GB2398926A

GB2398926A - Light emitting device

Info

Publication number: GB2398926A
Application number: GB0304627A
Authority: GB
Inventors: Richard Knight
Original assignee: Individual
Current assignee: Individual
Priority date: 2003-02-28
Filing date: 2003-02-28
Publication date: 2004-09-01
Also published as: GB0304627D0

Abstract

A light emitting device 10 comprises a plurality of spaced energisable light sources A,B,C and a main Holographic Optical Element (HOE) or a Diffractive Optical Element (DOE) 12. Light beams emitted from the light sources A, B, C are incident on and directed by the HOE or DOE 12, so that a light beam 14 output from the HOE or DOE is a uniform or substantially uniform mixture of the incident light beams. The HOE or DOE could be replaced by a main optical element which does not cause refraction and/or reflection of the incident light beams.

Description

LIGHT EMlTTIN(] DF,VICE This invention relates to a light emitting device.

Light emitting devices are well-known and come in all manner of shapes and sizes. One type of light emitting device is a Light Emitting Diode (LED). LED is a general term which is used to refer to any light emitting device where the light source is formed of one or more optical semi- conductor diodes.

An individual optical diode is referred to as a 'die', and a collection of die is referred to as 'dies' or 'dice'. The term 'chip', or 'chips', can be interchangeably used and have the same meaning.

In addition a 'package' or 'device' containing LED(s) can be referred to as a lamp-type device (typically these LEDs are 5mm diameter), a surfacemount device or diode (SMD), or an Emitter, such as made by Lumiled. Furthermore, the terms 2-in 1, 3-in-1, 4-in-1, and so forth, refer to more than one die being present in one package. The term 'gun', or 'guns', is also known to be used, particularly when there is the presence of more than one diode in a single device.

Herein, the term LED is taken to include any or all of the above terms and configurations, and any others not included above where the source of light is generated by an optical diode, or more than one optical diode. . . t 6 C, A,

C

it is known in an LF,D, which is one particular kind of light emitting device, to use a plurality of dies which emit light of different wavelengths to form a light beam of a colour definable by the intensity of the plurality of dies. Such produced light may include white as the sum of the plurality of wavelengths.

One known device is the Kingbright 4-in- 1 LED as shown in Figure I (R=Red, B=lilue, G=Green, C=Common). This device is only able to partially mix the light beams emitted from the dice due to the light sources being spaced apart from each other. This leads to a non-uniform colour distribution across the generated output beams, which are not collimated into a single beam, but remain, particularly at the outer edge of the beam, not mixed with any other colour.

Another known device is the Nichia 3-in-1 5 mm diameter NSTM515AS LED (not shown). This device requires the use of a diffuser on the transparent optical lens to mix the light beams emitted from the dice, so that a single light beam having a colour distribution of greater uniformity can be produced. However, when using a diffuser, a narrow light beam which is required in some applications is difficult and/or costly to achieve.

A 3-in-1 device without using diffusion, has the same inability to mix the separate colours together as shown by the Kingbright 4-in-1 LED represented in Figure 1. The Kingbright has a square layout in plan, and as such, although not shown in Figure 1, another blue die resides behind the shown blue die. The inability of the eolours to mix at the outer edge of each source beam is compounded by a greater À . À À .. À . :: e: À À number of light sources being present in a device.

If a plurality of separate LEDs, each with either a die or dice, is used, the problem of collimating and uniformly colour mixing the emitted light beams is further compounded as the distance between the centres of the sources, i.e. die or dice, is even greater by way of the placement of the LED.

The present invention seeks to overcome these problems.

According to a first aspect of the present invention, there is provided a light emitting device comprising two spaced energisable light sources and a main Holographic Optical Element (HOE) or a Diffractive Optical Element (DOE) arranged such that light beams emitted from the light sources are incident on and directed by the HOE or DOE so that a light beam output from the HOE or DOE is a uniform or substantially uniform mixture of the incident light beams.

Preferable and/or optional features of the invention are set forth in claims 2 to 18, inclusive.

According to a second aspect of the present invention, there is provided a light emitting device comprising two spaced energisable light sources and a main optical element arranged such that light beams emitted from the light sources are incident on and directed by the main optical element without being refracted and/or reflected by the main optical element, so that a light beam output from the main optical element is À e e c À a uniform or substantially uniform mixture of the incident light beams.

