GB2466787A

GB2466787A - A light emitting diode lamp with reflective optical diffuser

Info

Publication number: GB2466787A
Application number: GB0900065A
Authority: GB
Inventors: John Stephen Allard; John Matcham
Original assignee: Greengage Lighting Ltd
Current assignee: Greengage Lighting Ltd
Priority date: 2009-01-05
Filing date: 2009-01-05
Publication date: 2010-07-14
Also published as: GB0900065D0

Abstract

An electric lamp comprises a light emitting diode, LED, unit 18 for producing a divergent beam of light and a curved optical diffuser 20 arranged so that light from the LED unit is projected against an inner surface of the curved diffuser. A significant proportion of light which is incident on the diffuser's inner surface is transmitted through the diffuser to the outside and a significant proportion of light which is incident on its inner surface is reflected within the curved diffuser. A reflective surface 80 is arranged so that at least some of the light which is reflected by the inner surface of the curved diffuser and which impinges upon the reflective surface is reflected back to the inner surface of the curved diffuser. As a result of the partial reflections at the diffuser and the reflections at the reflective surface, light impinges on the inner surface of the diffuser from different directions in addition to the direction directly from the LED unit, and proportion of that light is transmitted to the outside. Accordingly, the lamp can readily be arranged so that its view half-angle as a whole is greater than the view half-angle of the bare LED unit and so that its apparent source area is greater than the bare LED unit, without any excessive hot-spots.

Description

TITLE

LED lamps

DESCRIPTION

This invention relates to light-emitting diode (LED) lamps.

LED devices having a single diode junction have been known for many years and used as, for example, indicator lights. Their advantages include high efficiency and long life.

Traditionally, however, disadvantages have included low light output, a narrow view half-angle and a restricted range of colours. More recently, LED units have been manufactured with a large number of diode junctions formed on a single substrate and connected in series.

Obviously, this increases the potential light output, and the LED units can be designed to run from the mains with few additional electrical components. Also, different junctions can be designed to produce different colours, thus affecting the overall colour of light produced by the LED unit.

An example of a high voltage LED unit that has been placed on the market is the Acriche AN3220 manufactured by Seoul Semiconductor. It is designed to require an input voltage of about 195 V AC to draw an optimum current of 20 mA and produces a luminous flux at that current typically of 150 lumens. This LED unit has its own lens which is approximately hemispherical with a small diameter of about 10 mm. The unit produces a divergent beam of light with a view half-angle of about 55 degrees. In other words, the luminous flux when viewed at an angle of about 55 degrees to the primary axis of the beam is one half of the luminous flux when viewed along the primary axis. At an angle of about 90 degrees to the primary axis of the beam, the relative luminous flux is substantially zero. When viewed along the primary axis, the LED unit appears as a very bright spot source of very small area.

Although such an LED unit may be used with no or few additional optical components and with few additional electrical components to emulate a mains-powered spotlight, it would be desirable also be able to use such an LED unit to emulate more closely a conventional electric light bulb, such as a tungsten filament light bulb with a pear-shaped or round glass envelope. In particular, it would be desirable to be able to use such an LED unit in an electric lamp which has a greater view half-angle, which appears as a light source of greater area and which does not have any excessive hot-spot' within that source area. It would also be desirable to be able to use an LED unit which produces a substantially linear, rather than spot, source of light to emulate more closely a tungsten filament strip light or fluorescent strip light. An aim of the present invention, or at least of specific embodiments of it, is to provide such a lamp.

