Light sources

Abstract: A light source 1 comprises a first layer 2 of plastics material, such as polycarbonate, incorporating two organic fluorescent dyes for emitting respectively, red and green light when excited by radiation of a shorter wavelength. A second layer comprising a phosphor material 3a is provided for emitting blue light when excited by an ultra violet radiating discharge caused by energisation of an electrode source electrode 6, the blue light serving to excite the organic fluorescent dyes of the layer 2. A barrier layer 3, such as phosphate glass, is provided between the first and second layers 2, 3a which serves to absorb ultra violet radiation and therefore prevents degradation of the first layer 2 of plastics material. The barrier layer 3 may also serve as a gas-impermeable layer to overcome the porosity of the plastics material of the layer 1. The red, green and blue light emissions from the device provide a white light source, although light of other colours may also be obtained.

Description: This invention relates to light sources, and it relates in particular to such light sources as utilise the mechanism of fluorescence to generate part at least of their light output. Hitherto, light sources based upon the use of fluorescence have typically exhibited problems in a number of areas, such as cost (particularly if specific colour rendition of good quality is desired), longevity and design flexibility. Some of the aforementioned problems have their origins in different aspects of conventional light sources but the present invention, by adopting a wholly different approach to the construction of and materials used in such sources, aims to provide light sources in which at least one of the above-mentioned problems is reduced. According to the invention there is provided a light source comprising a hollow body provided with a layer of phosphor material and with electrode excitation measn for exciting the phosphor material, characterised in that (a) the hollow body is made of a plastics material containing at least one organic fluorescent dye, (b) an inorganic barrier layer for absorbing ultra violet radiation generated by the excitation means, is attached to the inner surface of the hollow body, and (c) said layer of phosphor material is attached to the inner surface of said barrier layer. The use of plastics containing at least one organic, fluorescent dye as a basic constructional material for light sources in accordance with the invention assists in reducing the costs associated with obtaining good and reliable colour rendition as compared, for example, with the costs associated with conventional fluorescent light sources which incorporate powdered phosphors which emit light of characteristic wavelengths when excited by (typically) a mercury vapour discharge. It is sometimes preferred, when attempting to generate so-called "white light", to utilise three different phosphors, each of which fluoresces to emit light of a respective one of the primary colours. Currently, for good reliable colour rendition, it is necessary that a light source incorporates phosphors containing expensive rare-earth dopants, and that each of the phosphors emits light which is substantially invariant in amplitude, and wavelength. The present invention avoids the need for some at least of such expensive raw materials yet still provides light sources exhibiting good colour rendition because of the useful and reproducible spectral characteristics of light generated by organic fluorescent dyes. A further advantage which accrues from the use of plastics, instead of glass, as the basic constructional material of light sources in accordance with the invention, lies in the flexibility of design that is achievable. Light sources can be made in very many shapes once it is possible to avoid the constraints imposed by the use of glass. From Review of Scientific Instruments 43 (1972), pages 1199-1200, it is known per se, to increase the intensity of visible emission from gas discharge lamps having strong lines in the uv by encircling the lamp with a piece of fluorescing plastic. The plastics material is, in accordance with the invention, sealed by the application thereto of an inorganic barrier layer, typically of phosphate glass, that can be deposited in any convenient manner once the shaping of the plastics material to a desired form has been completed. Such deposition of the barrier layer can be effected by low temperature processes that do not adversely effect the shape of the plastics material. The layer of phosphor material may comprise phosphor powder or, alternatively, a phosphate glass layer incorporating phosphor powder. The light source may also include a further barrier layer for absorbing ultra violet radiation and arranged to overlie the layer comprising the plastics material. The use of the further barrier layer reduces the attendant risk of decomposition of the plastics material or of the organic dye or dyes therein from ambient ultra violet radiation to which the light source may be exposed. Preferably, the light source includes end caps for forming closure members for the region containing the excitation means, the end caps having threaded portions for co-operating with threaded portions of the plastics material. In a further embodiment, reflector means is provided for shielding the layer of phosphor material from heat generated by a thermionic electron source of the excitation means. In order that the invention may be clearly understood and readily carried into effect, one embodiment thereof will now be described with reference to the drawings of which:- Figure 1 shows a longitudinal schematic cross-sectional view of part of a light source in accordance with one example of the invention, and Figure 2 shows a transverse cross-section through the light source of Figue 1, taken on view arrows II-II thereof. Referring now to the drawings, a tubular light source is indicated generally at 1. In this example, the light source 1 comprises a linear tube but this need not be the case, and one of the advantages