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, wherein: Figure 1 is schematic view of a prior art Kingbright 4-in-1 LED, Figures 2A and 2B are schematics showing a transmission-type and a reflection-type holographic optical element, respectively, Figure 3 is a schematic showing a first embodiment of a light emitting device with the three light sources incident on the plane of an optical element, in accordance with the present invention, Figure 4 is schematic showing a first possible arrangement of the light sources of the light emitting device shown in Figure 3, Figure is a schematic showing a second possible arrangement of the light sources of the light emitting device shown in Figure 3, Figure 6 is a plan view showing a second embodiment of a light emitting device having seven light sources, in accordance with the present invention, Figure 7 is a plan view of the shapes of the light beams emitted by the light c À À À . À À À À À e À À À À À sources in Figure 6, Figure 8 shows a third embodiment of a light emitting device, in accordance with the present invention, Figure 9 shows a modification to the third embodiment of the light emitting device, in accordance with the present invention, Figure 10 shows a fourth embodiment of a light emitting device, in accordance with the present invention, Figures 11 and 12 show a fifth embodiment of a light emitting device, in accordance with the present invention, and Figure 13 shows a sixth embodiment of a light emitting device, in accordance ] 5 with the present invention.

Referring firstly to Figures 2A and 2B, a Holographic Optical Element (HOE) and a Diffractive Optical Element (DOE) are similar devices and utilise aspects of diffraction gratings and holograms. A HOE and a DOE can diffract an incident light beam into its separate wavelength components. Since the HOE and DOE can be manufactured to produce a set amount of diffraction in three dimensions at a particular point on its surface, the HOE and the DOE can cause the distinct wavelengths (colours) in a single incident light beam to be focussed to separate and predetermined positions or points and that set amount of deflection or diffraction can be distinct for À a ' À e: :: e: one wavelength compared to another wavelength There are two distinct types of HOE and DOE, referred to as Transmission (see Figure 2A) and Reflection (see Figure 2B). [n the transmission-type of HOE / DOEI, incident light 2 is transmitted through the HOE/DOEI. In the reflection type of HOE/DOE 3, incident light 4 is reflected by a reflective surface which is distinct from but used in conjunction with the HOE/DOE.

Manufacture of a HOEis presently by the same technique used for making a holographic image. Two mutually coherent light beams are arranged to be incident on a recording medium, typically film emulsion. 'I'he interference pattern between the two beams is then recorded on and in the film emulsion.

A DOEis presently manufactured using micro-etching, similar to used to produce a diffraction grating. The grooves or etchings function in the same diffractive manner as the recorded interference pattern in a HOE. A DOEis advantageous over a diffraction grating in that there is no requirement for the etched lines to be parallel.

The micro-etchings can follow any appropriate configuration, including a series of concentric circles, avoids and/or ellipses.

The usage of a HOE and a DOE in the present invention is as a device which deflects or directs or 'reverse diffracts' the component wavelengths to different or specific or predetermined spatial points. The invention is therefore not limited to the use of a HOE and/or a DOE, and any optical element that achieves this could À À À À e therefore be used. As such, the term optical element (OF) is used hereinafter and is intended to include a HOE, a DOE or any other suitable device where the deflection or redirection is not caused by refraction, such as in the case of a lens which affects incident light by way of differing medium densities, or reflection. However, a mirrored surface can be used in conjunction with the OF to enable reflection.

Referring to Figure 3, a light emitting device 10 shown therein comprises a plurality of energisable light sources, being in this case three A, B and C, connected to an electrical circuit (not shown). Each light source A, B. C is spaced from the other light sources and emits light of a different wavelength or light having at least one constituent wavelength which is different from that or those of the light emitted by the other light sources. The two light sources A and B are arranged equidistant either side of the centre light source C. The light sources A, B and C are arranged so that the light beams emitted by light sources A and B are superposed with the light beam emitted by light source C on a plane xl-x2. Specifically, the centres al-a2 and bl-b2 of the emitted light beams of light sources A and B are angled to intersect the centre cl-c2 of the emitted light beam of light source (I. The centre cl-c2 of the light beam emitted by light source C lies on the centre yl-y2 of the light emitting device.

This arrangement of the light beams can be accomplished by positioning the light sources A, B and C at an angle to each other, as shown in Figure 4, and by using a lens, called an initial lens aL, bL, cL, in front of each light source A, B. C. À À À e ' À À , e e e Alternatively, referring to Figure 5, the angling of the light beams of the two outer light sources A and B in relation to the centre light source C may also be achieved using 'offset' initial lenses aL', bL', cL'. By offsetting, it is meant that the centres of the initial lenses aX and bX are offset relative to the centre of the respective light beams at source. In this case, the 'offset' is in a direction towards the centre yl y2 of the light emitting device. The three light sources can therefore be mounted on a single plane Z and in parallel spaced relationship.