In accordance with the present invention, there is provided an electric lamp comprising: an LED unit for producing a divergent beam of light; a curved optical diffuser disposed relative to the LED unit so that light from the LED unit is projected against an inner surface of the curved diffuser, the diffuser being such that a significant proportion of light which is incident on its inner surface is transmitted through the diffuser to the outside and a significant proportion of light which is incident on its inner surface is reflected within the curved diffuser; and a reflective surface disposed relative to the curved diffuser so that at least some of the light which is reflected by the inner surface of the curved diffuser and which impinges upon the reflective surface is reflected back to the inner surface of the curved diffuser. As a result of the partial reflections at the diffuser and the reflections at the reflective surface, light impinges on the inner surface of the diffuser from different directions in addition to the direction directly from the LED unit, and at each reflection at the diffuser a proportion of that light is transmitted to the outside. Accordingly, the lamp can be arranged so that its view half-angle as a whole is greater than the view half-angle of the bare LED unit and so that its apparent source area is greater than the bare LED unit, without any excessive hot-spots.

The diffuser is preferably disposed relative to the LED unit so that all, or substantially all, of the light produced by the LED unit impinges on the inner surface of the diffuser.

The inner surface of the diffuser (i) preferably diffusely reflects the light incident thereon and (ii) preferably reflects between 15 and 50% of the light incident thereon.

The reflective surface (i) preferably diffusely reflects the light incident thereon; (ii) is preferably opaque; and (iii) is preferably substantially flat.

In one embodiment in which the LED unit provides a substantially spot-like source of light, the diffuser is preferably domed, and more preferably generally part-spherical. In this case, the reflective surface is preferably generally circular. The diffuser preferably has a diameter which is substantially greater (for example at least four times greater) than the diameter of the spot at the LED unit.

The diffuser may be mounted at one end of a body of the lamp, and a connector may be provided at the opposite end of the body for connecting the lamp to a mains lighting outlet.

In another embodiment in which the LED unit produces a substantially linear source of light, the diffuser is preferably substantially part-cylindrical, and more preferably part-circularly-cylindrical. In this case, the reflective surface is preferably generally rectangular. In the case where the line of light produced by the LED unit has a particular width at the LED unit, the diffuser preferably has a diameter which is substantially greater (for example at least four times greater) than the width of the line at the LED unit.

The diffuser and/or the reflective surface are preferably disposed substantially symmetrically relative to a primary axis or plane of the beam of light produced by the LED unit.

Although LEDs are efficient light producers, the AN3220 LED unit, for example, typically produces about 3.3 W of heat, and it is important with such a small device to dissipate that waste heat effectively, otherwise the temperature of the junctions of the LED unit will rise above the rated maximum of 125 C. The lamp of the invention preferably has a heat sink on which the LED unit is mounted, and in this case the reflective surface is preferably provided by a surface of the heat sink. The heat sink is preferably provided by a body of the lamp.

A specific embodiment of the present invention will now be described, purely by way of example, with reference to the accompanying drawings, in which: Figure 1 is a side view of an electric lamp; Figure 2 is an underplan view of the lamp; Figure 3 is a sectioned side view of the lamp; Figure 4 is an exploded sectioned side view of the lamp; Figure 5 is an isometric view of an LED unit of the lamp; Figure 6 is a circuit diagram of the lamp; Figure 7 is a graph of relative spectral power against wavelength for the light produced by the lamp; Figure 8 is shows a radiation pattern of the beam produced by the LED unit; Figure 9 is a ray diagram illustrating reflections and transmissions at a diffuser and reflector of the lamp; and Figure 10 is a diagram illustrating the light produced by the lamp.

Referring to Figures 1 to 5 of the drawings, an LED lamp 10 comprises front and rear body portions 12,14 secured together by three screws 16, an LED unit 18, a domed diffuser 20, a connector cap 22 and electrical circuitry 24 connecting the LED unit 18 to the connector cap 22.

The front body portion 12 is moulded from a white opaque plastics material having a matt surface so that it diffusely reflects incident light. The front body portion 12 has a circular wall 26 of a diameter of, for example, 60 mm. A cylindrical wall 28 projects forwardly from the periphery of the circular wall 26. A further cylindrical wall 30 of smaller diameter than the front cylindrical wall 28 projects rearwardly from the circular wall 28. A pair of holes 32 are formed in the circular wall 26 within the internal diameter of the rear cylindrical wall 30. A plurality of heat-radiating fins 34 beneath the circular wall 28 radiate from the rear cylindrical wall 30.

