GB2497768A - Multi-LED arrays - Google Patents

Abstract

A multi-LED array (10) comprising plurality of spaced apart LED light emitters (12), wherein at least two of the plurality of LED light emitters (12) have a different rotational orientation with respect to one another. The LED light emitters (12) comprise discrete LED dies disposed on a substrate (14), or LED elements formed in a single LED die, having different rotational orientations with respect to one another. The array produces relatively rotated overlapping beams with an overall circular or elliptical illumination pattern.

Description

MULTI-LED ARRAYS AND LIGHTING INCORPORATING THE SAME

Description:

This invention relates to multi-LED lighting, and in particular, but without limitation to multi-LEDs, multi-LED floodlighting and spotlighting.

LED (Light Emitting Diode) lighting technology has improved considerably in recent years with the development of more efficient and brighter LED emitters. In the past, the use of LED lights as alternatives to conventional filament light bulbs has been hampered by the poor light output, brightness and colour spectrum of LEDs compared to filament bulbs. However, nowadays, high-efficiency, and high-output LEDs can be mass produced relatively cheaply and, as a result, the benefits of using LED lighting can, in certain circumstances, now outweigh more traditional lighting methods. The uptake of LED lighting is therefore on the increase.

LEDs are solid-state semiconductor transducers that convert electrical current directly into a light output. By the appropriate use of "band gap engineering", that is to say, by the appropriate selection of the dimensions, geometry, materials and layer structure of the LEDs, the nominal output light frequency, frequency distribution and intensity can be "tuned" to particular applications. It is now possible to manufacture high-efficiency LED devices with a range of visible (and invisible) light output colours, which can be adapted for use in domestic and commercial light fittings.

The main advantage of LED lighting, compared to filament lighting and cold cathode (neon) lighting is its energy efficiency. This derives from the direct conversion of electrical current into light, as opposed to heat -as in a filament bulb, whereby the light is a by-product of the heat produced. As a result, modern LED devices can convert a much higher percentage of their energy input into a useful light output than can an equivalently-rated filament light bulb or a cold cathode tube.

Therefore, LED lighting elements having particular nominal power ratings can provide a much higher useful light output than can a filament light bulb or cold cathode tube having the same nominal power rating. This means that LED lighting can be more energy efficient for a given lighting requirement, and/or can be much brighter for a given power consumption requirement. Furthermore, the operating life of an LED can be 10 to 1,000 times that of an equivalent filament light bulb meaning that the maintenance and replacement costs associated with LED light fittings are much lower over extended periods of time than their filament and cold cathode counterparts.

Unfortunately, LEDs are not yet available that can provide the same light output as some of the higher-powered filament light bulbs. This is especially apparent in floodlighting situations where, say, a 500W filament bulb cannot, as yet, be replaced by a single LED of an equivalent light output. The solution to this problem is simply to multiplex a number of lower-powered LEDs in a single light fitting so that the sum of the LED's outputs matches that of the equivalent, single filament bulb. In such a manner, a single high-powered filament bulb can be substituted by an array of lower-powered LEDs to achieve a desired light output.

LED light fittings have therefore been developed that incorporate a substrate or circuit board onto which a number of LEDs can be soldered or coriductively affixed to provide a desired total light output. However, owing to the physical construction of LEDs, their light output tends to be non-uniform, that is to say, having different intensities along different directions relative to the circuit board. As such, it is often necessary to provide lenses that overlie the LED elements to focus and/or direct light in a given direction -normally away from the circuit board. Furthermore, reflectors may also be used to direct light in a desired direction, which can be achieved by making the front surface of the circuit board reflective using, for example, an aluminium foil overlay, or by forming the circuit board itself from an optically reflective material.

The lenses of an LED light fitting are commonly integrated into a transparent fascia, which overlies the circuit board and which provides a physical barrier between the environment and the circuit board. Such a construction reduces the part count" of a light fitting so constructed in addition to facilitating consistency in the manufacturing process because each lens does not need to be individually positioned and aligned. Needless to say, a considerable amount of effort is usually invested in the design of the circuit board, lenses and fascia to achieve a desired light distribution for the light fitting.