A combination of arrangements shown in Figures 4 and 5 can be used.

Alternatively, or additionally, the initial lens aL/aL', bL/bL', and/or cL/cL' could be an aspheric lens.

Figure 3 shows the arrangement described with regard to Figure 5.

The initial beam (or field) angles of the light sources A, B. C are the same or substantially the same. Therefore, the emitted light beams are incident and superposed, or substantially superposed, on the plane xl-x2. The light beams from light sources A and B will be ovoid in shape on the plane xl-x2, and the light beam from light source C will be circular. Each distinct wavelength will arrive at the surface xl-x2 at a differing angle of incidence to that of a differing wavelength from a differing location, arriving at the same point on the surface.

The light emitting device further comprises an optical element (OK) 12, as hereinbefore defined. The OF is positioned on the plane xl-x2. The light beams . À ces . À * À À À . À À c incident on plane xl-x2 therefore 'reverse diffract' and so deflect or deviate at this surface by way of their previous path being interrupted by the OK. The correct angle of required deflection, by way of 'reverse diffraction', is created by the OE such that the separate incident light beams are uniformly or substantially uniformly mixed and collimated, or substantially collimated, to form a single output light beam 14 at the exit pupil 16 of the OK.

By exit pupil it is meant the active area of the HOE or DOE surface being the overlap of the incident light rays.

Referring to Figures 6 and 7, a second embodiment of the invention is shown.

The light emitting device 10 of this embodiment comprises seven light sources 101 107, each spaced from the other and, when energised, emitting light of different wavelengths (i.e. colour) to each other or light having at least one constituent wavelength which is different from that or those of the light emitted by the other light sources. The seven light sources are arranged so that six of the light sources 101-106, being perimeter light sources, are equiangularly and equidistantly spaced around a centre source 107. The emitted light beams are arranged to travel from their respective locations in such a manner as to strike the OE 12 such that the constituent wavelengths of the light beams are superposed, or substantially superposed, at the plane of incidence of the OE 12. The centre source would produce a circular shape at the OE with each of the other sources producing an ovoid shape, the exact shape of which would be determined by their distance from the OE and their position relative to the centre of the OK. Each superposed light beam emitted by a perimeter light .- - À . c e À À C source overlaps the other superposed light beams at the OF (see Figure 7) due to the spaced relationship of the light sources. However, the centre of each ovoid shape formed by the six perimeter light sources on the OE are arranged to approximate to the centre of the circular shape from the centre light source. The OE directs or 'reverse diffracts' each incident ray of the light beams by an amount required to create a single collimated or substantially collimated output light beam composed of the constituent wavelengths.

It should be noted that the other embodiments described herein could also have seven light sources, similar to the second embodiment, or in fact any suitable number of light sources.

The part of the emitted light beam having the greatest angle of incidence, when incident with the OK, will require the greatest deflection by 'reverse diffraction' to create a substantially collimated output light. Conversely, the part of the emitted light beam having the smallest angle of incidence, when incident with the OK, will require the least deflection. As such, the part of the light beam requiring the least deflection could be arranged parallel to the centre line yl-y2 of the light emitting device, and so pass through the exit pupil of the OE uninterrupted and not deflected. The degree of required deflection at each and any point over the OE exit pupil is different for each present wavelength.

A specific wavelength of a light beam, arriving at a specific position, at a specific incident angle, has a specific angle of diffraction or 'reverse diffraction' :ce: ece e À . À . À e À À À requirement at the exit pupil of the OK. An optical element, such as a HOE or [JOE, is able to provide this at any given point on the exit pupil because it can discriminate between the wavelengths of incident light beams, for example Red, Green and Blue, arriving at the same point, by way of both the differing wavelengths and the differing incident angles. As such, at any point on the OK, the OE deflects these incident light beams by a bespoke and unique degree for each distinct wavelength.

A Diffractive Optical Element is advantageous over a Holographic Optical Element in that it is easier to mass produce, since the etched grooves are open on one side of the DOE whereas aspects of the recorded interference pattern in a HOE are contained within the recording medium.

rl'he use of more than one OE where a large number of distinct wavelengths are involved is not precluded as there is a potential limit to the number of wavelengths one OE can efficiently diffract or 'reverse diffract' at any given point on its surface.

Referring now to Figures 8 and 9, a third embodiment of a light emitting device 10 is shown. In this embodiment, an OE performs the function of a lens. An initial beam reduction of the sources using a secondary OE 18 directs the light beams onto the primary collimating and wavelength mixing OE 12, being the exit pupil 16.

