EP4205165A1

EP4205165A1 - Far uv emitting device and use thereof

Info

Publication number: EP4205165A1
Application number: EP21762505.2A
Authority: EP
Inventors: René Theodorus WEGH; Martinus Petrus Joseph PEETERS
Original assignee: Signify Holding BV
Current assignee: Signify Holding BV
Priority date: 2020-08-26
Filing date: 2021-08-19
Publication date: 2023-07-05
Also published as: WO2022043167A2; WO2022043168A1; WO2022043167A3

Abstract

Far UV emitting device comprising a vacuum ultraviolet, VUV, source configured to generate during operation VUV radiation in a wavelength range of 100-200 nm. The far UV emitting device further comprises a far UVC phosphor being configured to be excited by said VUV radiation. Said far UVC phosphor having at least 50%, preferably at least 70%, of its photon emission during operation at least in a far UVC sub-range of a far UVC wavelength range of 200-230 nm. The vacuum UV source can be a lamp, comprising a lamp envelope comprising a UV-transmissive wall and enclosing a space in a gastight manner, said space comprising a discharge gas filling configured to generate during operation said VUV radiation in a wavelength range of 100-200 nm. Alternatively the VUV source is a laser or a LED.

Description

1 Far UV emitting device and use thereof FIELD OF THE INVENTION The invention relates to a far UV emitting device emitting in the far UVC- range, 200-230 nm. The invention further relates to use of the far UV emitting device for disinfection. BACKGROUND OF THE INVENTION With the increasing threat of aggressive viruses like Covid-19, there is a renewed interest in UV based disinfection. From various studies it has been shown that UVC radiation, i.e. wavelengths in the range of 200 to 280 nm, can effectively render viruses ineffective even at relatively low doses. This makes UV treatment of surfaces and air an interesting candidate to fight the spread of these kind of viruses through contact with contaminated surfaces or aerosols. At the same time, however, it is also known that short wavelength UV radiation can give rise to DNA mutations if the radiation hits the unprotected skin. For lamps/devices using mercury 254 nm radiation, which, for example, are commonly used disinfection low-pressure mercury discharge lamps, this limits the allowable exposure in areas where humans are present or may be present, rendering these state-of-the-art UVC lamps/devices using mercury as the active radiator to have the disadvantage of being essentially unsuitable for disinfection of public spaces in an intrinsically safe manner. It is known also that far UVC wavelengths, i.e. wavelengths in the wavelength range of 200 to 230 nm, show a higher absorption in the skin and hence have only limited penetration into the living skin. At sufficiently low wavelengths this radiation does not penetrate further than the upper skin layers which consist of dead skin cells. As a result, usage of these very short wavelengths has significantly less detrimental side-effects than the 254 nm radiation that emanates from state-of-the-art UVC lamps/devices using mercury as the active radiator. Many far UVC lamps or devices based on excimer radiation, for example from KrCl excimer, having a main emission peak at 222 nm, or for example KrBr excimer, having a main emission peak at 207 nm, however, additionally produce a substantial amount, i.e.7 to 16%, of longer, deep UVC radiation, i.e. wavelengths in the 230-280 nm range, as a by-product of the intended far UVC wavelengths. In current far UVC lamps/devices these 2 harmful, longer deep UVC wavelengths are suppressed by a relatively expensive filter, preventing the emission of the longer wavelengths into the exterior. This renders the known UVC lamps/devices, having said combination of an excimer lamp/device discharge and especially designed external filters, for example dielectric multilayer band-pass filters, to have the disadvantages of being relatively expensive and laborious to manufacture. Furthermore, these excimer lamps have as another disadvantage that their efficiency is relatively low, only in the order of 4%. SUMMARY OF THE INVENTION It is an