of this invention, as has already been mentioned, is the ease with which various configurations including flat panels may be constructed. The tube comprises a first layer 2 of plastics material, such as polycarbonate, incorporating two organic fluorescent dyes. Inside the layer 2, and closely juxtaposed therewith, is a barrier layer 3 which is formed of phosphate glass, on the inner (exposed) surface of which is deposited a second layer in the form of a powdered phosphor material 3a. The phosphate glass is conveniently Al³⁺ or Fe³⁺ phosphate, deposited from a solution of Al (NO₃) 3 or Fe (NO₃)₃ and anhydrous phosphoric acid in anhydrous methanol and baked for around 1 hour at about 80°C. The inter-relationship of the wavelengths of light emitted by the two dyes in layer 2 and the powdered phosphor material 3a and the manner of their excitation will be described more fully hereinafter. The powdered phosphor material 3a and any binder/suspending agent associated therewith is preferably anhydrous to minimise the rigours of a subsequent baking process. The tube is provided with end caps such as 4 which are required to be impermeable and are hermetically sealed to and act as closure members for the tube 1. The end caps such as 4 can conveniently be attached initially to the tube by means of a coarse, single turn screw thread 5 co-operative parts of which are formed in the outward facing surface of the plastics material of the layer 2 and the inward facing surface of an axially extending part 4a of the end cap 4. A gasket 5′ of porous material which can be loaded in-situ with a suitable solvent for the materials of end cap 4 and plastics layer 2, or which will chemically bond or adhere to one or both of these, is inserted between these two components in order to permit the end cap to be effectively welded to the tube, forming an hermetic seal. The solvent may be one which requires a small degree of heat treatment to ensure its full operation. The end caps such as 4 have to be impermeable, as already mentioned, and to this end they are conveniently formed either of a material such as soda glass, which is impermeable of itself, or of a permeable material such as polycarbonate which is rendered impermeable by the application of a layer of suitable material such as phosphate glass. In this latter case, however, appropriate measures have to be taken to ensure that the polycarbonate material is not deformed or otherwise damaged by the temperature to which filament leads which pass therethrough are subjected in use. The end caps such as 4 may be formed with inwardly extending and self-supporting reflector cylinders such as 5a to shield the layer 3 and the general area adjacent the seal between end cap 4 and the tube from the heat generated by the operation of a conventional thermionic electron source electrode 6 which, in combination with a conventional fill within a core region of the tube 1, provides an excitation means for the phosphor material 3a, as described below. The reflector 5a is conveniently made of aluminium, although other materials may be used if desired. Furthermore, the reflector 5a need not be self supporting or attached to the end cap 4 and may instead be provided in the form of a layer deposited on the powdered phosphor material 3a or, if the material 3a is locally removed, on the layer 3 of phosphate glass. Enclosed within the core region of the tube 1 is a conventional fill comprising an inert gas and mercury vapour this fill being introduced in the usual way via an exhaust tubulation (not shown) in one of the end caps. The fill supports, in known manner, a discharge when the source electrode 6 is energised, the discharge giving rise to ultra violet radiation for exciting the phosphor material 3a. Other fills, containing for example, deuterium and/or tritium may be used if desired. The powdered phosphor material 3a is chosen so as to absorb substantially all of the ultra violet radiation generated by the discharge in the tube and to generate by fluorescence, in response to such absorption, visible blue light. This blue light is in turn incident upon the enveloping layer 2 which contains organic dyes arranged to generate by fluorescence red and green light respectively. The dye concentrations in the layer 2 are arranged so that some of the blue light generated by the powdered phosphor material 3a is not absorbed and can emerge directly from the light source 1. A chosen arrangement of dyes, phosphors and relative quantities thereof can therefore be used to define a given colour rendition for the light source 1. The use of plastics material, such as polycarbonate, as the host material for the dyes of layer 2 is advantageous in that it is relatively inexpensive and is easily formed into various shapes. Furthermore, the organic dyes are relatively inexpensive and reliable in colouration. Dye-loaded plastics materials have, however, two major disadvantages when considered for use in a lighting environment such as that described herein, namely a susceptibility to degradation of the dyes and the plastic (by decomposition) if exposed to the ultra violet radiation generated by the gaseous discharge over prolonged periods and a tendency to be porous, thus permitting undesirable ingress and egress of gaseous components. In the construction of this embodiment of the invention, both disadvantages are addressed by the use of the phosphate glass layer 3 and the phosphor material 3a; the susceptibility to ultra violet degradation being overcome by ensuring that the phosphor material in layer 3a absorbs substantially all of the ultra violet radiation generated as a result of the discharge (in particular that radiation of wavelength 254 nm and 185 nm). The porosity of the plastics layer 2 is overcome by the use of the impermeable phosphate glass material as layer 3. Phosphate glass acts as a sealant and, advantageously, requires low processing temperatures that are compatible with and not damaging to the integrity of layer 2. It also acts as an ultra violet filter at wavelengths of 254 nm and 185 nm, and thus