Ultra-violet radiation source and method for identification of a necessary dose of a fluorescent material required to improve a visual appearance of an ultra-violet radiation source

Abstract

Ultra-violet radiation source (2), in particular an LED emitter or LED device, comprising at least one LED-element (4) with a principle band gap radiation at a wavelength smaller or equal than 380nm and a preferably translucent substrate (6), on which the at least one LED-element (4) is arranged; wherein the at least on LED-element (4) and the translucent substrate (6) are encapsulated in a translucent material (12); characterized in that an predetermined amount of a fluorescent material (14) to cause a fluorescent-material-radiation in a visible wavelength range from 380-780nm, preferably from 400-500nm, is at least partially dispersed in the translucent material (12) and / or onto the at least one LED-element (4) and / or onto or into the translucent substrate to convert a part of the total radiant power of the at least one LED-element (4) into visible light; and wherein the converted part is between 0.1% and 10% of the total radiant power of the at least one LED-element (4).

Description

[0001] Ultra-violet radiation source and method for identification of a necessary dose of a fluorescent material required to improve a visual appearance of an ultra-violet radiation source

[0002] Description

[0003] The present invention relates to an ultra-violet radiation source according to the subject-matter of claim 1.

[0004] The present invention specifically relates to an ultra-violet radiation source, in particular an LED-emitter or an LED-device and a method for identification of a necessary dose of a fluorescent material required to improve a visual appearance of an ultra-violet radiation source. The present invention is applicable to conventional linear LED light sources in which multiple LEDs are used in the LED units, as well as to linear LED filament light sources.

[0005] Prior art ultraviolet LEDs having their peak emission at wavelengths below 380nm are plagued by variations in the colour of emitted light between individual LEDs, as well as from batch-to-batch. This results in an unsightly appearance. The root cause of this problem is that LEDs typically radiate two or more distinct spectra. The vast majority of their radiation is at a particular wavelength range that is determined by the electronic band gap of the semiconductor material - this is known as the band gap radiation. However, all semiconductor devices, including LED chips, contain traces of impurities as well as lattice defects in the crystalline matrix. Both can lead to the generation of parasitic luminescence at wavelengths other than the intended band gap radiation. For the popular Indium or Aluminium Gallium Nitride (AIGaN or InGaN) based LEDs which are generally manufactured to produce a relatively narrow band gap radiation in the ultraviolet to green wavelength range of approx. 250-550nm, in all measured samples we also see a broad but rather weak parasitic luminescence in the green-yellow-red wavelength ranges from approximately 480-700nm. This band is typical for GaN-based LEDs and is due to deep level radiative transitions caused by Gallium vacancies.

[0006] In case the principal band gap radiation is in the visible wavelength range, then the parasitic luminescence is greatly overpowered by the former and is of no consequence.

[0007] However, in case the principal band gap radiation is in the ultraviolet wavelength range, which is invisible to the human eye, then the only detectable visible radiation is the parasitic luminescence caused by the impurities and lattice defects. Moreover, since this parasitic luminescence is caused by very tiny defects that are difficult to control, its magnitude can be highly variable both between individual LEDs from the same batch, as well as between different production batches.

[0008] Additionally, there may be undesirable fluorescence effects of other LED components, such as sapphire substrates, which can themselves fluoresce when stimulated by wavelengths below 380nm.

[0009] It is challenging to produce LED chips having sufficiently consistent impurity and / or defect levels to eliminate this variation in visible light emission. Consequently, prior art UV LEDs often have a white, yellow or green colour light. This is not always accepted by consumers of UV products, who are used to the typically violet-blue appearance of conventional traditional UV sources, and there is considerable market interest for UV-A LEDs that also have a similar blue appearance.

