for holding adjacent PCBs in registration.

Figure 2 shows four PCBs held together with the spacers.

Figures 3a and 3b shows a housing and bus bar arrangement.

Figure 4 shows a corner piece for the housing. Figures 5a and 5b show detailed views of a cover for an LED.

Figures 6a and 6b show the cover with the LED in situ.

Figure 7 shows a cross-sectional view of one of the LEDs.

Figures 1 a and 1 b show a spacer for coupling display PCBs together such that they are maintained in perfect registration. The spacer comprises a pair of L- shaped arms 1a and 1 b. These are joined together by links 2a and 2b.

As is best seen from Figure 1 b, the first L-shaped arm 1 a has one integral clip 3a on its underside and the second L-shaped arm has three integral clips 3b to 3d on its underside. The clips 3a to 3d are for engagement with through-holes on a display PCB.

The first link 2a is also provided with two clips 4a and 4b on its top side, and the second link 2b is provided with two holes 5a and 5b. It is clearly visible in Figure 1 b that the plane in which link 2a lies is offset and lower than the plane in which link 2b lies. This allows two spacers to be joined together by pushing the clips 4a and 4b on the first spacer into the holes 5a and 5b on the other spacer to form a substantially planar assembly (i.e. one where the underside of both spacers and the clips 3a to 3d on both spacers all lie in the same plane).

The clips 3a to 3d and 4a and 4b are made from resilient material and are allowed to deform radially inwardly by separating them into four circular sector-shaped portions. Thus, the clips 3a to 3d and 4a and 4b may be inserted into corresponding holes on a PCB or other spacer. Once inserted, the clips remain in place by virtue of the locking flange which tapers towards the free end of the clips.

The through-holes are drilled in the PCB at the correct location so that, when attached, the spacer lies at the desired location with respect to the PCB. This location is chosen so that when the clip is attached to another identical clip fitted to another PCB, the two PCBs will be brought into, and remain in, perfect registration. In order to assist alignment and retention of the PCB edge, there are also provided four locking tabs 6a to 6d. These are also made of resilient material and have a flange tapering towards the free end. They engage the edges of the PCB as the spacer is coupled to it.

The spacer is made by injection moulding so that the L-shaped arms 1 a and 1 b, links 2a and 2b, clips 3a to 3d and 4a and 4b, and locking tabs 6a to 6d form one integral unit.

Figure 2 shows four PCBs 7a to 7d held together by four spacers 8a to 8d. In order to construct this assembly, the clips 3a to 3d of each spacer 8a to 8d are pushed through the corresponding through-holes on PCBs 7a to 7d. This couples each spacer with a respective PCB. The holes 5a and 5b on spacer 8a are then engaged with the clips 4a and 4b on spacer 8b, the holes 5a and 5b on spacer 8b are engaged with the clips 4a and 4b on spacer 8c, the holes 5a and 5b are engaged with the clips 4a and 4b on spacer 8d and the holes 5a and 5b are engaged with the clips 4a and 4b on spacer 8a.

The four PCBs 7a to 7d are thus held in a spaced relationship with respect to each other. However, the gap between each PCB 7a to 7d is defined entirely by the location of the through-holes and the dimensions of the spacers 8a to 8d. These can be very accurately controlled in manufacturing so that the registration of the PCBs 7a and 7d is cheaply and accurately set.

The PCBs 7a to 7d are populated with LEDs, each of which represents a pixel of the display system. The spacing between these LEDs on each PCB 7a to 7d is constant, and the positioning of the LEDs around the periphery of each PCB 7a to 7d is set so that the pixel pitch is maintained across the gap between the PCBs 7a to 7d. This ensures that the image created has a uniform, constant pixel pitch as well as ensuring perfect registration between the PCBs 7a to 7d.

The spacer provides a truly scaleable solution to the problem of building display systems of different sizes because it allows a small standard size PCB to be placed alongside as many others as are required whilst maintaining pixel registration.

Figure 3a shows a housing assembly for display PCBs. The housing typically receives an assembly of PCBs coupled together with the spacers described above.

