Abstract:
An arrangement for emitting light includes a hermetically sealed casing with a window, a layer of a fluorescent substance arranged within the casing covering at least a major part of the window, an electron emitting cathode arranged within the casing, and an anode. The casing is filled with a gas suitable for electron avalanche amplification. In operation, the cathode and anode are held at an electric potential such that said emitted electrons are accelerated and avalanche amplified in the gas. The layer of the fluorescent substance is arranged to emit light through the window in response to avalanche amplified electron bombardment and/or ultraviolet light emitted from the gas.

Description:
FIELD OF INVENTION 
   The present invention generally relates to cathodoluminescent light sources. More particularly, the invention relates to an arrangement and a method for emitting light by use of electron emission cathodes and fluorescent substances. 
   BACKGROUND 
   One type of a light source is the fluorescent tube. In the fluorescent tube a gas discharge emits ultraviolet (UV) light onto a fluorescent material. The light source suffers from serious drawbacks. For instance, there is always a delay after the power has been turned on until the light source radiates at full power. Further, it needs complicated control equipment, which requires space and adds cost. Also, it is unfortunately necessary to use material having negative environmental effects, such as mercury. Furthermore, the choice of fluorescent material is limited to UV-sensitive materials. Most of these fluorescent materials emit light of a spectral shape, which is not optimal for the eye and human comfort. Finally, this kind of light source is often rather temperature sensitive in that the emission intensity is significantly weaker for a long time after switch-on at low temperatures compared to at high temperatures. 
   Another type of light source is the cathodoluminescent light source. In a cathodoluminescent light source electrons are emitted from a cathode either by heating the cathode, thus thermally emitting the electrons, or by employing a strong electric field in the vicinity of the surface of the cathodes, thus emitting electrons through field emission. 
   Examples of field emission cathode light sources employing a strong electric field in the vicinity of the surface of a cathode are disclosed in U.S. Pat. Nos. 5,877,588 and 6,008,575. 
   The main drawback of a thermally emitting cathode is that large amounts of energy are lost in heating the cathode. The main drawback of both field and thermal emission cathodes is that high emission currents cause the cathode to wear out as all electrons producing the light have to be emitted from the cathode. This implies that a high electron current has to be emitted from the surface of the cathode, which complicates the cathode structure and production thereof. Further, the current cathodoluminescent light sources only operate in vacuum, which requires thick walls around the light source. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to provide an improved light source and method, respectively, which provide brighter light compared to prior art light sources, and which lack at least some of the drawbacks described above. 
   This object, among others, is according to the present invention attained by arrangements and methods, respectively, as defined in the appended claims. 
   By providing a gas suitable for electron avalanche amplification in a cathodoluminescent light source brighter light can be achieved. Furthermore, the emission current from the cathode is reduced as a majority of the electrons are liberated from the gas and not emitted from the surface of the cathode, which simplifies the construction of the cathode and prolongs its life time. 
   As the pressure in a gas-filled light source is considerably higher than vacuum, typically atmospheric pressure, the walls of the light source can be made thinner, which makes the light source lighter. 
   As during the avalanche amplification, besides electrons, also UV light is emitted that may stimulate the fluorescent material, causing it to emit light, the total electron current per unit light output is smaller than in a conventional cathodoluminescent light source, which simplifies the design of the light source. 
   As it is easy to vary the emission current in a field emission cathode and/or the avalanche amplification by varying an avalanche voltage the light source may readily be dimmed. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Further features and advantages of the present invention will be evident from the following detailed description of embodiments given below and the accompanying Figures, which are given by way of illustration only, and thus, are not limitative of the present invention, wherein: 
       FIGS. 1–6  are cross-sectional side views of light sources according to six different embodiments of the present invention; and 
       FIGS. 7–9  are perspective views of three different lamp housings which may be used together with the light sources of the present invention. 
   

   DETAILED DESCRIPTION OF EMBODIMENTS 
   A first embodiment of the present invention will now be described with reference to  FIGS. 1A and 1B . 