[0010] Another drawback of prior art UV LEDs is that some appear to be much brighter than others - and sometimes if the impurity content is very low, the visible light output might also be very low. This is often misinterpreted by the end users as meaning that there are also variations in the UV output even though that is typically quite consistent - but of course not visible to the human eye. This leads to a fear of the customers that the LEDs having lower visible output are inferior or maybe also delivering so little LIV as to be useless. The present invention solves both these technical problems by a novel method

[0011] To improve the LED chip manufacturing process and raw materials such that the concentration of impurities and defects is consistently held as low as possible, thereby minimising the magnitude and variation of parasitic luminescence at undesirable wavelengths.

[0012] However, improving the LED chip manufacturing process typically requires raw materials having elevated chemical purity, and / or manufacturing equipment that is able to operate within narrower process tolerances. The result is increased product cost - however the process is also very challenging and another result is that many suppliers seem unable to translate these improvements, which can be achieved in the lab, into their manufacturing processes. Moreover, it is apparent that gradual improvements are achieved over time, which results in a continual decrease of the parasitic luminescence. As such it is not possible to combine LEDs made at one period of time with those made at a later date. This presents challenges in the field if products of different manufacturing date are mixed in the same installation. This can effectively block continual improvement in other aspects of the manufacturing process, amidst fears of producing LEDs having undesirable visual differences vs earlier production batches.

[0013] To sort the entire production of ultra-violet LEDs into different bins based on the dominant wavelength or chromaticity in the visible part of the spectrum, and to use only those LEDs having a visible light colour within an acceptable range.

[0014] Binning of LED emitters according to color is a slothful process which adds cost. Moreover, a certain percentage of the binned LEDs remain unusable in the application. It is logistically undesirable to use only some color bins of UV LEDs within individual products where the color appearance is important, and to reserve the balance of the color bins for other less-critical applications. Moreover, since the color is influenced by the inherent variability of the manufacturing process and raw materials, it cannot be guaranteed that sufficient yield will be produced of LEDs having the selected visual appearance. Some LEDs are so strongly colored that they have no application and must eventually be disposed of, which brings additional potentially undesirable financial implications It is therefore an object of the present invention to provide an improved ultraviolet radiation source and a method for determination of a necessary dose of a fluorescent material required to improve a visual appearance of an ultra-violet radiation source.

[0015] The above object is solved by a ultra-violet radiation source according to the subject-matter of claim 1 and a method according to the subject matter of claim 7. Preferred embodiments of the invention are indicated by the subject-matter of the dependent claims.

[0016] Specifically, the present invention provides an ultra-violet radiation source, in particular an LED emitter or LED device, comprising:

[0017] - at least one LED-element with a principle band gap radiation at a wavelength smaller or equal than 380nm and

[0018] - a preferably translucent substrate, on which the at least one LED-element is arranged; wherein the at least on LED-element and the translucent substrate are encapsulated in a translucent material; characterized in that an predetermined amount of a fluorescent material to cause a fluorescent-materialradiation in a visible wavelength range from 380-780nm, preferably from 400- 500nm, is at least partially dispersed in the translucent material and / or onto the at least one LED-element and / or onto or into the translucent substrate to convert a part of the total radiant power of the at least one LED-element into visible light; and wherein the converted part is between 0.1% and 10% of the total radiant power of the at least one LED-element.

[0019] The at least one LED-element may comprise an individual or a plurality of semiconductor p-n junctions. The term "principle band gap radiation" can be understood as a particular wavelength range, at which the vast majority of the radiation occurs and that can be determined by the electronic band gap of the semiconductor material. The at least LED-element may further be a LED-Chip which is typically disposed within a package for an electro-thermal-optical-mechanical containment to form a complete LED emitter. The LED-emitter is typically equipped with an optical window consisting of UV-transparent material, e.g. silicone, for the out-coupling of the LIV radiation. A plurality of these LED emitters may be disposed within an LED light source or built directly into the final product. The fluorescent material is excited by the UV-radiation of the at least on LED-element.

[0020] The ultra-violet radiation source according to the invention has advantages regarding costs, color stability, increased luminous flux and regarding the spectral freedom.