One aspect of light fitting design that can impact greatly on the quality of the is light emitted lies in the number, and locations, of the individual LEDs in the array. For example, locating the LEDs relatively close together on the circuit board can give rise to a much more intense and "punchy" lighting effect than can the same number of LEDs spaced further apart, the latter often giving rise to more diffuse and "ambient" lighting effect. In addition, the lenses can be selected to provide a desired beam angle for each of the individual LEDs, and the fascia itself may comprise curved surfaces to spread, collimate, converge or overlap the light emitted from each of the individual lenses at a given "focal plane" or "working distance" in front of the fitting. The design and construction considerations that need to be taken into account when designing LED light fittings are well-known to those skilled in the art and did not require detailed consideration here.

It should, nevertheless, be borne in mind that LED light fittings are mass-produced items that are often manufactured using robots or automation technology.

One particular stage of the manufacturing process involves placement of the LEDs on the circuit board prior to them being soldered or glued to the circuit board. This is often accomplished using a "pick and place" machine, which, as its name suggests, picks individual LEDs from a receptacle and accurately places them on the circuit board on top of their contact pads ready for soldering in the next step. The use of "pick and place" machines is very widespread, in particular, in the LED light fitting sector.

Known multi-LED light fittings generally comprise a generally planar circuit board having a number of LED5 disposed thereon at predetermined locations, which are selected in accordance with the design requirements of the light fitting into which it will be inserted. This is shown, schematically, in Figures ito 3 of the drawings.

In Figure 1, a side view of a multi-LED lighting system i0 is shown schematically, in which a number of surface-mount LEDs (and lenses) 12 are provided at spaced apart locations on a circuit board 14. The LEDs (and lenses) 12 each emit a beam of light i6 away from the circuit board 14 having a beam spread 18 dictated by the LED's and lenses' geometries. The beams 16 are divergent, and so overlap a certain distance 20 in front of the circuit board 14 to form a "beam pattern" in a plane 22 spaced from the circuit board 14.

The spatial distribution of the light intensity of the beam pattern is shown, schematically, by the line 24 in the overlaid plot of light intensity 26 against position 28. The beam pattern 24 comprises regions of relatively higher light intensity 30, where two or more beams 16 overlap, and regions of relatively lower light intensity 32 where just one 16, or no beams, impinge on the plane 22 in question. The beam pattern is two-dimensional, that is to say, the light intensity 26 varies at different horizontal and vertical positions in the plane 22.

This is shown in Figure 2, which is a projection of the beam pattern for a single LED 12 on the plane 22. In Figure 2, it can be seen that a rectangular LED 12 (most surface-mount LEDs are formed as square or rectangular prisms) and its associated lens emits a beam 16 having a generally rounded-rectangular beam outline 34, whose intensity decreases radially with distance from the LED 12. In other words, each portion of the LED's surface emits light, giving rise to a cone of light from that point. The cones of light, when summed over all of the points on LED's surface, give rise to a beam pattern that is larger than the LED's surface and having a rounded periphery 34.

In a conventional multi-LED array, as shown in Figure 3, for example, a number of spaced-apart LEDs and lenses 12 are disposed on the surface of the circuit board 14. This produces a beam pattern 40 in a given plane 22 that is made up of the overlapping beam patterns of each of the individual LEDs and lenses 12.

As can be seen in Figure 3, there are regions of relatively higher light intensity 30, and regions of relatively lower light intensity 32 bounded by a periphery 42 being a line at which the light intensity has decayed to a minimal level. In a situation where the LED's and lenses have a relatively broad spread 18, this effect can be barely noticeable to the naked eye. However, when the LEDs and lenses 12 are configured with relatively narrow beam angles or where the LEDs 12 are spaced close together, the effect can be quite noticeable. Moreover, the shape of the outer periphery 42 of the beam pattern can become easily discernible, in particular when the LEDs are being used as spat lighting.