The rear body portion 14 is moulded from plastics material and is generally annular.

The front face 36 of the rear body portion 14 is flat, and three holes 38 are provided, through which, upon assembly of the lamp 10, the screws 16 are passed into engagement with three screw-threaded holes 40 in the rear of the rear cylindrical wall 30 of the front body portion 12.

A plurality of ventilation holes 42 are spaced apart around the rear body portion 14. The rear end of the rear body portion 14 is formed as a cylindrical spigot 44 with a external crimping groove 46.

The connector cap 22 is of conventional design and in the drawing is shown as a BC' or B22' cap, but other designs of cap 22 may alternatively be employed. The cap 22 has a pair of terminals 48, and a pair of bayonet pins 50. Upon assembly of the lamp 10, the spigot 44 of the rear body portion 14 is fitted into the mouth 52 of the cap 22, and the cap is then crimped in the region 54 into the crimping groove 46 of the rear body portion 14.

The LED unit 18 has a disc-shaped heat-sinking base 56 onto which a semi-spherical lens 58 is formed. A large number of LEDs (not shown) are formed on the base 66 under the lens 58. The LEDs are series connected between a pair of solder pads 60. The LED unit 18 is attached by its solder pads 60 to a small printed circuit board 62. A thermally conducting paste may be applied between the base 56 of the LED unit 18 and the printed circuit board 62. The printed circuit board has two holes 64 for connection to the electrical circuitry 24. An example of the LED unit 18 is the Acriche AN3220 manufactured by Seoul Semiconductor.

The diffuser 20 is moulded from translucent plastics material which has optical transmission and reflection characteristics that will be described in more detail below. The diffuser 20 has the form of a dome with a generally uniform wall thickness. A main portion 66 of the diffuser 20 is part-spherical, being slightly less than a hemisphere, and having an external radius of, for example, about 30 mm. At the rear edge of the main portion 66, a connecting portion 68 is provided in the form of a cylindrical skirt having a external diameter which is about the same as the internal diameter of the front cylindrical wall 28 of the front body portion 12.

The electrical circuitry 24 comprises: a printed circuit board 70 on which are mounted a number of components 72 including a large capacitor Cl; a pair of wires 76 for connecting the printed circuit board 70 to the terminals 48 of the connector cap 22; and a pair of wires 78,79 for connecting the printed circuit board 70 to the LED unit 18.

During assembly of the lamp 10: the connector cap 22 is crimped to the rear body portion 14; the ends of the wires 76 are soldered to the connecting cap terminals 48 and to the printed circuit board 70; and the wires 78 are soldered to the printed circuit board 70. Also, the printed circuit board 62 of the LED unit 18 is bonded to the centre of the front face 80 of the circular wall 26 of the front body portion 12, using a thermally-conductive adhesive, with the holes 64 of the LED circuit board 62 aligned with the holes 32 through the circular wall 26 of the front body portion 12. The electrical circuitry 24 is then inserted in the cavity 82 formed by the rear cylindrical wall 30 of the front body portion 12, and the front ends of the wires 78 are guided through the holes 32 in the wall 26 and through the holes 64 in the LED circuit board 62. The screws 16 are then used to secure the front and rear body portions 12,14 together. The front ends of the wires 78 are then soldered to the LED circuit board 62. The diffuser 20 is then fitted to the front body portion 12 using a suitable adhesive, with the skirt 68 being fitted inside the front cylindrical wall 28 of the front body portion 12.