The housing comprises four side pieces 10a to 1Od, each of which is extruded from a plastic material. The side pieces 10a to 10d are held together by way of connecting corner pieces 17 (one of which is shown in Figure 4).

The side pieces 10a to 10d are extruded so as to provide a recess 16 on the underside around the whole inside edge of the housing when it is assembled. This recess receives a back panel 11 , which may be either screwed or glued to the side pieces 10a to 10d.

Two of the side pieces 10b and 10d have opposing apertures for receiving four copper bus bars 12a to 12d. This arrangement is shown in more detail in Figure 3b in which bus bar 12c has been hidden to reveal one of the apertures 21. The bus bars 12c is supported within the side piece 10b in this aperture. Each bus bar 12a to 12d is connected at each end to a respective threaded stud, some of which are shown by reference numerals 13a, 13b, 14a, 14b and 15a. These threaded studs 13a, 13b, 14a, 14b and 15a fit through holes in the underside of side pieces 10b and 10d and screw into holes drilled in the bus bars 12a to 12d. This holds the bus bars 12a to 12d in position and allows electrical connections to be made to the bus bars 12a to 12d.

The bus bars 12a to 12d are drilled at various places along their lengths to provide holes which correspond with plated through-holes on the PCBs when fitted to the housing. A nut and bolt can then be inserted through the holes on bus bars 12a to 12d and the corresponding plated through-holes on the PCBs. This provides electrical connection between the bus bars 12a to 12d and the PCBs and also helps to hold the PCBs in the correct position. Figure 4 shows one of the corner pieces 17. This has two protruding sections 18 and 19. The protruding section 18 is an interference fit in side piece 10b and the protruding section 19 is an interference fit in side piece 10a.

In order to construct a display assembly using this frame, the PCBs are firstly joined together (using the spacers described above). Plastic insulating clips are then fitted to the outer edge of each PCB in the PCB assembly to provide an insulating standoff around the whole periphery of the PCB assembly. These insulating clips are fitted to through-holes in the PCBs in the same manner as the spacers described above.

The frame is partially constructed by joining three side pieces (e.g. side pieces 10a to 10c) together using two corner pieces 17, and then the assembly of PCBs can be slid into a groove in the three joined side pieces, the insulating standoffs ensuring that there is no undesired electrical connection between the PCBs and the side pieces 10a to 10c which will be required if these are manufactured from a metal. The last side piece (in this example, 10d) can then be attached using two more corner pieces 17, again being kept electrically isolated from the PCBs by way of the insulating standoffs. The corner pieces 17 may be screwed to the side pieces to ensure that the assembly does not fall apart.

The copper bus bars are then electrically coupled to the PCBs using nuts and bolts as described above. Each of the PCBs is provided with copper pads of the same width as the bus bars and running across the whole width of the PCBs in the same direction as the bus bars to provide a good electrical coupling between the bus bars and the PCBs.

Finally, threaded studs 13a, 13b, 14a, 14b and 15a are inserted through the holes in the underside of side pieces 10b and 10d and screwed into holes drilled in the bus bars 12a to 12d.

If the bus bars are to be fitted to side pieces 10b and 10d made from a metal or other conductive material, then a plastic insulator is fitted into the side pieces 10b and 1 Od and the bus bars are inserted into the side pieces 10b and 10d through the plastic insulator. Separate plastic insulators are provided in the holes in side pieces 10b and 10d which receive the threaded studs 13a, 13b, 14a, 14b and 15a.

A transparent polycarbonate front cover is fitted in the side pieces 10a to 10d over the display PCBs. This provides some environmental protection whilst allowing the display PCBs to remain clearly visible. The polycarbonate front cover is typically provided with an infra-red reflective coating on its outer surface. This prevents the display PCBs from absorbing too much infrared radiation when placed in direct sunlight, which can otherwise lead to a dramatic rise in temperature of the display PCBs (which are normally black in colour) and consequent reduction in operating life of the LEDs.