[0021] In detail, regarding the cost advantage, prior art techniques attempt to tackle the root cause of the problem, that is, to try and eliminate the undesirable parasitic luminescence or to maintain it within consistent levels by improved manufacturing techniques and / or improved raw materials. Or to simply remove that portion of the production having undesirable colour, by a separate binning operation. All these conventional methods have significant cost implications. Meanwhile, the cost of adding an intentional trace amount of a fluorescent material to an LED emitter is almost negligible. Procedures for the addition of fluorescent materials to LEDs are already well established for the white LED industry, in which white light is produced by dosing a particular blend of different fluorescent materials onto a blue-emitting or UV-emitting LED chip.

[0022] In view of the color stability, the ultra-violet radiation source according to the present invention makes no attempt to minimise or remove the root cause of the colour variation problem. Instead, a deliberate extra addition of radiant energy at visible wavelengths is arranged. It will be shown that when the quantity of phosphor to be added is above a certain critical minimum concentration, it is sufficient to unify the visual appearance ultraviolet LEDs having a wide range of different hues. This method is not only easier to control, it also allows the possibility to determine at which wavelengths the visible light will be generated by a deliberate process, rather than accepting whatever colour of visible light happens to be produced as a spinoff of process and raw material variations. For instance, traditional ultra-violet light sources based on gas discharge or fluorescent technologies tend to produce a violet-blue colour light, to which consumers of such products have become accustomed. Such a visible light colour is easy to replicate by the technique of phosphor addition.

[0023] It is another drawback of conventional ultra-violet LED sources < 380nm that it is not always apparent whether or not the light sources are energized, especially when used in environments having higher ambient light levels. This is due to the extremely small quantity of visible light that is produced by the LIV LEDs. In some applications, an auxiliary indicator must be added to the equipment, to provide a clear visual signal that the light sources are operating and the device is energized. Moreover, as all light sources age, the quantity of emitted radiation decreases. With conventional ultra-violet and visible light sources, both gradual degradation of output as well as complete failure is easily noticeable, because such light sources also produce some visible light emission and the decrease or extinction of the visible radiation is readily noticeable. However, with ultra-violet LED sources this degradation may not be apparent, due to the very small quantity of visible radiation. Specialized measurement equipment may be necessary to monitor the reduction in radiant energy over time. When a deliberate visible light addition is arranged by converting a part of the UV flux to visible radiation with a fluorescent material, e.g. phosphor, it is instantly noticeable whether or not the light source is operating or has substantially decreased in output.

[0024] As already mentioned above, at least a part of the radiation of the fluorescent material is in the visible wavelength range from 380-780nm, preferably in the range from 400-500nm. The fluorescent material preferably has its emission in the visible spectrum. More preferably, the fluorescent material has its peak emission in the blue wavelengths that is in the range from 400-500nm so as to naturally counter the yellow-green light typically produced by UV LED elements, and to produce a visual appearance that is similar to traditional non-LED ultraviolet light sources with which the end customers are familiar.

[0025] In a preferred embodiment the converted part is with regard to the radiant power predominantly in the blue range between 400nm and 500nm, that is that more than 70%, preferably more than 80% of the radiant power of the radiation in the visible range between 380nm and 780nm is in the blue range, that is between 400nm and 500nm, more preferably the remaining radiant power within the visible range with wavelengths above 500nm, that is wavelengths between 500nm and 780nm is suppressed to have an overall radiant power with regard to the total emitted radiant power of the LED element to be below 1%, preferably to be below 0.4%, further preferably to be below 0.1%.

[0026] Moreover, in an embodiment, the predetermined amount of the fluorescent material comprises a dose weight between O.lmg and 2.5mg. More preferably the dose weight comprises one of the following values: O. lmg, 0.2mg, 0.3mg, 0.4mg, 0.5mg, l.Omg, 1.5mg or 2,5mg. The different dose weight amounts allow an adaption of the converted part of the total radiant power of the at least one LED-element into visible light and therefore to an adaption to the radiation of the at least one LED- element.