object of the invention to provide a far UV emitting device in which at least one of the disadvantages of the known far UVC emitting device is counteracted. Thereto the far UV emitting device according to the invention having a photon emission, during operation, for a majority in the far UVC range, the far UV emitting device comprises: - at least one of a vacuum ultraviolet, VUV, source configured to generate during operation mainly VUV radiation in a wavelength range of 100-200 nm and a cathode luminescent lamp, configured to generate during operation accelerated electrons having an enrgy in the range of 2-30 keV; - a far UVC phosphor being configured to be excited by said VUV radiation and/or said accelerated electrons and to exhibit said photon emission; - the far UVC phosphor having a majority, preferably at least 70% of its photon emission during operation at least in a far UVC sub-range of a far UVC wavelength range of 200-230 nm. The far UV emitting device can be integrated into, for example, a HVAC, a washing machine, television screen or display, a luminaire. It can be combined with a separate light source providing visible light or it can be integrated into a single light source emitting both UV and visible light. For further improvement of the desired disinfection, the far UV emitting device can be combined with a separate ion generating device, such as an ionizer, or integrated into a single unit with an ionizer. Typically, a vacuum UV source is a lamp, comprising a lamp envelope comprising a UV-transmissive wall and enclosing a space in a gastight manner, said space comprising a discharge gas filling configured to generate during operation said VUV radiation in a wavelength range of 100-200 nm. Alternatively the VUV source is a laser or a LED. A further alternative for generating the far UVC radiation is a cathodeluminescent lamp comprising a space evacuated from gas in which a cathode on one side of said space and an 3 anode with a far UVC generating phosphor as the far UVC source on another, opposite side of said space are arranged, wherein, during operation, electrons are emitted from the cathode and accelerated towards the anode, which electrons upon hitting the far UVC phosphor release energy which is converted into far UVC radiation by the far UVC phosphor. Also the far UV emitting device could comprise a CathodeLuminescent Lamp (CLL). Cathodoluminescence typically takes place when a phosphor emits light upon excitation by accelerated electrons striking the phosphor. Electrons are typically emitted via either thermionic emission or field emission from a cathode arranged in an evacuated space. In thermionic emission, the cathode is heated to make it emit electrons and it could take several minutes to “warm up”. In contrast, the field emission enables switching on instantly by placing the cathode in a strong electric field. Carbon fibers, for example, work well as field emission cathode material. A compact power source for the CLL fits around the glass light bulb with minimal effect on its size. CLLs rely on the same principle as cathode-ray tube televisions. In cathodoluminescent technologies, a vacuum tube contains a negatively charged electrode (cathode) at one end and a positively charged, phosphor-coated electrode (anode) at another, opposite end. The cathode serves as an electron gun from which emitted electrons accelerate toward the anode at the opposite end, giving the electrons an energy in the range of 2-30 keV, and striking the far UVC generating phosphor, which converts the energy, at least partly, into far UVC radiation. Hence, the far UVC source is the far UVC generating phosphor excited by the electrons. In the context of this invention the following definitions apply: Vacuum UV, also referred to as VUV, is UV in the wavelength range of 10- 200 nm; Far UVC is UV in the wavelength range of 200-230 nm; Deep UVC is UV in the wavelength range of 230-300 nm or 230-280 nm; UVB is UV in the wavelength range 300-320 nm or 280-320 nm; UVA is UV in the wavelength range of 320-380 nm; Visible light is light in the wavelength range of 380-700 nm; IR light is light in the wavelength