The bus bars 12a to 12c each carry a different supply voltage and the bus bar 12d is a common OV return rail for each of these supplies. Between them, the bus bars carry a 3.3V supply, a 4.5V supply and a 5V supply. The 3.3V supply and 4.5V supplies are used to provide power to the LEDs on the PCBs which carry tricolour LEDs (such as the LP6-NPP1 -01 -N1 from Cotco Luminant Device (Huizhou) Limited), each of which is effectively a combination of discrete red, green and blue LEDs in the same device. Two supply voltages are used because the red LEDs operate at a lower forward voltage than the green and blue LEDs. The red LEDs operate at a forward voltage of around 2.8V derived from the 3.3V supply, whereas the blue and green LEDs operate at a forward voltage of around 4.1V derived from the 4.5V supply. The PCBs carry LED driver circuitry, based on the TLC5941 LED Driver from Texas Instruments, which can selectively provide forward current to each LED at the correct forward voltage. The 5V supply is used to power the logic circuitry on the PCBs.

By supplying different supply voltages to the LEDs in this way, the efficiency of the display system with respect to prior art systems may be dramatically increased. In prior art systems, all the LEDs are supplied with power at a common voltage, typically 5V, through respective bias resistors which lower the voltage across the LEDs to the forward voltage when the forward current is flowing. Not only does this approach require a vast quantity of resistors to be fitted to the display PCBs, but the bias resistors also dissipate a large amount of heat as the forward current flows through them.

The use of bus bars carrying appropriate supply voltages not only improves the efficiency but also allows the display system to occupy a much smaller volume. In fact, we have been able to make a display system with a thickness of only 19mm, which would simply be impossible using standard wiring looms as the current flowing in the OV return can reach values as high as 900A.

Figures 5a and 5b show a cover for a surface mount LED such as may be fitted to the PCBs discussed above. Figures 6a and 6b are similar to Figures 5a and 5b but show the LED in situ.

The cover is injection moulded from a black plastic material. It comprises four side walls 30a to 3d and a top wall 31. The top wall has a bevelled section 32 with an aperture 33 at its centre. The bevelled section mitigates the limiting effect that the wall thickness of top wall 31 might otherwise have on viewing angle.

The side walls 30a to 3Od and the top wall 31 together define a recess which allows the cover to be placed over an LED 34. Two of the side walls 30a and 30c form an interference fit with the LED 34, thereby preventing the cover from falling off.

Within the recess, there are some ribs 35a to 35d which interdigitate with the legs 36a to 36f of the LED 34 as shown in Figure 6b. The ribs 35a and 35d thereby ensure that the aperture 33 is aligned with the light emitting part 37 of LED 34.

The cover provides a very cheap way of enhancing the contrast ratio, and when used in conjunction with black PCBs a very effective, high contrast display PCB may be manufactured. The LEDs used in the display system shown in the Figures are manufactured without lenses. That is to say, that the LEDs do not have optical elements for focussing the beams of emitted light. In place of this, a layer of clear epoxy filler is provided over the semiconductor dice in the LED. The layer of clear epoxy is flattened on its outer surface and does not carry diffusing glass particles.

Figure 7 shows a cross-sectional view of one of the LEDs from which the structure is apparent. The LED is built on a substrate 40 on which conductive tracks 41 a and 41 b are deposited. These conductive tracks are used to carry power to LED die 42 which is deposited on track 41a. A conical reflector 43 is provided to ensure that as much of the light emitted by the LED die 42 as possible is directed through the clear front plate 45. The volume between the front plate 45, reflector 43 and the conductive tracks 41 a, 41 b is filled with a clear epoxy filler 44 as described above. This filler 44 does not cause any focussing or redirection of the light emitted by the LED die 42.

As already explained, this helps to widen the viewing angle and when used in conjunction with the cover described above results in very high contrast display viewable from almost an entire hemisphere.

Claims

1. A spacer for holding a first printed circuit board (PCB) in registration with a second PCB, the spacer comprising at least one attachment member, the or each attachment member being adapted for engagement in use with a corresponding through-hole provided at a predetermined location in the first PCB; and at least one spacer coupling member, the or each spacer coupling member being adapted for engagement in use with a corresponding hole in a second spacer, which is adapted for coupling to a second PCB in use, whereby the first and second PCBs may be coupled together in use and held in registration with each other.