Referring now to Figure 6, the wires 76 from the connector cap terminals 48 are connected to the AC inputs of a rectifier bridge Dl on the printed circuit board 70. The positive output from the bridge Dl feeds via a diode D2, positive temperature coefficient thermistor Ri, resistor R2 and one of the wires 78 to the LED unit 18. From the LED unit 18, the return path is via the other wire 79, a choke Li and a diode D3 to the negative output of the bridge Dl.

The large capacitor Ci is connected between the wire 78 and the negative output of the bridge Di. The charging and discharging of the capacitor Ci is controlled by a sub-circuit 84 fed from the positive output of the bridge Di and comprising resistors R3,R4, capacitor C2, transistor Qi, resistors R5,R6, capacitor C3 and transistor Q2.

In operation, when full mains voltage is initially suddenly applied to the cap terminals 48, substantially full, full-wave-rectified mains voltage is applied to the series connection of the thermistor Ri, resistor R2, LED unit 18 and choke Li. However, the large capacitor Ci is initially discharged. Therefore, the voltage applied to the LED unit 18 rises progressively from zero as the large capacitor Ci charges. The charging of the large capacitor Ci is moderated by the sub-circuit 84 in which transistor Qi progressively turns on, and in which transistor Q2 immediately turns on but is then progressively turned off by transistor Qi. The values of the circuit components are chosen so that the turn-on time of the LED unit 18 is about 0.5 seconds, so that the LED unit 18 emulates a standard tungsten light bulb being switched on.

In operation of the lamp 10 in its steady on' state, transistor Qi is on, but the resistance of resistor R5 is high so that negligible current flows through transistor Qi, and transistor Q2 is off. The thermistor Ri and resistor R2 serve to drop the voltage applied to the LED unit 18 to a suitable value, and the large capacitor Ci is charged and smoothes the voltage applied to the LED unit 18. The thermistor Ri regulates the supply to the LED unit 18 by increasing its resistance if the lamp 10 becomes too hot or if the current through the thermistor Ri becomes too high, for example due to prolonged over-voltage applied to the cap terminals 48. The capacitor Ci and choke Li also serve to protect the LED unit 18 from any fast transient spikes.

When the supply to the connector cap terminals 48 is suddenly switched off, the large capacitor Ci is initially fully charged and progressively discharges through the LED unit 18.

The discharging of the large capacitor Ci is assisted by the sub-circuit 84 in a controlled manner, in which transistor Qi progressively turns off, and in which is transistor Q2 is progressively turned on by transistor Qi before finally turning off. The values of the circuit components are chosen so that the turn-off time of the LED unit 18 is about 2 seconds, so that the LED unit emulates a standard tungsten light bulb being turned off.

If an LED unit such as the unit 18 were connected to a triac dimmer rather than directly to the mains, with some designs of triac dimmer the LED unit would not come on and go off smoothly. In particular, at low settings, the circuit can hunt so that the LED unit would flicker sporadically at less than mains frequency. For example, when the dimmer setting is progressively increased from fully off, the LED unit changes from an off state to a flickering state and then to a low state in which is light output of the LED unit is greater than would be desired. However, with the circuit of Figure 6, and particularly the large capacitor Ci, this flickering phenomenon can be avoided and, with progressive increase of the dimmer setting, the light output of the LED unit 18 progressively increases from zero to full without any flickering or significant step changes in the light output form the LED unit 18.

In an example of the circuit of Figure 6 designed to operate with a nominal mains voltage of 230V AC, the LED unit 18 may be provided by an Acriche AN3220 manufactured by Seoul Semiconductor, which draws a current of about 2OmA at an applied voltage of about 195V. Example values of the other circuit components are: thermistor Ri, 300 Ohms cold; resistor R2, 500 Ohms; resistor R3, 300 kOhms; resistor R4, 20 kOhms; resistor R5, 300 kOhms; resistor R6, 1 kOhm; capacitor Cl, 220 microF; capacitor C2, 4.7 microF; capacitor C3, 47 microF; and choke Li, 68 microH. It should be noted that the Acriche 3220 contains two chains of LEDs of opposite polarities connected between its terminal tags 60, and that with the circuit of Figure 6, only one of those chains of LEDs is driven.