range of 700-100000 nm. Downstream is defined as the direction from the discharge or accelarated electrons inside the far UV emitting device, where the UV radiation is generated, to the exterior/environment of the far UV emitting device. The wall of the lamp envelope being transmissive for ultraviolet radiation, i.e. UV, means that the wall is at least transmissive for far UVC radiation, but typically also is 4 transparent for UVB, UVA, visible and Infrared (IR) radiation. Suitable materials for such a wall are fused quartz glass like Suprasil; sapphire, which is hard durable, yet expensive, but is transmissive for radiation down to about 140 nm; CaF₂ which is resistant to Fluorine corrosion, yet is hygroscopic and brittle, but is transmissive to about 120 nm; and MgF₂ which doesn't absorb water, yet has transmission less than CaF₂ and is brittle, but is transmissive to about 120 nm. Other suitable materials are, for example, LiF₂, BaF₂, NaF, KBr, Y₃Al₅O₁₂ (YAG = Yttrium Aluminate Garnet), spinel (MgAl₂O₄), AlON (Al₂₃O₂₇N₅), and PCA (polycrystalline alumina). A phosphor relates at least to a luminescent, a phosphorescent, and a fluorescent material. The percentage of photon emission by the far UV emitting source in the far UVC range is considered to relate to the total emission of the far UVC emitting source in the wavelength range from 200 nm to 700 nm, only, i.e. only to the visible light wavelength range and the UVA, UVB and UVC range (hence, the vacuum UV range and IR range being excluded). The vacuum UV source can be a laser, such as Ar2, Kr2, F₂, Xe2 or ArF lasers, emitting respectively at 126 nm, 146 nm, 157 nm, 172 & 175 nm, and 193 nm. The emitted wavelength is essentially monochromatic, yet, if also deep UVC is emitted, a deep UVC converting phosphor preferably is still used to filter out by conversion deep UVC radiation. The vacuum UV source, can also emit deep UVC radiation. Any deep UVC radiation generated by the vacuum UV source and/or by the far UVC phosphor is then converted by the deep UVC converting phosphor into longer wavelengths. Preferably the deep UVC converting phosphor has a transmissivity for wavelengths in the far UVC wavelength range of 200-230 nm of at least 50%, preferably at least 75%, more preferably at least 90%, such as at least 95%. Additionally the deep UVC converting phosphor preferably also converts any VUV radiation unintentionally escaping /having passed through the far UVC phosphor. Suitable lamp types as UV emitting device are: A) Low pressure discharge lamp with a rare gas or rare gas mixture and with coiled electrode with emitter, i.e. lamps similar to a normal fluorescent lamp, but without mercury in its filling. Examples include pure Xe, a Ne/Xe mixture preferably having a Ne percentage of >= 90%, typically with a prime emission wavelength at 147 nm. B) A pure Xe discharge lamp using external electrodes and capacitively operated, either in a coaxial geometry, i.e. ‘industrial’ type discharge lamps, with a radial discharge, or 5 an axial geometry as for example marketed by Ushio, or planar geometry as for example marketed as the Osram Planon lamp. All these lamps emit primarily at 172 nm. C) A pure Xe discharge lamp based on a micro-hollow cathode design, for example of an Eden-Park type, emitting primarily at 172 nm. Hence, typically the invention relates to two basic configurations, i.e.: 1) A combination of: - a low pressure excimer lamp, for example Kr2, F₂, ArBr, ArCl, KrI, ArF excimers lamps, or Xe or Ne/Xe discharge lamp which produces vacuum UV light, also referred to as VUV light, at 147 nm, or a medium pressure Xe discharge lamp which produces VUV light at 172 nm, with - a far UVC emitting phosphor that converts VUV light into the desired far UVC light, and possibly with - a deep UVC converting phosphor for converting undesired by-product emission radiation of the far UVC emitting source in the deep UVC wavelength range of 250-275 nm, preferably in the wavelength range of 230-280 nm, more preferably in the wavelength range of 230-300 nm, into radiation of wavelengths longer than 300 nm, for example in the UVB range for vitamin D generation