2. A spacer according to claim 1 , wherein the spacer further comprises at least one spacer coupling hole for receiving the spacer coupling member from a third spacer in use.

3. A spacer according to either of the preceding claims, wherein the or each attachment member comprises a plurality of resilient mounting fingers provided in a spaced relationship and adapted to be introduced through the corresponding through-hole in the first PCB, and locking portions on the free ends of the resilient mounting fingers gripping behind the edge of the through-hole in the first PCB.

4. A spacer according to claim 3, wherein the resilient mounting fingers are integral with a base section, which itself is integral with the spacer.

5. A spacer according to claim 4, wherein the base section is cylindrical.

6. A spacer according to any of claims 3 to 5, wherein the resilient mounting fingers are circular sector-shaped.

7. A spacer according to any of claims 3 to 6, wherein each locking portion comprises a radially outwardly projecting flange which tapers towards the free end of the resilient mounting finger.

8. A spacer according to any of claims 3 to 7, wherein the spaced relationship is such that the resilient mounting fingers are circumferentially separated.

9. A spacer according to any of the preceding claims, wherein the or each spacer coupling member comprises a plurality of resilient mounting fingers provided in a spaced relationship and adapted to be introduced through the corresponding through-hole in the second spacer, and locking portions on the free ends of the resilient mounting fingers gripping behind the edge of the hole in the second spacer.

10. A spacer according to claim 9, wherein the resilient mounting fingers of the or each spacer coupling member are integral with a base section, which itself is integral with the spacer.

11. A spacer according to claim 10, wherein the base section of the or each spacer coupling member is cylindrical.

12. A spacer according to any of claims 9 to 11 , wherein the resilient mounting fingers of the or each spacer coupling member are circular sector-shaped.

13. A spacer according to any of claims 9 to 12, wherein each locking portion of the or each spacer coupling member comprises a radially outwardly projecting flange which tapers towards the free end of the resilient mounting finger.

14. A spacer according to any of claims 9 to 13, wherein the spaced relationship of the resilient mounting fingers of the or each spacer coupling member is such that the resilient mounting fingers are circumferentially separated.

15. A spacer according to any of the preceding claims, wherein the spacer comprises at least one integral standoff for holding the first PCB at a predefined height above a supporting substrate.

16. A spacer according to any of the preceding claims, wherein the spacer comprises a pair of L-shaped arms, the first ends of the L-shaped arms being joined by a first link and the second ends of the L-shaped arms being joined by a second link.

17. A spacer according to claim 16, wherein each L-shaped arm comprises two PCB attachment members projecting from the L-shaped arms in a first direction.

18. A spacer according to claim 17, wherein the first link comprises two spacer attachment members projecting from the first link in a second direction which is opposed to the first direction, and the second link comprises two spacer attachment holes.

19. A spacer according to claim 17 or 18, wherein the two L-shaped arms and the second link lie in a first plane and the first link lies in a second plane disposed form the first plane in the first direction.

20. A display assembly comprising a plurality of display PCBs, each of which has a plurality of light sources and is attached to a corresponding spacer according to any of the preceding claims, the spacers on the PCBs being coupled together such that the display PCBs are held in registration with each other and in a desired arrangement.

21. A display assembly according to claim 20, wherein the light sources are disposed such that each light source in the assembly lies at an identical predetermined distance from adjacent light sources.

22. A housing for a display comprising a frame having a pair of apertures, each of which receives a respective end of a bus bar for coupling power to display PCBs disposed within the frame in use.

23. A housing according to claim 22, wherein the frame is assembled from a plurality of side pieces, each side piece being joined to neighbouring side pieces in the frame by a connecting piece.

24. A housing according to claim 23, wherein each of the pair of apertures is provided in a respective one of two side pieces, which oppose each other in the frame.

25. A housing according to claim 24, wherein each of the two opposing side pieces comprises a second aperture through which a coupling terminal can be passed, in use, to engage and electrically connect with the bus bar.

