TECHNICAL FIELD

The present invention relates to an improved field emission lighting arrangement. More specifically, the invention relates to a field emission lighting arrangement which has been adapted for improved heat dissipation during operation.

BACKGROUND OF THE INVENTION

There is currently a trend in replacing the traditional light bulb with more energy efficient alternatives. Florescent light sources also in forms resembling the traditional light bulb have been shown and are often referred to as compact fluorescent lamps (CFLs). As is well known, all florescent light sources contain a small amount of mercury, posing problems due to the health effects of mercury exposure. Additionally, due to heavy regulation of the disposal of mercury, the recycling of florescent light sources becomes complex and expensive.

Accordingly, there is a desire to provide an alternative to florescent light sources. An example of such an alternative is provided in

WO 2005074006, disclosing a field emission light source containing no mercury or any other health hazardous materials. The field emission light source includes an anode and a cathode, the anode consists of a transparent electrically conductive layer and a layer of phosphors coated on the inner surface of a cylindrical glass tube. The phosphors are luminescent when excited by electrons. The electron emission is caused by a voltage between the anode and the cathode. For achieving high emission of light it is desirable to apply the voltage in a range of 4 - 12 kV.

The field emission light source disclosed in WO 2005074006 provides a promising approach to more environmentally friendly lighting, e.g. as no use of mercury is necessary. However it is always desirable to improve the design of the lamp to prolong the life time, and/or to increase the luminous efficiency of the lamp. SUMMARY OF THE INVENTION

According to an aspect of the invention, the above is at least partly met by a field emission lighting arrangement, comprising an evacuated envelope inside of which an anode structure comprising a phosphor layer and a field emission cathode are arranged, the anode structure being configured to receive electrons emitted by the field emission cathode and to generate light when a voltage is applied between the anode structure and the field emission cathode, wherein the evacuated envelope comprises a light exit portion and a light reflective portion being adapted to reflect light generated at the anode structure, and the field emission lighting arrangement further comprises a heat sink arranged outside of the evacuated envelope and thermally coupled to the light reflective portion.

Prior art field emission lighting arrangements are generally configured such that, during operation, the cathode emits electrons, which are

accelerated toward the complete phosphor layer of the field emission lighting arrangement. The phosphor layer may provide luminescence when the emitted electrons collide with phosphor particles. The luminescence process is accompanied by the production of heat which may reduce the lifetime of the field emission lighting arrangement.

As a comparison and according to the invention, the field emission lighting arrangements is rather configured such that light generated during operation the field emission lighting arrangement is reflected out from the field emission lighting arrangement through a light exit portion. This is achieved by providing a light reflective portion which serves a dual purpose, both reflecting light and being thermally connected to a heat sink for dissipating heat generated during emission of light.

The light reflecting portion is preferably arranged in the vicinity of the anode structure, e.g. being an adjacent layer to the phosphor layer of the anode structure or being arranged outside of but in close contact to the envelope, thereby providing a direct or indirect thermal coupling to the light reflective portion. Independently, heat generated at the anode structure may according to the invention advantageously be dissipated using a heat transfer route starting at the phosphor layer, passing through the light reflective portion and being dissipated by the heat sink being thermally coupled to the light reflective portion. By dissipating heat generated during operation of the field emission lighting arrangement it is in turn possible to decrease the temperature at the phosphor layer of the anode structure. This thus provided the advantage of an increased lifetime of the field emission lighting

arrangement, possibly also reducing the lighting cost for the end user as the field emission lighting arrangement can be replaced at a lower rate.

In an embodiment, the heat sink is configured to reflect light generated at the anode structure out from the evacuated envelope through the light exit portion. Accordingly, in such an embodiment both the reflective functionality and the heat sink may be arranged outside of the envelope, possibly being provided as a single component.

In another embodiment the anode structure may alternatively be configured to reflect light generated at the anode structure out from the evacuated envelope through the light exit portion, thus providing the reflective functionality inside of the envelope and the heat sink outside of the envelope. Such a reflective anode structure may for example comprise a metal layer arranged inside of the envelope, for example aluminum or one of its alloys, or other metallic materials deposited on the inner surface of the envelope.