The same considerations apply to multi LEDs, which comprise a substrate and a number of LED dies" or LED elements thereon. A single LED can therefore have a number of individual LED elements or light emitters housed within a single housing such that the light output of the LED as a whole can be increased. In such a situation, the LED dies are disposed in a geometric, or grid-like array, usually a square or rectangular grid, such that the beam emitted by the LED has a rounded square or rounded rectangular outline. This is what can give rise to the shape of the beam pattern illustrated schematically in Figure 2.

As can be seen in Figure 3, therefore, the effect of the overlapping beam patterns of the individual, rounded rectangular beams is to create a rounded-rectangular "spot" of light in the plane 22 (e.g. on a surface upon which the light is projected). To the human eye, the rounded-rectangular periphery of the "spot" 42 can be visually unattractive because humans have become accustomed to seeing the rounded spots of light that would be created by a single light source, for example a single filament light bulb, of the same overall light output.

A problem with multiple-LEDs and multiple-LED light fittings therefore subsists in the need to use a plurality of LEDs in each LED or array of LEDs to obtain a similar light output to a single filament bulb. However, the use of an array of spaced-apart light sources can give rise to an unappealing light pattern, which can sometimes be seen on an illuminated object or surface.

It is therefore an object of the invention to address one or more of the problems highlighted above, and/or to provide an improved and/or alternative multi-LED array.

According to a first aspect of the invention, there is provided a multi-LED array comprising plurality of spaced apart LED light emitters, wherein at least two of the plurality of LED light emitters have a different rotational orientation with respect to one another.

The invention pertains not only to light fittings comprising a number of LEDs arranged on a circuit board, but also to individual LEDs that comprise a number of LED light emitters or dies on a substrate within an LED housing.

A second aspect of the invention provides a light fitting comprising a multi-LED array, the multi LED array comprising plurality of spaced apart LEDs, wherein at least two of the plurality of LEDs have a different rotational orientation with respect to one another.

By rotating the LEDs relative to one another, the beam patterns created by the individual LEDs are relatively rotated. This may enable the outer periphery of the beam pattern of the LED array to be more rounded, substantially circular or elliptical, as opposed to a conventional multi-LED array, whose beam pattern has a generally rounded rectangular periphery. The effect of the invention may be to improve the light distribution or beam pattern of light emitted from the multi-LED array.

The LEDs may be mounted on a circuit board. Conveniently, this can be achieved by the use of surface-mount LEDs that can be soldered, or glued, to the circuit board.

The circuit board, where provided, preferably comprises a plurality of spaced-apart contact pads to each of which contact pads, at least one LED is electrically connectable. Given that at east two of the LEDs will be rotated relative to one another, at least two of the plurality of contact pads will have a different rotational orientation with respect to one another.

Each contact pad preferably comprises a cathode contact region and an anode contact region each being electrically connectable, in use, to the cathode and anode of an LED, respectively. The cathode contact region and the anode contact region may each be electrically connectable, by soldering, or by an electrically conductive adhesive, to the cathode and anode of a surface-mount LED, respectively. Each contact pad may additionally comprise a mounting region to which mounting region a mounting portion of a surface-mount LED can be affixed.

The use of a separate mounting region enables the weight of the LED to be borne by a dedicated, structural connection to the circuit board, rather than via the electrical connections, which may improve the longevity of the light fitting or array.

A third aspect of the invention provides a method of manufacturing a multi-LED array, the method comprising the steps of positioning a plurality of LEDs each on an electrical contact pad of a circuit board, and adhering the LED5 to their respective contact pads using an electrically conductive adhesive or solder, the positioning step including laterally spacing the LEDs relative to one another, and the method being characterised by additionally rotating at least some of the LEDs relative to one another during the positioning step.

The method can be carried out automatically using a pick-and-place" machine or robot, the pick-and-place machine being configured for translation of the LEDs in the plane of the circuit board, for translation of the LEDs normal to the circuit board, and for rotation of the LEDs about an axis extending perpendicularly from the circuit board.