The secondary OE 18 therefore performs the initial adjustment of the emitted light beams and replaces the initial lenses aL/aL', bL/bL', and/or cL/cL' of the first and second embodiments. ? a '

., ' The secondary OF, is generally arranged on a single piece of OF, material The secondary OE can thus be folded or shaped to obtain the required output light beam.

As shown in Figure 8, the secondary OE 18 is symmetrically positioned with respect to the light sources a, B. C, and each light source is arranged as described previously with reference to Figure 4.

Alternatively, as shown in Figure 9, the light emitting device 10 could be arranged as described with respect to Figure 5, with an appropriately configured planar secondary OE 20 replacing each offset or aspheric initial lens aL/aL', bL/bL', and/or cL/cL'. An advantage of manufacturing accrues with a single plane, single piece, flexible material OE over separate individual initial narrowing lenses as previously described, and an OE used in this manner is lighter and thinner than a more typical lens array.

In a modification to the third embodiment, a plurality of separate secondary OEs could replace the initial lenses aL/aL', bL/bL', and/or cL/cL'. These separate or individual secondary OEs could be non-planar to provide the required characteristics of the initial lenses being replaced.

By the inclusion of a tertiary OE 22, as shown in Figure 1O, a light emitting device of a fourth embodiment can function as if incorporating a set of lenses. In this case, the secondary OE 20 or OEs replace the initial lenses aL/aL', bL/bL', and/or cL/cL' of the first and second embodiments, the primary OE 12 acts to collimate and mix the incident light beams, and the tertiary OE 22 acts to converge or, as shown in l # r. a a

Figure 10, diverge the collimated and mixed beam output from the primary OF 12.

Referring to Figures 11 and 12, a fifth embodiment of a light emitting device is shown. In this embodiment, the primary collimating and mixing OE 12 and the secondary initial beam adjustment OE 20, as shown in Figure 11, and the primary, secondary and tertiary OEs 12, 20, 22, as shown in Figure 12, are formed as a single layer.

In a modification to the fifth embodiment, a single OE could be formed to function as the primary and secondary OEs 12, 20, as the primary and tertiary OEs 12, 22, and/or as the primary, secondary and tertiary OEs 12, 20, 22.

It is known that a conventional refracting lens has a disadvantage in that differing wavelengths are refracted by a differing amount. This phenomenon is referred to as Chromatic Aberration.

As an HOE or DOE can be designed to operate as a lens or series of lenses, an advantage accrues by way of a layer, of HOE being used to concentrate or diverge the collimated output, as an overarching single or multiple lens layer. As the HOE can be designed to affect differing incident wavelengths in a unique manner, such usage would avoid the Chromatic Aberration problem associated with conventional refracting lenses.

The above embodiments have been described with reference to a tMnsmission

- c

type OK. For completeness, Figure 13 shows a final embodiment of a light emitting device which utilises a reflection-type OE 24. This light emitting device is shown as a 2-in-1 type device having a primary collimating and mixing OE 24 which operates in conjunction with a reflective surface 26, such as a mirror, distinct from the OE 24 to enable reflection. However, other configurations of light sources and OEs, similar to those described above, are entirely possible.

The output light beam is therefore in the reverse, or substantially reverse, direction of travel of the input light beams. This embodiment advantageously provides a smaller height to the light emitting device.

Although the primary, second and/or tertiary OEs have, for the most part, been described as being planar, it would be entirely possible to shape or flex one or each OE so that it takes a non-planar configuration, such as convex or concave.

The primary collimating OE can be dispensed with, leaving only the secondary and/or tertiary OEs, if only a uniform, or substantially uniform, mixing of the incident light beams, and not collimation, is required. This would allow a convergent or divergent output light beam to be formed without the need for collimation at any point.

The primary use of a light emitting device hereinbefore described is seen as being for Theatrical Lighting, Architectural Lighting and/or Domestic Lighting However, such a light emitting device is not limited to use in these fields and could be ce' e.A- À À À À 1 À À r

used in any suitable field.

It is thus possible to provide a light emitting device which utilises a Holographic Optical Element or a Diffractive Optical Element to reverse diffract a plurality of incident light beams. It is further possible to provide a light emitting device in which the degree of reverse diffraction for each differing wavelength of the input light sources at the HOE or DOE can be designed to be specific and exclusive to one particular wavelength and different for another wavelength. The same point on an HOE or DOE can thus produce a differing degree of reverse diffraction for each wavelength present at that point and can do so for all other point on the exit pupil of the HOE or DOE. The HOE or DOE can be designed to produce a different reverse diffractive response for the incident wavelengths at any points over its exit pupil for each of the differing wavelengths being present at any point on the exit pupil.