and/or skin treatment, or in the UVA or visible wavelength range; 2) A combination of: - a cathodeluminescent lamp generating accelerated electrons in an evacuated space, with - a far UVC emitting/generating phosphor that converts energy of the accelerated electrons striking the far UVC emitting/generating phosphor, and possibly with - a deep UVC converting phosphor for converting undesired by-product radiation of the far UVC emitting source in the deep UVC wavelength range of 250-275 nm, preferably in the wavelength range of 230-280 nm, more preferably in the wavelength range of 230-300 nm, into radiation of wavelengths longer than 300 nm, for example in the UVB range for vitamin D generation and/or skin treatment, or in the UVA or visible wavelength range.. 6 So, disclosed is a far UV emitting device, such as a lamp, with use of a phosphor with Pr³⁺, Nd³⁺ or Tm³⁺ as activator in combination with a medium pressure Xe excimer lamp or a low pressure Xe or Ne/Xe lamp. The lamp produces vacuum UV light (< 200 nm) which is converted by the phosphor into far-UVC radiation. Pr³⁺, Nd³⁺ and Tm³⁺ phosphors show strong 4f →5d emission in the UV which can be tuned to the far-UVC range by careful choice of the host matrix. The overall system efficacy can be 2 to 4 times higher, and the system can potentially be cheaper and safer, i.e. no or less radiation is produced in the 250-290 nm range compared to a KrCl or KrBr excimer lamp. Typically the lamp is a low pressure lamp which is in many ways similar to the current UV disinfection lamps, using a fused quartz lamp envelope, a low pressure rare gas filling with a pressure in-between 100 Pa and 4000 Pa, and a lamp filling comprised of either Xe or a mixture of Xe and Ne, or a mixture of Ne, Ar and Xe. In all cases no mercury is dosed, yielding a discharge emission that is mainly concentrated around the Xe 147 nm resonance line. In this case low-pressure (Ne-)Xe lamp in essence the same lamp geometry can be used, as for a normal fluorescent lamp, using wounded tungsten electrodes with an oxide emitter, and low- or high frequency alternating current (AC) running through the lamp. For the gas mixture a 0.5-5% fraction of Xe in Ne is preferred, as this makes it easy to start the lamp. Depending on the inner diameter, a 100-1000 Pa total pressure is preferred for diameters of 8-20 mm, whereas 600-4000 Pa are preferred for lamps with inner diameters of less than 8 mm. Instead of wound tungsten electrodes with oxide emitter coating it is also possible to use cold-cathode electrodes, especially in narrow-diameter lamps. In these types of discharge lamps, it is also preferred to use AC operation of the discharge in order to avoid cataphoretic effects that could deplete the discharge of Xe atoms on one side of the tube. Alternatively, a higher-pressure lamp filled with pure Xe can be used in a dielectric barrier discharge configuration. In this type of lamp, the spectrum has a peak around 172 nm, originating from the Xe₂* excimer. This kind of lamp configuration has the advantage that the Stokes shift of the far-UV phosphor is smaller, which can lead to a high overall lamp efficiency. Besides these preferred excitation sources, there are multiple other VUV lamp sources that emit radiation < 200 nm, which could be used to excite phosphors that emit in the 200-230 nm range. These include various excimer sources using mixtures of halogens and rare gases, and deuterium lamps. In either of the lamps preferably the phosphor coating is deposited on the inside of the discharge vessel, which converts the discharge radiation into the desired 7 spectrum, which should preferably be concentrated around 200-230 nm in order to avoid skin damage on the one side (λ > 230 nm) and ozone generation on the other side (< 200 nm). Having the phosphor on the inside of the discharge vessel also has the advantage that the vessel only needs to be transparent to λ > 200 nm which makes it possible to use cheaper wall materials like fused quartz. Alternatively, the phosphor coating can be deposited on the outside of the discharge vessel or on a separate tube which is placed around the discharge