Preferred embodiments of the invention shall now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a schematic side view of a multi-LED lighting array and its beam pattern; Figure 2 is a schematic projection of the beam pattern of a single LED of Figure 1; Figure 3 is a schematic projection of the beam pattern of the array of LEDs of Figure 1; Figure 4 is a schematic projection of the beam pattern of an array of LEDs in accordance with the invention; Figure 5 is a plan view of a circuit board suitable for use with the invention; Figure 6 is a schematic perspective view of the circuit board of Figure 5; and Figure 7 is an example of a radial coordinate system for the placement of the LEDs shown in Figure 6.

In contrast to the beam pattern of the known multi-LED array shown in Figure 3, the LEDs 12 of the invention are rotated relative to one another. This is shown in Figure 4, which uses the same number and types of LED5 and lenses 12 as shown in Figure 3.

As can be seen in Figure 4, four LEDs and lenses 12 are provided on a circuit board having identical centre positions to those shown in Figure 3. As in Figure 3, each LED 12 in Figure 4 similarly creates its own beam pattern, which is that same as that shown in Figure 2.

In Figure 4, however, it will be noted that the LEDs 12 have been rotated relative to one another so their respective beam patterns are relatively rotated and overlap to form an overall beam pattern having a much rounder outer periphery 46.

This gives rise to the appearance of a more circular "spot" of light on an object or surface illuminated by the LEDs 12, which can be more visually appealing to a human observer.

As such, the invention addresses one of the hitherto perceived drawbacks of LED lighting systems, namely poor light quality or visual appearance compared to a conventional, single filament light source. The invention can be put into effect using a new type of circuit board or substrate for the LED5 12, such as that shown in Figure 5.

In Figure 5, a circuit board 50 is shown that is suitable for use in an LED light fitting having an array of thirteen LEDs (not shown). The circuit board 50 is manufactured from a substantially circular disc having metallised contact and conductor regions arranged on its front surface, that is to say, the surface that faces outwardly through the light fitting's transparent or translucent fascia, in use. The circuit board 50 can be held in place and aligned relative to a light fitting (not shown) by screws extending through pre-drilled screw holes 52, and by alignment notches 54 provided on its peripheral edge, which register with corresponding features of the light fitting (not shown).

The circuit board 50 comprises thirteen LED mounting pads 56, each mounting pad 56 comprising an anode contact region 58, a cathode contact region and a mounting region 62, which can be soldered or glued, respectively, to the anode, cathode and mounting surface of a surface-mount LED (not shown). The anode and cathode contact regions 58, 60 are connected, in series, by electrically conductive tracks 64 to enable electrical current to pass from a first connection 66, through all of the LEDs (not shown) and back to a second connection 68. A pair of supplementary mounting pads 70 are provided adjacent to the first and second connections 68,70 to which a plug-and-socket type connector (not shown) can be affixed, enabling a power fly lead (not shown) to be connected to the circuit board 50 to power the LEDs.

A further pair of electrical connections 72, 74 are provided having supplementary tracks 70 leading to the central LED mounting pad only, which enables the central LED only (not shown) to be illuminated independently of the remaining LEDs (not shown). Such an arrangement optionally enables the light fitting to be independently wired to a supplementary power supply, for example to provide emergency lighting in the event of a master power failure.

In Figure 6, a most preferred embodiment of the invention is shown, which produces a substantially circular beam pattern using an array of spaced apart LEDs 12. In Figure 6, it can be seen that the LEDs 12 are located on the circuit board according to a generally polar coordinate scheme, which is shown separately in Figure 7.

Specifically, each LED 12 is located at a radius 80 from the centre 82 of the circuit board, which centre 82 corresponds, in the illustrated example of Figure 6, to the pole 84 of the polar coordinate scheme. The LEDs 12 therefore each have a position defined by its radius 80 from the pole 84 and an azimuth angle 86 from the polar axis 88.

Each LED 12 is also rotated about its own geometric centre, in the plane of the circuit board 50, about a line 90 which is perpendicular to the polar axis 88 or the plane of the circuit board 50. The angle of rotation 92 of each LED 12 corresponds substantially to its azimuth angle 86 such that the LED's midlines appear to lie parallel to tangents of concentric rings centred on pole 84 of the polar coordinate scheme. As such, the beam pattern created by each individual LED 12 (as shown in Figure 2) is rotated in a viewing plane 22 about an angle 92 corresponding to its azimuth 86, which causes the overall beam pattern to be substantially circular.