   A planar cathodoluminescent light source comprises a planar cathode  1 , a planar anode  2  parallel to the cathode  1  and a fluorescent layer  3  inside a casing  4 . The casing  4  has a window  10  to allow light to emerge from the light source. The fluorescent layer  3  is arranged on the inside of the window  10 , and the anode  2  is arranged on a surface of the fluorescent layer  3 , which faces the cathode  1 . 
   The casing  4  is hermetically sealed and filled with a gas suitable for electron avalanche amplification. A diffuser may be arranged outside the casing  4  (not illustrated). A diffuser provides leveling of luminous intensity to compensate for different luminous intensity from different areas of the light source. 
   The planar cathode  1  may be any type of cathode that can be stimulated to emit electrons from its surface  1 A facing the anode  2 . It may have a smooth or an irregular surface. Irregularities in the surface  1 A may e.g. be formed by irradiating the surface with laser light, etching, mechanical roughening, or deposition of material producing irregular shapes such as e.g. carbon nanotubes, fulerenes, etc. Emission of electrons is provided either by heating the cathode  1 , causing the electrons to be thermally emitted, or by applying a strong electric field in the vicinity of the surface of the cathode  1  causing electrons to be emitted by field emission. It is further possible to heat a field emission cathode to provide emission of electrons by applying a lower electric field, as compared to a non-heated field emission cathode. 
   The planar anode  2  is permeable to high-energy electrons, to allow such electrons to penetrate the anode and bombard the fluorescent layer  3 . The planar anode  2  may e.g. be a thin foil or may have a meshed shape. 
   Alternatively, the anode  2  is arranged between the fluorescent layer  3  and the casing  4  as illustrated in  FIG. 1B . The planar anode  2  has then to be transparent to light and may be made of a transparent conductor or may have a meshed shape. However, the anode has not to be transparent to electrons. The anode  2  can in this case be part of the casing  4  where e.g. the casing  4  can be made of a conductive material, e.g. conductive glass or plastic. 
   The fluorescent layer  3  may consist of a single material or a mixture of materials, e.g. a mixture of Y 2 O 2 S:Eu, ZnS:Cu;Al and ZnS:Cl. 
   A gas suitable for electron avalanche amplification may e.g. be any noble gas, nitrogen or a noble gas mixed with a hydrocarbon gas such as 90% argon and 10% methane. The gas is preferably at atmospheric pressure, but may be at under- or overpressure, preferably in the range 0.001–20 atm. 
   A voltage U is, during use, applied between the anode  2  and the cathode  1 . The voltage U should be high enough to cause electrons to be emitted from the cathode  1  in the case of field emission. The voltage U should in all cases be high enough to avalanche amplify the electrons in the gas. The avalanche amplified electrons are accelerated towards the anode  2  and thus the fluorescent layer  3 . The electrons are absorbed in the fluorescent layer  3  and thus excite the fluorescent material thereof. During relaxation the fluorescent layer  3  emits bright visible light. 
   As during the avalanche amplification, besides electrons, also UV light is emitted that may stimulate the fluorescent material, causing it to emit light. This physical process may be used together smith the electron bombardment or separately for producing the light. 
   An advantage of using avalanche amplification in a gas is that electrons emitted from the cathode are accelerated by an electric field between the cathode  1  and the anode  2  and ionize the gas and new electrons are emitted from the gas, which in turn are accelerated and ionize the gas further. Thus, the main part of the electrons providing light is derived from the gas and not from the cathode, which lessen the wear of the cathode. The gas functions as a catalyst as positive ions formed during the ionization of the gas drift toward the cathode where they are neutralized and revert to the gas. 
   Using a distance of 1 mm between the anode  2  and the cathode  1  in a gas of argon and methane at a pressure of 1 atm a voltage of typically 1000 V is sufficient to emit electrons from the cathode  1 , and to avalanche amplify the emitted electrons. 
   The dimensions of the light source may vary tremendously, depending on the intended use and light source may be produced having quadratic to very elongated light emitting surfaces. 