[0027] In a preferred embodiment, the unconverted part of the total radiant power of the at least one LED-element is at least 70%, preferably at least 90%, more preferably between 99.9% and 90%. In other words, the quantity of the fluorescent material is in particular sufficiently large that its radiation overpowers the tiny amount of parasitic light generation by the at least one LED-element at undesirable visible wavelengths, but not so great that it leads to significant attenuation of the desirable ultraviolet radiation.

[0028] As a further embodiment the fluorescent material comprises or is phosphor, preferably Europium-activated Strontium Chlorophosphate (Sr3(PO4)3CI:EU2+). This material has its peak emission wavelength at 447nm with FWHM distribution of approx. 30nm. This material has its peak excitation wavelength at approximately 270nm but its excitation frequency also extends into the visible region, and still achieves a useful relative intensity of approx. 0.2 at for instance 380nm.

[0029] It has also been found advantageous that in an embodiment it is desirable for the fluorescent material to have small diameter grains. The larger the grain diameter, the greater the dose weight of phosphor to achieve a particular colour correction which results not only in elevated cost, but also a greater attenuation of the ultraviolet radiation. The D5o particle size diameter should preferably be smaller than 10 pm. D5O herewith means the mass-median diameter (MMD), that is MMD is considered to be the average particle diameter by mass, so that D5o = 10pm means that 50% of all particles by mass have a diameter smaller than 10pm, whereas 50% of all particles by mass having a diameter larger than 10pm.

[0030] One further potential application of ultra-violet LED emitters is in UV lamps and devices for the attraction and trapping of flying insects, for instance in shops, restaurants and food preparation areas. Certain insects are attracted solely by the ultra-violet radiation, whereas others are more sensitive to visible wavelengths in the violet, blue and green wavelength ranges. Consequently, in order to develop an effective trap that will catch multiple different species, it is customary to use a plurality of LED emitters of different wavelength ranges, in addition to the LIV LED emitters. The necessity to mix different discrete LED emitters within a single product is an additional complication, because of their different mechanical, electrical and thermal operating requirements. Moreover, the end result can be unsightly, due to the multiple different colored spots of lights from each of the different color LED emitters. The combination of a specific fluorescent blend, e.g. phosphor blend with an ultraviolet LED emitter, allows the possibility to design a single light source that directly generates a multi-wavelength spectrum, and in which the visual appearance of the end products is of consistent color, that is all light sources show the same uniform color.

[0031] Regarding the method, the object is in particular solved by a method for identification of a necessary dose of a fluorescent material required to improve a visual appearance of an ultra-violet radiation source, wherein the ultra-violet radiation source comprises at least one LED-element encapsulated by a translucent material; the method comprising the following steps: determining an absolute parasitic luminescence spectrum for the ultra-violet radiation source; obtaining a pure and uncontaminated band gap radiation; determining a radiation produced by the fluorescent material; identifying the necessary dose of the fluorescent material depending on the determined radiation of the fluorescent material and on a diameter of the translucent material.

[0032] It is therefore a fundamental principle of the present invention to disclose a mathematical procedure for identification of a necessary dose required to improve a visual appearance, by overpowering the parasitic luminescence caused by deep level radiative transitions at the Gallium vacancies.

[0033] It is further preferred that the absolute parasitic luminescence spectrum is determined by multiplying a spectral power distribution form the parasitic luminescence by its peak radiant flux at a wavelength of 550nm.

[0034] In a further preferable embodiment the pure and uncontaminated band gap is obtained by substraction of the parasitic luminescence spectrum from a measured spectrum of the ultra-violet radiation source. In an embodiment of the method the radiation produced by the fluorescent material is determined by the comparison of a plurality of LED-elements, wherein the only variable is a dose weight of the fluorescent material.