The heat sink may be selected from a plurality of known heat dissipating materials and be constructed in any suitable form. However, in an embodiment, the heat sink comprises a heat dissipating carbon based compound, possibly pressed to a suitable form matching the form of the field emission lighting arrangement.

Preferably, the field emission lighting arrangement further comprises a power supply connected to the field emission cathode and the anode structure and configured to provide a drive signal for powering the field emission lighting arrangement.

Depending on the structure of the field emission lighting arrangement and once the choices of the cathode and anode materials are made, the configuration and the physical dimensions of the field emission lighting arrangement are determined; the physical properties of the field emission lighting arrangement may be determined. From the electric circuit point of view, some of these properties may be identified with those of electronic components, like a diode, capacitor and inductor with predetermined resistance, capacitance and inductance. The field emission lighting

arrangement as a whole therefore manifests like these components in different ways, most importantly a resonance circuit under different driving conditions, such as DC, driving, low frequency driving and resonance frequency driving. Any frequency below the resonance frequency is defined as low frequency. By adjusting the capacitance and/or inductance inside and/or outside the lamp, it is possible to choose a desired predetermined (resonance) frequency and a phase relation between the input voltage and the current. This is further disclosed in EP09180155 by the applicant, which is incorporated by reference in its entirety. Accordingly, it may be preferred to select the predetermined frequency such that it is within a range

corresponding to the half power width at resonance of the field emission lighting arrangement.

Alternatively, the predetermined frequency may also be selected to depend on an emission decay of the phosphor layer. Generally, the emission decay for a phosphor layer suitable for a field emission arrangement takes place in a range of micro seconds thus indicating a high predetermined frequency. Taking into account the heat generated at the emission of light, the predetermined frequency is preferably selected to be above 10 kHz and preferably above 30 kHz.

Preferably, the envelop is made of glass and the voltage is preferably in the range of 2 - 12 kV. Furthermore, the power supply may be electrically connected or in physical contact to the field emission arrangement, such as for example within a socket/base/side in the case the field emission arrangement is a field emission light source or placed in the vicinity of the field emission arrangement.

Also, the field emission lighting arrangement may be compactly integrated as a single component, e.g. as a luminaire for lighting, or as a backlight for a display. Additionally, the field emission lighting arrangement according to the invention may preferably forms part of any lighting requiring application, including for example a field emission display, an X-ray source. Further features of, and advantages with, the present invention will become apparent when studying the appended claims and the following description. The skilled addressee realize that different features of the present invention may be combined to create embodiments other than those described in the following, without departing from the scope of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

The various aspects of the invention, including its particular features and advantages, will be readily understood from the following detailed description and the accompanying drawings, in which:

Fig. 1 illustrates a prior art field emission lighting arrangement;

Fig. 2 illustrates a currently preferred embodiment of the field emission lighting arrangement; and

Fig. 3 illustrates an alternative embodiment of the field emission lighting arrangement.

DETAILED DESCRIPTION

The present invention will now be described more fully hereinafter with reference to the accompanying drawings, in which currently preferred embodiments of the invention are shown. This invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided for thoroughness and completeness, and fully convey the scope of the invention to the skilled addressee. Like reference characters refer to like elements throughout.

Referring now to the drawings and to Fig. 1 in particular, there is depicted a field emission lighting arrangement 100. The field emission lighting arrangement 100 is based on the concept of using a transparent field emission anode, such as an ITO layer 102 being provided on a transparent envelope, such as an evacuated cylindrical glass tube 104 (envelope). For emission of light, a layer of phosphor 106 is provided on the inside of the ITO layer 102, facing a field emission cathode 108. The field emission cathode 108 comprises a base structure onto which pluralities of sharp edges are arranged, for example a ZnO based emitter structure as disclosed in

EP10159139 by the applicant, which is incorporated by reference in its entirety.