   A second embodiment of the present invention will next be described with reference to  FIG. 2 . This second embodiment is identical with the first embodiment apart from the following. 
   The planar cathodoluminescent light source of  FIG. 2  further comprises a modulator electrode  5  positioned between the anode  2  and the cathode  1 , preferably closer to the anode  2  than to the cathode  1 . Preferably, the modulator electrode  5  has a meshed shape to allow electrons to pass through. 
   An electric field necessary to emit an electron from a cathode through field emission is normally lower than an electric field for avalanche amplification of electrons. Thus, by providing the modulator electrode  5  close to the anode  2  a sufficiently high electric field may be obtained without applying very high voltage for the electrons emitted from the cathode  1  to be avalanche amplified close to the anode  2 . 
   By providing a modulator electrode in the light source the positive ions formed during the ionization of the gas drift toward the modulator electrode where they are neutralized and revert to the gas. 
   A first voltage U 1  is, during use, applied between the modulator electrode  5  and the cathode  1 , and causes emission of electrons from the cathode  1  and/or acceleration of emitted electrons from cathode  1 . A second voltage U 2  is applied between the anode  2  and the modulator electrode  5 , and is high enough to avalanche amplify the emitted electrons in the gas and give them sufficiently high kinetic energy such that the avalanche amplified electrons are capable to penetrate the anode  2  and bombard the fluorescent layer  3 , which in response thereto emits light. 
   Next, a third embodiment of the present invention is described with reference to  FIG. 3 . This third embodiment is identical with the second embodiment except for the following. 
   The planar cathodoluminescent light source further comprises an avalanche electrode  6  positioned between the anode  2  and the modulator electrode  5 , preferably closer to the modulator electrode  5  than to the anode  2 . Preferably, the avalanche electrode  6  has a meshed shape to allow electrons to pass through. Gratings may be used to make up the meshed shapes of the modulator electrode  5  and the avalanche electrode  6 . The electrodes  5  and  6  should preferably be positioned parallel with each other and having apertures aligned with each other. 
   A dielectric  21 , such as a polyamide film, may be positioned between the modulator electrode  5  and the avalanche electrode  6  to keep them apart at a well defined distance. The dielectric  21  may have apertures precisely matching the apertures of the gratings or have apertures that are wider or narrower than the apertures of the gratings  5  and  6 . When a dielectric  21  is utilized to stabilize the electrodes  5  and  6  the gratings of the electrodes may be manufactured by means of metallizing the dielectric  21 . 
   By providing a modulator electrode and an avalanche electrode in the light source the positive ions formed during the ionization of the gas drift toward the modulator electrode and the avalanche electrode, respectively, where they are neutralized and revert to the gas. 
   A first voltage U 1  is, during use, applied between the modulator electrode  5  and the cathode  1 , and causes emission of electrons from the cathode  1 , and/or acceleration of emitted electrons from the cathode  1 . A second voltage U 2  is applied between the avalanche electrode  6  and the modulator electrode  5  and accelerates the emitted electrons in the gas, possibly the voltage U 2  may be high enough to achieve avalanche amplification of the emitted electrons. A third voltage U 3  is applied between the anode  2  and the avalanche electrode  6 , and is high enough to either further avalanche amplify the previously amplified electrons or to drift the electrons towards and through the anode  2  and bombard the fluorescent layer  3 , which in response thereto emits light. 
   Provided that the second voltage U 2  avalanche amplifies the electrons, the third voltage U 3  may have a reversed electrical field, collecting the electrons on the avalanche electrode  6  instead of on the anode  2 . In the gap between the electrodes  5  and  6  UV-light is formed by means of the avalanche effect, which illuminate the fluorescent layer  3  without bombarding it with electrons. This is particularly advantageous when the anode  2  is positioned between the fluorescent layer  3  and the window  10  or when the anode  2  is part of the casing  4 . 
   A fourth embodiment of the present invention will next be described with reference to  FIGS. 4A and 4B . 