[0035] The above and further features and advantages of the invention will become more readily apparent from the following detailed description of preferred embodiments of the invention with reference to the accompanying drawings, in which like reference signs designate like features, and in which:

[0036] Fig. la shows a first embodiment of an ultra-violet radiation source according to the invention;

[0037] Fig. lb shows a cross-sectional view of the ultra-violet radiation source according to Fig. la;

[0038] Fig. 2a shows a second embodiment of an ultra-violet radiation source according to the invention;

[0039] Fig. 2b shows a cross-sectional view of the ultra-violet radiation source according to Fig. 2a;

[0040] Fig. 3a shows a third embodiment of an ultra-violet radiation source according to the invention;

[0041] Fig. 3b shows a cross-sectional view of the ultra-violet radiation source according to Fig. 3a;

[0042] Fig. 4 shows a plot of the relationship of a radiant power at 550nm versus 490nm for ultra-violet radiation sources without any fluorescent material;

[0043] Fig. 5 shows a table of fit coefficients.

[0044] It is noted that in the figures parts with the same effect are always shown with the same reference signs. io

[0045] In a first preferred embodiment according to Fig. la, the ultra-violet radiation source 2 comprises a plurality of UV-emitting LED-elements 4 disposed along a translucent substrate 6. The LED-elements 4 are electrically connected with wirebonds 8 to a pair of end terminals (10a, 10b) so as to create a so-called LED filament. The whole is encapsulated in a translucent material 12 in which a number of particles of a fluorescent material 14 dispersed. In the first embodiment shown in Fug. la and lb, the fluorescent material 14 is uniformly dispersed in the translucent material 12. The translucent material 12 can be a silicone mantle.

[0046] In the second embodiment as shown in Fig. 2a and 2b, the ultra-violet radiation source 2 is substantially the same as that already described with reference to Fig. la and lb, except that the dispersion of the fluorescent material particles 12 is not uniformly within the translucent material. The translucent material 12 located above the LED-elements 4 is substantially transparent and does not contain particles of the fluorescent material 14. However, the translucent material 12 located on the rear surface of the LED-elements 4 (below the translucent substrate 6) is dosed with particles of the fluorescent material 14. This arrangement facilitates the unhindered transmission of UV radiation in the upward direction of the LED-elements 4 and maintains an LED filament having substantially unaffected UV irradiance on its active surface. However, a portion of the rearward UV radiation from the LED-elements 4 is intercepted by the fluorescent material disposed behind the LDE-elements 4 or even behind the translucent substrate 6, thereby converting a portion of their invisible ultra-violet radiation to visible light.

[0047] The net effect is similar to the arrangement described with reference to Fig. la and lb, but with a beneficially asymmetric radiation pattern of UV and visible radiation. It will be appreciated that for certain applications, the side of the LED filament onto which the fluorescent material is dispensed may be inverted.

[0048] In a third preferred embodiment shown in Fig. 3a and 3b the ultra-violet radiation source 2 is substantially the same as that already described with reference to Fig. la and lb, except that the fluorescent material 14 is dispensed directly onto the surface of the LED-element 4 and / or onto the translucent substrate 6. The density of the fluorescent material 14 on different surfaces of the LED-elements 4 is in this case not necessarily uniform, so as to ensure maximum conversion to visible wavelengths with minimum absorption of the forward ultra-violet flux. This arrangement has certain benefits that are mainly specific to the chosen manufacturing method. In not shown embodiments it is also possible that the ultra-violet radiation source 2 is substantially the same as that already described, except that the plurality of LED-elements 4 take the form of a conventional single-sided LED package, rather than the filament package or that the LED-elements 4 or the ultra-violet radiation source 2 may be conventional ultra-violet emitters without any fluorescent material 14, and a fluorescent material 14 coated surface may be located remotely to the LED-elements 4 or the ultra-violet radiation source 2 and positioned elsewhere within the final product containing said LED-elements 4 or the ultra-violet radiation source 2, such that a portion of the UV wavelengths are intercepted and re-radiated at visible wavelengths.

[0049] Wirth view to Fig. 4 to 9 the method for determination of a necessary dose of a fluorescent material 14 required to improve a visual appearance of an ultra-violet radiation source 2 is further described.