In Figure 6, it will be noted that there is a centre LED and two surrounding rings of six LEDs. The total light output of each ring is given by the sum of the light outputs of the LEDs, and the apparent intensity of the light output of each ring varies inversely with the ring's radius 80. As such, the beam pattern produced by the LED array shown in Figure 6 has an intense central portion and a relatively less intense outer portion, which gives rise to a pseudo Gaussian or "bell shaped" beam pattern similar to that obtained from a single point source. Of course, the number of LEDs in each ring, the number of rings and the ring radii can all be varied to produce different beam patterns having different desired beam patterns and fall-offs.

It will be readily appreciated that the position of the LEDs on a circuit board, as shown in Figure 6, could equally be applied to the location of individual LED dies on a substrate of a multi-emitter LED. In such a situation, of course, the individual LED dies would be relatively rotated such that the LED as a whole emits a rounder beam than a conventional multi-element device in which each of the LED dies are arranged in a grid pattern and are not relatively rotated.

It will also be appreciated that it is the orientation of the beams emitted by the LEDs on the substrate, or the individual LED dies of the LED, that is important, rather than the physical orientation of the LEDs or dies themselves. In particular, it may be possible to use LEDs or dies whose beam patterns are rotated relative to their casings to achieve a similar effect, or to use LEDs mounted on fly-leads or "legs" that can be twisted to create an array of relatively rotated beams to achieve the same, or a similar, effect to the invention.

The LEDs 12 can be placed on the circuit board using an automated, four-axis pick-and-place machine, that is, a pick-and-place machine having X, Y and Z translation capabilities, and also the ability to rotate the LEDs at each X-Y-Z position.

The invention is not restricted to the details of the foregoing embodiments, which are merely exemplary. For example, the number of LEDs may be varied, the materials of manufacture may be changed, the relative spacings and rotations of the LEDs may be varied without departing from the scope of the invention.

Claims (1)