   A cylindrical cathodoluminescent light source comprises a rod cathode  1  having a circular cross section, a cylindrical anode  2  having an annular cross section and a cylindrical fluorescent substance  3  inside a casing (not illustrated). The casing has a window to allow light to emerge from the light source. The fluorescent layer  3  may be arranged to cover the inside of the window. The anode  2  is preferably arranged on the cylindrical fluorescent substance  3  facing the cathode  1 . 
   The casing is hermetically sealed and filled with a gas suitable for electron avalanche amplification. A diffuser (not illustrated) may be arranged outside the casing, to provide leveling of luminous intensity to compensate for different luminous intensity from different areas of the light source. 
   The rod cathode  1  may have a surface similar to the cathode surface described above in connection with the first embodiment, i.e. smooth or irregular. Alternatively, the cathode  1  may consist of a plurality of fibers, e.g. carbon fibers, carbon nanotubes, fulerenes etc, extending radially, thus forming a plurality of disks forming a rod-shape as illustrated in  FIG. 4B . 
   The anode  2  is permeable to high-energy electrons, allowing such electrons to penetrate the anode  2  and bombard the fluorescent cylindrical layer  3 . The anode  2  may e.g. be a thin foil or have a meshed shape. 
   Distances, fluorescent substance, gas contents and applied voltages may be identical with those of the first embodiment described above. 
   This fourth embodiment has been described as having cylindrical symmetry, but may alternatively have spherical symmetry. 
   Further, this embodiment may include a modulator electrode as described in the second embodiment, and yet further include an avalanche electrode and a dielectric as described in the third embodiment. 
   A fifth embodiment of the present invention, illustrated in  FIG. 5 , is identical with the fourth embodiment except for that the cathode  1  has a square cross section and that the anode has a square-shaped cross section  2 . 
   A sixth embodiment of the present invention will next be described with reference to  FIG. 6 . This sixth embodiment is identical with the first embodiment apart from the following. 
   The cathode  1  is heated by means of a heater  20  to boost the emission of electrons from the cathode  1 . 
   The anode  2  is not planar, but has a surface partly parallel with the cathode  1  and partly perpendicular to the cathode  1 . Thus, providing an electrical field (illustrated by arrows in  FIG. 6 ) causing emission of light in non-parallel planes. 
   Further, this embodiment may include a modulator electrode as being comprised in the second embodiment, and may yet further include an avalanche electrode and a dielectric as described in connection with the third embodiment. 
   Different types of lamp housings will next be described with reference to  FIG. 7–9 . A diffuser as described above may be included in such a lamp housing. 
   A first type of lamp housing is illustrated in  FIG. 7 , and includes a lamp fitting part  7  and a glass part  8 . The lamp fitting part  7  is non-transparent and holds a light source as e.g. one of the first to third embodiments or the sixth embodiment within the lamp housing and includes means to fix the lamp housing to a wall, a ceiling or other support. The lamp housing may also house the electronics associated with the light source. The glass part  8  is transparent or translucent and is arranged to protect the light source and to admit light to be transmitted from the light source. 
   Another design of lamp housing is illustrated in  FIG. 8 , and includes a lamp fitting part  7  and a glass part  8 . The lamp fitting part  7  is arranged to hold a light source as e.g. the fourth or fifth embodiment in the lamp housing and the lamp housing. The glass part  8  is transparent, translucent or non-transparent radial to an axis of symmetry of the cylinder and open upwards and/or downwards. 
   Yet another design of lamp housing is illustrated in  FIG. 9 , and includes a lamp fitting part  7  and a glass part  8 . The lamp fitting part  7  is non-transparent and arranged to hold a light source as e.g. the spherical alternative of the fourth embodiment in the lamp housing and the lamp housing to a ceiling. The glass part  8  is transparent or translucent. 
   All the embodiments described above may easily be provided with a dimmer. By varying a voltage applied to the light source the emission current and/or the avalanche amplification may be varied, which in turn varies the intensity of the emitted light from the light source. 
   It will be obvious that the present invention may be varied in a plurality of ways. Such variations are not to be regarded as departure from the scope of the present invention.