[0050] To establish a model, a series of prototype LED filaments were constructed in accordance with the embodiment as shown in fig. 1, in which the LED-elements 4 were based on an AIGaN chemistry, having a peak wavelength of 370nm, dimensions 300pm x 300pm and 120pm thick, with the usual Bragg interference coating on the lower surface, and a radiative power of approximately 3mW per chip when driven at 5mA with a junction temperature of 50°C. The LED elements 4 were distributed uniformly along a polycrystalline alumina substrate measuring 29.5mm x 0.76mm and 0.40mm thick and all connected electrically in series by the established practise of gold wire bonding, including the provision of two end terminals (cf. fig. 1).

[0051] An encapsulation in translucent material was provided (here a transparent silicone material) so as to yield a total filament diameter of approximately 1.60mm, with an encapsulated length of 32.0mm.

[0052] Numerous LED filaments were fabricated in each case with the LED-elements 4 of approximately constant parasitic luminescence, and in which the aforementioned phosphor was uniformly dispersed in the encapsulation of translucent material 12 with difference total dose weights of 0.2mg, 0.5mg, l.Omg, 1.5mg and 2.5mg.

[0053] The resulting LED-elements 4 are operated in pulse mode with a forward current of e.g. 5mA and constant junction temperature of e.g. 50°C, leading to a power dissipation of 375mW. This condition is representative of the typical operating parameters for such kind of ultra-violet radiation sources 2 in a real application. The absolute spectral power distribution is recorded from 340-760nm, and the total radiant power is derived for the three distinct wavelength ranges 340-410nm (band gap radiation from the LED-elements 4), 500-760nm (parasitic luminescence due to Ga vacancies), and 411-499nm (violet-blue emission from the fluorescent material 14). The chromaticity co-ordinates CCx and CCy according to the CIE (1931) method are also derived.

[0054] When considering first the undoped LED-elements 4 without any fluorescent material 14, it can be observed that irrespective of the LED-element 4 batch, the shape of the spectral power distribution from the parasitic luminescence is always constant, and only varying in its magnitude as can be seen from Fig. 4.

[0055] Therefore, the radiant flux at 550nm (the peak wavelength of the parasitic radiation) vs the radiant flux at 490nm is plotted. The result is a linear correlation, therefore proving that certainly as low as 490nm, the radiation is still solely produced by the deep-level radiative transitions of the Ga vacancies. This is illustrated in Fig. 5.

[0056] Moreover, it is not possible to visually distinguish the parasitic radiation extending below 490nm because of the increasing overlap with the band gap radiation. This is therefore calculated by first plotting the ratio of the radiant flux at every other wavelength between 400nm and 760nm vs the peak parasitic luminescence flux at 550nm as shown in Fig. 6. There is quite some noise in the data due to the very low signal strength from the parasitic luminescence, but the trend is clear. A 4th degree polynomial trendline is then applied to the averaged data, and extrapolated below 490nm to identify the full spectral power distribution being produced only by the parasitic luminescence. The fitted polynomial becomes negative below 478nm, therefore below that point the scaled parasitic radiation spectrum is set to zero.

[0057] The above derivation allows two distinct spectra to be mathematically calculated:

[0058] • For any LED-element 4 of similar construction, its absolute parasitic luminescence spectrum can be precisely determined by multiplying the above fitted trendline by its peak radiant flux at 550nm. • Similarly, the parasitic luminescence spectrum can be subtracted from the complete measured spectrum of any similar LED-element 4, to obtain the pure and uncontaminated band gap radiation.

[0059] These two spectra then allow a theoretical model of the complete spectrum to be built, for an arbitrary LED filament diameter.