During operation of the field emission lighting arrangement 100, an electrical field is applied between the cathode 108 and an anode structure, e.g. the transparent ITO layer 102 (acting as an electrode) for example by means of a control unit and power supply (not shown). By application of the electrical field, the cathode 108 emits electrons, which are accelerated toward a phosphor layer 106 of the anode structure. The phosphor layer 106 may provide luminescence when the emitted electrons collide with phosphor particles of the phosphor layer 106. Light provided from the phosphor layer 106 will transmit through the transparent ITO/anode structure 102 and a glass cylinder 104 acting as a substrate for the anode structure. The light is preferably white, but colored light is of course possible. The light may also be UV light.

Turning now to Fig. 2, which illustrates a currently preferred

embodiment of a field emission lighting arrangement 200. The field emission lighting arrangement 200 provides similarities with the field emission lighting arrangement 100 shown in Fig. 1 but has been adapted for achieving a higher heat dissipation during operation of the field emission lighting arrangement 200, thus possibly increasing its lifetime. The main difference between the illustrated field emission lighting arrangement 200 and the field emission lighting arrangement 100 of figure 1 is that the field emission lighting arrangement 200 has been provided with a heat sink 204 for example comprising heat sink flanges 206. The heat sink 204 may for example be provided such that it encloses a portion of the glass cylinder 104, thereby being thermally coupled to each other.

Additionally, in the field emission lighting arrangement 200 only a portion of the glass cylinder 104 has been provided with an anode structure, thereby forming a portion of the glass cylinder 104 where the light generated during operation of the field emission lighting arrangement 200 may be lead out. Preferably, the portion of the glass cylinder 104 where the anode structure has been provided coincides with the portion of the glass cylinder 104 which is thermally coupled to the heat sink 204. In an embodiment the portion of the glass cylinder 104 being thermally coupled to the heat sink 204 is around 1/3 to 1/2 of the perimeter of the glass cylinder 104. Other arrangements are of course possible and within the scope of the invention.

A further difference between the field emission lighting arrangement 200 and the field emission lighting arrangement 100 shown in Fig. 1 is that the ITO electrode layer 102 has been replaced with a reflective anode electrode 202, for example in the form of a metal layer, such as aluminum, being provided on the inside of the glass cylinder 104. During operation of the field emission lighting arrangement 200, the electrons emitted from the cathode 108 will travel towards the reflective anode electrode 202 to strike the phosphor layer 106 such that light is emitted. The light will be reflected by the reflective anode electrode 202 towards the outside of the field emission lighting arrangement 200. Heat generated during operation of the field emission lighting arrangement 200 at the phosphor layer 106 and the reflective anode electrode 202 will be at least partly dissipated by means of the heat sink 204 and heat sink flanges 206 thermally coupled to the glass cylinder 104. The reflective anode structure 202 can be made of a highly reflective metallic layer such as aluminum.

In an alternative embodiment of the invention, and as is illustrated by the field emission lighting arrangement 300 in Fig. 3, again a transparent anode structure 302 is used, for example an ITO layer. Thus, instead of using a reflective anode structure as in Fig. 2, light generated during operation of the field emission lighting arrangement 300 will be allowed to be out coupled through towards the heat sink 204. However, the heat sink 204 has been provided with a reflective layer 304 which thereby will reflect the light out towards the side opposite of the heat sink 204. The reflective layer 304 may for example be provided onto a heat dissipating carbon based compound for example being formed to the heat sink 204. However, the reflective layer 304 may also be an integrated portion of e.g. a metal based heat sink. Other possibilities are of course possible and readily understood by the skilled addressee. Even though the invention has been described with reference to specific exemplifying embodiments thereof, many different alterations, modifications and the like will become apparent for those skilled in the art. Variations to the disclosed embodiments can be understood and effected by the skilled addressee in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. For example, even though the embodiments of the invention have been provided in terms of cylindrical field emission lighting arrangements, other forms are of course possible and within the scope of the invention, including for example multi angular field emission lighting arrangements. Furthermore, in the claims, the word "comprising" does not exclude other elements or steps, and the indefinite article "a" or "an" does not exclude a plurality.