Figure 7 shows the relative spectral power of the light produced by the lamp 10 when employing an Acriche AN3 220 for the LED unit 18 and a non-tinted diffuser 20. It is to be noted that the relative spectral power of ultra-violet light is substantially zero, and that the relative spectral power of infrared light is very low and decreases to substantially zero at 800 nm.

Figure 8 shows the light radiation pattern from the bare LED unit 18, both in polar form to the right and in Cartesian form to the left. It will be noted that the direction of maximum luminous flux coincides with the primary axis 86 of the LED unit 18. Also, the relative luminous flux falls to 50% at an angle A of about 55 to 60 degrees. At an angle of 90 degrees the relative luminous flux has fallen to substantially zero. In the case of the Ariche AN3220, the diameter DL of the lens 58 is relatively small, being 10 mm. It will be appreciated that the apparent area of the lens 58 of the LED unit 18, when viewed on the primary axis 86, is Api DL2, or 79mm2. The domed diffuser 20 and the reflective nature of the front face 80 of the circular wall 26 of the front body portion 12 serve to increase the apparent area of the source of light and also to increase the angles at which the relative luminous flux has fallen to 50% and substantially to zero, as will now be described.

Referring to Figure 9, the diffuser 20 is moulded or treated so that the transmission coefficient for light incident on its inner surface is about 70%, the reflection coefficient is about 30%, and the absorption coefficient is a close to zero as possible. The reflection is diffuse and may be Lambertian reflection or, as shown in Figure 9, hazy reflection in which the maximum of reflected power is in the specular direction, but nevertheless with a significant amount of scattering. Also, the front body portion 12 is moulded or treated so that the reflection coefficient for light incident on the surface 80 is as close as possible to 100%. Again, the reflection is diffuse and may be Lambertian reflection or hazy reflection. As a result of the partial reflectance of the diffuser 20 and the reflectance of the surface 80, the inside of the domed diffuser 20 fills with light travelling in all directions, and as each ray of light impinges on the inner surface of the main portion 66 of the diffuser 20, only about 70% of its radiant power is transmitted through the diffuser 20 to the outside.

In a prototype, it was found that the angle B at which the relative luminous flux falls to 50% of the luminous flux in the direction of the axis 86 was about 100 degrees. This compares with about 55 to 60 degrees for the bare LED unit 18. An example of a ray transmitted from the lamp 10 at an angle B of 100 degrees to the primary axis 86 is indicated by the reference numeral 88 in Figure 10. Examples of ray paths leading to the ray 88 are also shown in Figure 10, taking the paths abcdef, abgdef, hicdef, higdef. It will also be appreciated that the diameter DD of the diffuser, being slightly less than 60 mm, is almost six times greater than the diameter DL of the lens 58, and therefore that the apparent area of the of the diffuser 20, when viewed on the primary axis 86, is almost thirty-six times the apparent area of the lens 58. The overall effect of the diffuser 20 and reflective surface 80 is to increase the view angle of the LED unit 18 and to increase the apparent area of the light source whilst avoiding any excessive hot-spot' effect.

It will be appreciated that many modifications and developments may be made to the embodiment of the invention described above. For example, although the described embodiment of the invention generally resembles a pear-shaped light bulb, the invention may also be applied to a lamp resembling a strip light, employing an LED unit in which the LED junctions are arranged in a line rather than being concentrated in a spot. In this case, the reflective surface of the heat sink may be rectangular having a length slightly greater than the length of the LED unit, with the LED unit being mounted along the longitudinal centre-line of the reflective surface, and the diffuser may be part-circularly cylindrical, for example generally semi-circularly cylindrical, of similar length to the reflective surface and with its axis approximately coinciding with the line of LED junctions.