removes any such radiation not absorbed by the powdered phosphor 3a. In an alternative construction, the powdered phosphor material 3a is incorporated in a glassy support layer, which may be an ultra violet translucent phosphate glass, and a separate ultra violet absorbent is additionally provided in the phosphate glass layer 3. In any event, it is desirable (though not essential) to provide an outer barrier layer 7 to prevent external ultra violet radiation, such as from ambient sunlight, from impinging upon and thus damaging the layer 2 over a prolonged period of time. Conveniently this outer barrier layer 7 can be of phosphate glass, or other known ultra violet absorbing material, applied as a final stage of the lamp production process when the physical shape and configuration of the light source have been defined. The layer may, if desired, contain an additive to increase its ultra violet absorption capabilities. A significant advantage accrues from the use of the powdered phosphor material 3a, as opposed to using a third organic dye, in that it assists in liberating some of the light generated in layer 2 which becomes trapped therein by total internal reflection. The light so entrapped eventually encounters a powder particle and is scattered thereby at angles allowing it to escape from the tube. This mechanism greatly enhances the brightness and efficiency of the light source 1. It is not, however, essential to use a powdered phosphor material such as 3a. If the phosphor is applied as a thin film, for example, it is desirable for the plastics layer to contain a scattering pigment such as titania to reduce entrapment of light. It will be appreciated that, in the described embodiment, where white-light is assumed to be generated, the powdered phosphor material 3a is usually a blue phosphor because it has to excite fluorescence in the organic dyes in layer 2 and the physics of fluorescence dictates that a shift to longer wavelength radiation occurs. Thus the blue phosphor is excited by the predominance of ultra violet radiation in the mercury discharge and the blue light so generated excites the red and green organic dyes in layer 2. Some of the green light generated in layer 2 may also excite the red dye and thus the balance of amounts of powdered phosphor material 3a and red and green dyes in layer 2 are chosen to achieve a desired colour rendition in the light emitted by the source 1 as a whole. If white light is not the objective, the powdered phosphor material 3a need not generate blue light on fluorescence and the layer 2 need not contain two organic dyes. For example, a single dye might be sufficient for some colourations. Thus a green phosphor might pump a layer having a red dye to provide a red, orange or yellow emission from the light source 1 as a whole. It may also be advantageous to load the phosphate glass layer 3 with an ultra violet absorber to further reduce the risk of impingement of ultra violet radiation on the layer 2 with the attendant risk of decomposition of the layer 2 or of the organic dye or dyes therein. The dye or dyes incorporated in the layer 2 are preferably of high purity to reduce still further their susceptibility to decomposition. Although the invention has been described with respect to an embodiment in which a full length mercury discharge occurs in the tube, this need not be the case and the invention can be applied to any light source in which fluorescence is to be excited. For example, the invention can be used with so-called "negative glow" low pressure discharge tubes.

Claims: A light source comprising a hollow body provided with a layer of phosphor material and with electrode excitation means for exciting the phosphor material, characterised in that (a) the hollow body (2) is made of a plastics material containing at least one organic fluorescent dye, (b) an inorganic barrier layer (3) for absorbing ultra violet radiation generated by the excitation means (6) is attached to the inner surface of the hollow body (2), and (c) said layer of phosphor material (3 a ) is attached to the inner surface of said barrier layer (3). A light source as claimed in claim 1, characterised in that the barrier layer (3) is gas-impermeable. A light source as claimed in claim 1, characterised in that the barrier layer (3) is phosphate glass. A light source as claimed in claim 1, 2 or 3, characterised in that the layer of phosphor material (3 a ) is phosphor powder. A light source as claimed in claim 1, 2 or 3, characterised in that the layer of phosphor material (3 a ) is phosphate glass containing phosphor powder. A light source as claimed in any one of claims 1 to 5, characterised in that an additional barrier layer (7) for absorbing ultra violet radiation is attached to the outer surface of said hollow body (2). A light source as claimed in any one of claims 1 to 6, characterised by end caps (4) forming closure members for the hollow body which is tubular, the end caps carrying said electrode excitation means (6) and each cap having a screw threaded (5) engagement with said tubular hollow body (2). A light source as claimed in any one of claims 1 to 7, characterised by reflector means (5 a ) for shielding the barrier layer (3) and phosphor material layer (3 a ) from heat generated by the excitation means (6). A light source as claimed in any one of claims 1 to 8, characterised in that the hollow body contains a scattering pigment for scattering light generated by excitation of the phosphor material. A light source as claimed in any one of claims 1 to 9, characterised in that the plastics material (2) contains two organic fluorescent dyes for providing red and green light in response to excitation thereof and the phosphor material layer (3 a ) provides blue light for excitation of said dyes so that the light source generates white light.

Inventors: HOLDEN JOHN GRAHAM, SHANKS IAN ALEXANDER, HOLDEN, JOHN GRAHAM, SHANKS, IAN ALEXANDER
Applicants: GE LIGHTING LTD, GE LIGHTING LIMITED
Application year: 1990
Main classification: H01J61
IPC codes: H01J61, C09K11, B32B17, B32B9, C09K3