vessel, an option that might be attractive in case the phosphor degrades too much when it is in the discharge. It is known that certain phosphors are prone to degradation due to reaction with Xe*. Still further the invention relates to (the use of) a phosphor with Pr³⁺, Nd³⁺ or Tm³⁺ as activator. Pr³⁺ phosphors show strong 4f →5d emission in the UV under VUV excitation. For this reason, they have been proposed as UV phosphors for Xe discharge lamps. The phosphor with the shortest-wavelength emission is YPO₄:Pr³⁺ which emits in the 225-280 nm range with the maximum of the first emission band at 232 nm, as such is not suitable for our application. A better phosphor is CaSO₄:Pr³⁺,M+ where M+ is an alkaline metal ion like Li+ or Na+, its emission spectrum has four main emission bands between 220 and 260 nm with decreasing intensity. These emission bands all belong to 4f →5d transitions, to the 4f ground state and the lowest 4f excited states. There are some more emissions in the UVA and visible, but very weak relative to the UVC emission bands. So it is a better candidate than YPO₄:Pr³⁺, but still there is significant emission intensity in the unwanted deep-UVC range, so more ideal is to shift the emission to even shorter wavelength, preferably to an emission peak wavelength of approximately 202 nm, meaning a shift with respect to CaSO4:Pr³⁺,M+ of ~4500 cm-1. It is noted that CaSO4 has the disadvantage of having (some) absorption in the 200-230 nm wavelength range, rendering it less suitable for the intended use as a deep UVC converting phosphor. Some examples of suitable host materials are given in tables I-III comprising a dopant of respectively Pr³⁺, Nd³⁺ and Tm³⁺ . The crystal-field depression D(A) for the lowest 4f →5d level relative to the free-ion energy level, and the Stokes shift ΔS(A) for the 4f →5d emission, are given for many host matrices (taken from P. Dorenbos, J. Lumin.91 (2000) 155-176). In most cases they were determined from the 4f →5d excitation and emission spectra for the Ce³⁺-doped material, but these values are in approximation constant for all trivalent lanthanides and therefore can be used to calculate the lowest 4f →5d excitation and emission for all trivalent lanthanides. Using the ‘ideal’ shift of 4500 cm-1, or at least a 8 positive shift with respect to the CaSO4:Pr³⁺,Na+ emission, the following candidate materials were identified for Pr³⁺ doped materials, see Table I. Potentially suitable Pr³⁺ phosphors are given in the table I below: D(A) ΔS(A) exc em em Δem max1 onset max1 max1 [cm-1] [cm-1] E [cm- E [cm- E [nm] ΔE [cm-1] 1] 1] target 49485 202 4500 target min 9000 200 target max 12000 206 CaSO4 15556 780 46024 45244 221 259 SrSO4 50200 45249 221 264 BaSO4 50500 45249 221 264 La2(SO4)3 12303 3367 49277 45910 218 925 SrB6O10 14738 932 46842 45910 218 925 SrB4O7 14080 1560 47500 45940 218 955 LaB3O6 12286 3704 49294 45590 219 605 LaMgB5O10 12568 3542 49012 45470 220 485 SrAl12O19 11050 5201 50530 45329 221 344 CaAl12O19 11604 6090 49976 43886 228 -1099 ThF4.4H2O 11352 2900 50228 47328 211 2343 AlF3 11838 2900 49742 46842 213 1857 KThF5 11352 2900 50228 47328 211 2343 KMgF3 (Oh) 6605 4712 54975 50263 199 5278 LaF3 8751 4975 52829 47854 209 2869 GdF3 10878 5567 50702 45135 222 150 YF3 9915 5444 51665 46221 216 1236 LuF3 10573 4940 51007 46067 217 1082 LiLuF4 15140 1242 46440 45198 221 213 NaYF4 8854 1500 52726 51226 195 6241 Table I 9 A similar approach was used for another candidate ion, i.e. Nd³⁺. Host materials with a depression (D(A)) between about 18000 and 24000 cm-1 will yield Nd³⁺ emission in the wavelength range 200-230 nm, ideally between 210 and 217 nm. These materials are listed in table II. Potentially suitable Nd³⁺ phosphors are given in the table II below: 10 11 Table II A similar approach was used for yet another candidate ion, i.e. Tm³⁺. Host materials with a depression (D(A)) between 22000 and 29000 cm-1 will yield Tm³⁺ emission in the wavelength range 200-230 nm, ideally between 202 and 222 nm. These materials are listed in table III. 12 Potentially suitable Tm3+ phosphors are given in the table III below: 13 14 Table III 15 In the event that these