Each wavelength of the spaced light sources derives from a differing location, and therefore has a unique set of incident light ray angles arriving at all points on the HOE or DOE surface at the exit pupil. Each set of incident light ray angles differ from any other incident wavelength from a differing source. The design of the HOE or DOE is such that all points on the exit pupil act upon the unique combination of both the wavelength of any ray of light and the associated incident angle of that ray.

The embodiments described above are given by way of example only, and other modifications will be apparent to persons skilled in the art without departing from the scope of the invention as defined by the appended claims. For example, each À À À À r.

À À À r r À À r r light source could emit the same colour light (i.e. light having the same wavelength or constituent wavelengths) if required; and the light sources could be any type or combination of light sources, such as a filament light source and/or an electroluminescent light source.

Claims

À : : À À À À CLAIMS

1. A light emitting device comprising two spaced energisable light sources and a main Holographic Optical Element (HOE) or a Diffractive Optical Element (DOE) arranged such that light beams emitted from the light sources are incident on and directed by the HOE or DOE so that a light beam output from the HOE or DOE is a uniform or substantially uniform mixture of the incident light beams.

2. A light emitting device as claimed in claim 1, wherein the main HOE or DOE is a primary HOE or DOE and acts to collimate or substantially collimate the incident light beams.

3. A light emitting device as claimed in claim 1 or claim 2, wherein each light beam emitted from its respective light source has a wavelength or at least one of its constituent wavelengths which is different from the wavelength or wavelengths of the light beam emitted from the other light source.

4. A light emitting device as claimed in any one of claims 1 to 3, wherein the light sources are light emitting diodes (LEDs).

5. A light emitting device as claimed in any one of the preceding claims, further comprising a secondary HOE or DOE between the light sources and the main HOE or DOE and which performs an initial adjustment on the emitted light beams. À À À

À c.

À À À À 1

6. A light emitting device as claimed in claim 5, wherein the secondary HOF. or DOEis spaced form the main HOE or DOE.

7. A light emitting device as claimed in claim 5, wherein the secondary HOE or DOEis in contact with the main HOE or DOE.

8. A light emitting device as claimed in claim 5, wherein the secondary HOE or DOEis unitarily formed with the main HOE or DOE as a single optical element.

9. A light emitting device as claimed in any one of the preceding claims, further I comprising a tertiary HOE or DOE on which the light beam output form the main HOE or DOEis incident, the tertiary HOE or DOE performing a final adjustment on the light beam output from the main HOE or DOE so that a convergent or divergent light beam is output form the tertiary HOE or DOE.

9. A light emitting device as claimed in claim 8, wherein the tertiary HOE or DOEis spaced from the main HOE or DOE.

10. A light emitting device as claimed in claim 8, wherein the tertiary HOE or DOEis in contact with the main HOE or DOE.

11. A light emitting device as claimed in claim 8, wherein the tertiary HOE or DOEis unitarily formed with the main HOE or DOE as a single optical element.

he À - À . . À :: À .

12. A light emitting device as claimed in claim 5 or claim 9, wherein the, one or each HOE or DOE functions as a lens or as a set of lenses.

13. A light emitting device as claimed in any one of the preceding claims, wherein the or each HOE or DOE is planar or substantially planar.

14. A light emitting device as claimed in any one of claims I to 12, wherein the, one or each HOE or DOE is non-planar.

15. A light emitting device as claimed in any one of the preceding claims, wherein the, one or each HOE or DOE is a transmission-type HOE or DOE.

16. A light emitting device as claimed in any one of claims I to 14, wherein the, one or each HOE or DOE is a reflection-type HOE or DOE.

17. A light emitting device as claimed in any one of the preceding claims, wherein the number of spaced energisable light sources is three.

18. A light emitting device as claimed in any one of the preceding claims, wherein the number of spaced energisable light sources is seven.

19. A light emitting device comprising two spaced energisable light sources and a main optical element arranged such that light beams emitted from the light sources are incident on and directed by the main optical element without being refracted and/or À À . À À . À . À À À À reflected by the main optical element, so that a light beam output from the main optical element is a uniform or substantially uniform mixture of the incident light beams.

20. A light emitting device substantially as hereinbefore described with reference to Figures 3 to 5? Figures 6 and 7, Figure 8, Figure 9, Figure 10, Figure 11, Figure 12, or Figure 13 of the accompanying drawings.