26. A housing according to claim 24 or claim 25, wherein each of the two opposing side pieces comprises four apertures, each aperture receiving, in use, a respective end of a respective bus bar.

27. A housing according to any of claims 23 to 26, wherein each of the side pieces is a metal or plastic extrusion.

28. A housing according to any of claims 22 to 27, wherein the or each bus bar is insulated from the frame by way of insulating sleeves inserted in the apertures.

29. A housing according to any of claims 23 to 28, wherein each connecting piece comprises two projecting portions, each projecting portion engaging with a corresponding recess in the pair of adjacent side pieces joined by the connecting piece.

30. A housing according to any of claims 23 to 29, wherein each of the side pieces has a rebate such that the frame provides a recess for receiving a back panel.

31. A housing according to any of claims 22 to 30, wherein a transparent cover is fitted to the frame, the transparent cover being coated with an infrared reflective coating.

32. A kit of side pieces and connecting pieces as defined in any of claims 23 to 31 suitable for constructing a housing as set out in any of claims 22 to 31.

33. A display assembly comprising a housing according to any of claims 22 to 32, at least one display PCB bearing a plurality of light sources disposed within the housing, and a pair of coupling terminals for the or each bus bar.

34. A display assembly according to claim 33, wherein the or each display PCB bears a first set of light sources adapted to emit light of a first colour and a second set of light sources adapted to emit light of a second colour, the first set of light sources being electrically coupled to a first one of the bus bars which is supplied with power at a first voltage, and the second set of light source being electrically coupled to a second one of the bus bars which is supplied with power at a second voltage.

35. A cover for a surface mount light emitting diode (LED), the cover comprising at least one side wall and an end wall which together define a recess for receiving and engaging the LED in use, the cover further comprising an aperture within the end wall and a registration feature adapted to ensure that the aperture aligns with a light emitting portion of the LED in use, wherein the surface characteristics of the cover are such that substantially all incident visible light impinging on the cover is absorbed thereby providing an enhanced contrast ratio with respect to light emitted by the LED in use.

36. A cover according to claim 35, wherein the cover is black in colour.

37. A cover according to claim 35 or claim 36, wherein the surface area of the aperture is smaller than the surface area of the surface of the LED containing the light emitting portion.

38. A cover according to any of claims 35 to 37, wherein the aperture is at the centre of a depressed portion in the cover.

39. A cover according to claim 38, wherein the depressed portion is bevelled.

40. A cover according to any of claims 35 to 39, wherein the registration feature comprises a set of ribs within the recess which interdigitate with mounting legs on the LED when the cover is placed over the LED in use.

41. A cover according to any of claims 35 to 40, comprising two opposing walls spaced apart such that they make an interference fit with the LED when the cover is placed over the LED in use.

42. A display assembly comprising a plurality of surface mount LEDs, each of which is fitted with a cover according to any of claims 35 to 41.

43. A display assembly comprising a plurality of surface mount LEDs, wherein the LEDs do not have lenses.

44. A display assembly according to claim 43, wherein each of the LEDs is fitted with a cover according to any of claims 35 to 41.

45. A display assembly according to any of claims 42 to 44, further comprising a plurality of display PCBs on which the plurality of surface mount LEDs is mounted, each PCB being attached to a corresponding spacer according to any of claims 1 to 19, the spacers on the PCBs being coupled together such that the display PCBs are held in registration with each other and in a desired arrangement.

46. A display assembly according to claim 45, wherein the LEDs are disposed such that each LED in the assembly lies at an identical predetermined distance from adjacent light sources.

47. A display assembly according to claim 45 or claim 48, wherein the plurality of surface mount LEDs is mounted on front surfaces of the display PCBs, the front surfaces being black in colour.

48. A display assembly according to any of claims 45 to 47, further comprising a housing according to any of claims 22 to 31 , within which the plurality of display PCBs are disposed, and a pair of coupling terminals for the or each bus bar, whereby the coupling terminals are electrically connected to the display PCBs.