[0060] The model is now extended to include the additional radiation that is produced when a known concentration of the fluorescent material 14 is added. This is possible from an analysis of the spectra of a set of LED-elements 4 where the only variable is the dose weight of the fluorescent material 14, thereby allowing identification of the radiation produced by only the fluorescent material 14. The resulting model allows the calculation of the spectrum of any similarly-designed filament as a function of three parameters:

[0061] •d Diameter of the silicone mantle (mm)

[0062] •m Phosphor dose weight (mg)

[0063] •G Peak radiant power of the parasitic luminescence at 550nm (pW / nm)

[0064] A three-dimensional matrix of modelled spectra is set up with the following exemplary parameters:

[0065] •d = 1.0mm, 1.5mm, 2.0mm, 2.5mm

[0066] • m = O.Omg, O.lmg, 0.2mg, 0.3mg, 0.4mg, 0.5mg

[0067] •G = OpW / nm, 5 pW / nm, 10 pW / nm, 15 pW / nm, 20 pW / nm

[0068] This results in the generation of e.g. 120 spectra, and for each of these the CIE chromaticity co-ordinates can be calculated along with the radiant power in each of the three spectral bands 340-400nm, 401-499nm and 500-760nm. These results are then fitted as a function of the parameters d, m and G with a linear regression function:

[0069] Yi = cO + cld + c2m + c3d2 + c4dm + c5m2 + c6G + c7dG + c8mG + c9G2 where Yi stands for the different photometric variables and cj (j=0..9) are fit coefficients.

[0070] The coefficients for the respective variables are given in the table according to Fig. 7. It is noted that the blank spaces for terms that are not statistically significant for that specific variable.

[0071] The coefficients for m are referenced to d=1.55 mm and encapsulated filament length 32 mm, G is referenced to an LED filament with electrical power of 375 mW. The next step is to generalize the model to an arbitrary LED-element 4 or LED- filament diameter and power.

[0072] Fig. 8 shows a plot of the calculated chromaticity coordinates (CCx, CCy) on the CIE(1931) Chromaticity Space for the case of d=1.6 mm and G=0pW / nm and G=24pW / nm respectively. When the dose of the fluorescent material 14 is zero, the chromaticity of the filament with G=0pW / nm appears in the violet corner of the chromaticity space since the only radiation is the UV / violet from the band gap emission of the LED-element 4. Similarly, when G=24pW / nm the chromaticity is in the yellow-green region since the parasitic luminescence accounts for the majority of the visible light emission, and greatly overpowers the tiny amount of violet band gap radiation.

[0073] The dose of the fluorescent material 14 is then varied from 0.0 to 0.6 mg. The radiation of the fluorescent material 14 alone is substantially blue, and when its dose is increased for both G values, the chromaticity shifts from the violet and yellow / green extremes towards the central blue region. All intermediate G values also fall on the same line and the line shifts only slightly for different LED-element 4 or LED-filament diameters.

[0074] Finally, it is possible to define the minimum required dose of fluorescent material 14 to achieve a blue chromaticity, as a function of filament diameter (d) and the impurity radiation content (G). As a target a CCy value is set less than yO, wherein yO represents an y-value below which the color points represent a blue color (e.g. 0.2). Then the dose m of the fluorescent material 14 is calculated at which CCy=yO for a set of filament diameters d (mm) and impurity radiations G (nm-1). G is now expressed as the spectral radiation at 550 nm per electrical Watt of the filament, since the calculation has been generalized to apply to any filament power. The minimum phosphor dose rnminis then expressed as:

[0075] — b — V / )2— 4ac mmin~ 2ac where a = c5b = C2 + CsG c = Co + Cid + CsG + c?dG + CgG2-yo

[0076] Fig. 9 show the results of these calculations for y0= 0.2. Hereby the minimum dose is expressed in pg per mm filament length. So for the case of a 32 mm filament these doses have to be multiplied by 32.

[0077] As a function of filament (or LED-element 4) diameter: Larger diameters require less dose concentration of fluorescent material 14 as can be seen from the less- than-linear increase of the minimum dose.

[0078] This calculation can be repeated for any desired maximum CCy. Knowing the dose, the corresponding x coordinates can be calculated using the coefficients for CCx.