1. <claim-text>Claims: 1. A multi-LED array comprising plurality of spaced apart LED light emitters, wherein at least two of the plurality of LED light emitters have a different rotational orientation with respect to one another.</claim-text> <claim-text>2. An LED according to claim 1, the LED comprising an array of LED light emitters in the form of discrete LED dies disposed on a substrate, at least two of the plurality of LED dies having a different rotational orientation with respect to one another.</claim-text> <claim-text>3. A multi-LED array or LED as claimed in claim 1 or claim 2, wherein the beam patterns formed by at least two LED light emitters of the array are relatively rotated.</claim-text> <claim-text>4. A multi-LED array or LED as claimed in any of claims 1, 2 or 3, wherein the beam patterns formed by at least two LED light emitters are relatively rotated and overlap in a plane spaced apart from the array.</claim-text> <claim-text>5. A multi-LED array or LED as claimed in any preceding claim, wherein the outer periphery of the beam pattern formed by all of the LED light emitters in the array is any one or more of the group comprising: rounded; substantially circular; circular; and elliptical.</claim-text> <claim-text>6. A multi-LED array as claimed in any preceding claim, wherein the LED light emitters are mounted on a circuit board.</claim-text> <claim-text>7. A multi-LED as claimed in claim 6, wherein the LED light emitters comprise surface-mount LEDs.</claim-text> <claim-text>8. An LED array as claimed in any of claims 1 to 5, wherein the LED light emitters are LED dies disposed on a substrate.</claim-text> <claim-text>9. A multi-LED array or LED as claimed in any of claims 6 to 8, wherein the LED light emitters are located on the circuit board or substrate according to polar coordinate scheme, the position of each LED light emitter having a position defined by its radius and azimuth angle from a pole of the polar coordinate scheme, and wherein at least two of the LED light emitters of the array are rotated about an axis extending perpendicularly to the plane of the circuit board or substrate.</claim-text> <claim-text>10.A multi-LED array or LED as claimed in claim 9, wherein each of the LED light emitters in the array is rotated about an axis extending perpendicularly to the plane of the circuit board or substrate.</claim-text> <claim-text>11 A multi-LED array or LED as claimed in claim 9 or claim 10, wherein the angle of rotation of at least one LED light emitter of the array about the axis corresponds substantially to its azimuth angle.</claim-text> <claim-text>12.A multi-LED array or LED as claimed in any of claims 9, 10 or 11, wherein the angle of rotation of each of the LED light emitters of the array about their respective axes corresponds substantially to their respective azimuth angles.</claim-text> <claim-text>13.A multi-LED array or LED as claimed in claim 12, wherein a midline of each LED light emitter lies parallel to a tangent of a ring centred on the pole of the polar coordinate scheme.</claim-text> <claim-text>14.A multi-LED array or LED as claimed in claim 13, wherein the LED light emitters are arranged in groups arid wherein the midlines of the LED light emitters of each group lie parallel to tangents of a common ring, each group of LED light emitters sharing its own common ring, the rings of different groups of LED light emitters being concentric and centred on the pole of the polar coordinate scheme.</claim-text> <claim-text>15.A multi-LED array or LED as claimed in any of claims 11 to 13, wherein the beam pattern created, in use, by each individual LED light emitters is rotated in a viewing plane about an angle corresponding to its azimuth angle to cause, in use, the beam pattern of the array as a whole to have a substantially circular outer periphery.</claim-text> <claim-text>16.A multi-LED array or LED as claimed in claim 15, wherein the beam pattern is any one or more of the group comprising: Gaussian; bell-shaped; and substantially the same as that obtained from a single point source.</claim-text> <claim-text>17.A multi-LED array or LED as claimed in any of claims 7 to 16, wherein the circuit board comprises a plurality of spaced-apart contact pads to each of which contact pads, at least one LED is electrically connectable.</claim-text> <claim-text>18.A multi-LED array as claimed in claim 17, wherein at least two of the plurality of contact pads have a rotational orientation that is rotated with respect to one another.</claim-text> <claim-text>19.A multi-LED array as claimed in claim 17 or claim 18, wherein each contact pad comprises a cathode contact region and an anode contact region.</claim-text> <claim-text>20.A multi-LED array as claimed in claim 19, wherein each contact pad additionally comprises a mounting region to which mounting region a mounting portion of a surface-mount LED can be affixed.</claim-text> <claim-text>21 A multi-LED array as claimed in any of claims 17 to 20, wherein the anode and cathode contact regions are connected, in series, by electrically conductive tracks.</claim-text> <claim-text>22.A multi-LED array as claimed in any of claims 17 to 21, further comprising supplementary tracks leading to a single LED mounting pad only for powering the single LED independently of the remaining LEDs.</claim-text> <claim-text>23.A light fitting comprising a multi-LED array according to any preceding claim or a plurality of LEDs according to any of claims 2 to 22.</claim-text> <claim-text>24.A method of manufacturing a multi-LED array, the method comprising the steps of positioning a plurality of LEDs each on an electrical contact pad of a circuit board, and adhering the LEDs to their respective contact pads using an electrically conductive adhesive or solder, the positioning step including laterally spacing the LEDs relative to one another, and the method being characterised by additionally rotating at least some of the LEDs relative to one another during the positioning step.</claim-text> <claim-text>25.The method of claim 24, wherein the step of positioning and rotating the LEDs is performed, in use, by a pick-and-place machine or robot, the pick-and-place machine being configured for translation of the LEDs in the plane of the circuit board, for translation of the LED5 normal to the circuit board, and for rotation of the LEDs about an axis extending perpendicularly to the circuit board.</claim-text> <claim-text>26.The method of claim 25, wherein the pick-and-place machine comprises a four-axis pick-and-place machine being adapted for translation of the LEDs in three orthogonal exes and for rotation of the LEDs about an axis extending perpendicularly to circuit board.</claim-text> <claim-text>27.A multi-LED array, LED, light fitting or circuit board substantially as hereinbefore described, with reference to, and as illustrated in, Figures 4 to 7 of the accompanying drawfrigs.</claim-text>