It should be noted that the embodiment of the invention has been described above purely by way of example and that many other modifications and developments may be made thereto within the scope of the present invention.

Claims

-10 -CLAIMS(The reference numerals in the claims are not intended to limit the scope of the claims.) 1. An electric lamp (10) comprising: a LED unit (18) for producing a divergent beam of light; a curved optical diffuser (20) disposed relative to the LED unit so that light from the LED unit is projected against an inner surface of the curved diffuser, the diffuser being such that a significant proportion of light which is incident on its inner surface is transmitted through the diffuser to the outside and a significant proportion of light which is incident on its inner surface is reflected within the curved diffuser; and a reflective surface (80) disposed relative to the curved diffuser so that at least some of the light which is reflected by the inner surface of the curved diffuser and which impinges upon the reflective surface is reflected back to the inner surface of the curved diffuser.
2. A lamp as claimed in claim 1, wherein the diffuser is disposed relative to the LED unit so that all, or substantially all, of the light produced by the LED unit impinges on the inner surface of the diffuser.
3. A lamp as claimed in claim 1 or 2, wherein the inner surface of the diffuser diffusely reflects the light incident thereon.
4. A lamp as claimed in any preceding claim, wherein the inner surface of the diffuser reflects between 15 and 50% of the light incident thereon.
5. A lamp as claimed in any preceding claim, wherein the reflective surface diffusely reflects the light incident thereon.
6. A lamp as claimed in any preceding claim, wherein the reflective surface is opaque.
7. A lamp as claimed in any preceding claim, wherein the reflective surface is substantially flat.

-11 -
8. A lamp as claimed in any preceding claim, wherein the LED unit provides a substantially spot-like source of light, and the diffuser is domed.
9. A lamp as claimed in claim 8, wherein the diffuser is generally part-spherical.
10. A lamp as claimed in claim 8 or 9, wherein the reflective surface is generally circular.
11. A lamp as claimed in any of claims 8 to 10, wherein the spot of light produced by the LED unit has a particular diameter (DL) at the LED unit, and the diffuser has a diameter (DD) which is substantially greater than the diameter of the spot at the LED unit.
12. A lamp as claimed in any of claims 8 to 11, wherein the diffuser has a diameter which is at least four times as great as the diameter of the spot at the LED unit.
13. A lamp as claimed in any preceding claim, wherein the LED unit produces a substantially linear source of light, and the diffuser is substantially part-cylindrical.
14. A lamp as claimed in claim 13, wherein the diffuser is substantially p art-circularly-cylindrical.
15. A lamp as claimed in claim 13 or 14, wherein the reflective surface is generally rectangular.
16. A lamp as claimed in any of claims 13 to 15, wherein the line of light produced by the LED unit has a particular width at the LED unit, and the diffuser has a diameter which is substantially greater than the width of the line at the LED unit.
17. A lamp as claimed in any of claims 13 to 16, wherein the diffuser has a diameter which is at least four times as great as the width of the line at the LED unit.
18. A lamp as claimed in any preceding claim, wherein the diffuser is disposed substantially symmetrically relative to a primary axis (86) or plane of the beam of light produced by the LED unit.

-12 -
19. A lamp as claimed in any preceding claim, wherein the reflective surface is disposed substantially symmetrically relative to a primary axis or plane of the beam of light produced by the LED unit.
20. A lamp as claimed in any preceding claim, wherein the lamp has a heat sink (12) on which the LED unit is mounted, the reflective surface being provided by a surface of the heat sink.
21. A lamp as claimed in claim 20, wherein the heat sink is provided by a body (12) of the lamp.
22. A lamp as claimed in any preceding claim, wherein the diffuser is mounted at one end of a, or the, body of the lamp, and a connector (22) is provided at the opposite end of the body for connecting the lamp to a mains lighting outlet.
23. An electric lamp substantially as described with reference to the drawings.