[0079] The results from the above procedure are applicable to identify the required minimum dose of fluorescent material 14 to correct the colour appearance for any LED filament or LED-element 4 and therefore for any ultra-violet radiation source 2 having a mechanical construction as described, over any reasonable diameter, length, power dissipation, and parasitic luminescence radiant power. However it will be appreciated that the same procedure can be applied to any other construction of ultraviolet LED emitter to identify the required minimum dose of fluorescent material 14 to correct the well-known problems of undesirable visual colour deviations. List of reference signs ultra violet radiation source LED-element translucent substrate wire bonds a, b terminals translucent material fluorescent material

Claims

Claims1. Ultra-violet radiation source (2), in particular an LED emitter or LED device, comprising:- at least one LED-element (4) with a principle band gap radiation at a wavelength smaller or equal than 380nm and- a preferably translucent substrate (6), on which the at least one LED- element (4) is arranged; wherein the at least on LED-element (4) and the translucent substrate (6) are encapsulated in a translucent material (12); characterized in that an predetermined amount of a fluorescent material (14) to cause a fluorescent-material-radiation in a visible wavelength range from 380- 780nm, preferably from 400-500nm, is at least partially dispersed in the translucent material (12) and / or onto the at least one LED-element (4) and / or onto or into the translucent substrate to convert a part of the total radiant power of the at least one LED-element (4) into visible light; and wherein the converted part is between 0.1% and 10% of the total radiant power of the at least one LED-element (4).

2. Ultra-violet radiation source (2) according to claim 1, wherein the fluorescent material (14) comprises a peak radiation wavelength in the blue wavelengths, namely between 400-500nm.

3. Ultra-violet radiation source (2) according to claim 1 or 2, wherein the converted part of the radiation is with regard to the radiant power predominantly in the blue range between 400nm and 500nm, that is that more than 70%, preferably more than 80% of the radiant power of the radiation in the visible range between 380nm and 780nm is in the blue range that is between 400nm and 500nm.

4. Ultra-violet radiation source according to any of the preceding claims, wherein the predetermined amount of the fluorescent material (14) comprises a dose weight between O.lmg and 2.5mg.

5. Ultra-violet radiation source (2) according to any of the preceding claims, wherein the unconverted part of the total radiant power of the at least one LED-element (4) is at least 70%, preferably at least 90%, more preferably between 99.9% and 90%.

6. Ultra-violet radiation source (2) according to any of the preceding claims, wherein the fluorescent material (14) comprises or is phosphor, preferably Europium-activated Strontium Chlorophosphate (Sr3(PO4)3CI:EU2+).

7. Ultra-violet radiation source (2) according to any of the preceding claims, wherein a grain diameter of the fluorescent material (12) is smaller than 10pm.

8. Method for determination of a necessary dose of a fluorescent material (14) required to improve a visual appearance of an ultra-violet radiation source (2), wherein the ultra-violet radiation source (2) comprises at least one LED-element (4) encapsulated by a translucent material (12); the method comprising the following steps:- determining an absolute parasitic luminescence spectrum for the ultraviolet radiation source (2);- obtaining a pure and uncontaminated band gap radiation;- determining a radiation produced by the fluorescent material (14);- identifying the necessary dose of the fluorescent material (14) depending on the determined radiation of the fluorescent material (14) and on a diameter of the translucent material (12).

9. Method according to claim 7, wherein the absolute parasitic luminescence spectrum is determined by multiplying a spectral power distribution form the parasitic luminescence by its peak radiant flux at a wavelength of 550nm.

10. Method according to claim 7 or 8, wherein the pure and uncontaminated band gap is obtained by substraction of the parasitic luminescence spectrum from a measured spectrum of the ultra-violet radiation source (2).

11. Method according to any of claims 7 to 9, wherein the radiation produced by the fluorescent material (14) is determined by the comparison of a plurality of LED-elements (4), wherein the only variable is a dose weight of the fluorescent material (14).

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