phosphors have an undesired emission in the deep UVC wavelength range, such as in the range of 230-280 nm, an additional deep UVC converting phosphor can be provided in the far UV emitting device for conversion of deep UVC radiation as emitted by the far UVC phosphor, into longer wavelengths of >= 290 nm, such as wavelengths > 300 nm, such as preferably UVB, UVA, visible radiation, wherein the deep UVC converting phosphor is substantially transmissive for the emission of the far UVC source in the far UVC range. These phosphors need to have strong absorption in 230-280 nm wavelength range and essentially no absorption in the 200-230 nm wavelength range. The exact absorption range depends on the emission spectrum of the first phosphor emitting the far-UVC radiation. Subsequent emission in the visible or in UV-A by said deep UVC converting phosphor is preferred. The ultraviolet emitting device, UV-emitting device, may have the feature that it comprises a combination of a far UVC emitting phosphor and a deep UVC converting phosphor in combination with a vacuum ultraviolet source (= VUV source) or with a source of accelerated electrons. The far UVC phosphor converting vacuum UV radiation generated by the VUV source into far UVC radiation, and sometimes also into deep UVC radiation, said deep UVC radiation is converted into longer wavelengths, typically at least one of UVB, UVA, visible and IR radiation, by the deep UVC converting phosphor. Said deep UVC converting phosphor being at least transparent for far UVC radiation. In view of the foregoing description and advantages of the far UV emitting device according to the invention, the far UV emitting device could have the following features: A far UV emitting device comprising: - a vacuum ultraviolet, VUV, source configured to generate during operation VUV radiation in a wavelength range of 100-200 nm; - a far UVC phosphor being configured to be excited by said VUV radiation; - the far UVC phosphor having at least 40%, preferably at least 50%, more preferably at least 70%, of its photon emission during operation at least in a far UVC sub- range of a far UVC wavelength range of 200-230 nm. The far UV emitting device could have the feature that it further comprises a lamp envelope comprising a UV-transmissive wall and enclosing a space in a gastight manner, said space comprising a discharge gas filling configured to generate during operation said VUV radiation in a wavelength range of 100-200 nm. 16 The far UV emitting device could have the feature that the far UVC phosphor has an activator chosen from Pr³⁺, Nd³⁺ and Tm³⁺. The far UV emitting device could have the feature that the far UVC phosphor comprises a host lattice or a mixture of host lattices, said host lattice being at least one of the group consisting of fluorides, chlorides, bromides, iodides, halo-oxides, oxides, oxy-sulfides, halo-sulfides, selenides, and oxy-nitrides. The far UV emitting device could have the feature that the far UVC phosphor comprises an oxide host lattice or a mixture of oxide host lattices, said oxide host lattice being at least one of the group consisting of nitrates, sulfates, carbonates, phosphates, hydrates, hydroxides, borates, silicates, germanates, aluminates, gallates, and transition metal complexes. The far UV emitting device could have the feature that the activator is Pr³⁺ and has a highest peak emission in the wavelength range of 200-210 nm, i.e. wherein the host lattice is The far UV emitting device could have the feature that the activator is Nd³⁺ and has a highest peak emission in the wavelength range of 210-217 nm, i.e. wherein the host lattice is chosen from 17 The far UV emitting device could have the feature that the activator is Tm³⁺ and has a highest peak emission in the wavelength range of 202-222 nm, i.e. wherein the host lattice is chosen from 18 19 The far UV emitting device could have the feature that the far UVC phosphor is provided on an inner surface of the UV-transmissive wall. The far UV emitting device could have the feature that the discharge gas filling is comprised in a lamp, wherein the lamp is chosen from the group consisting of a deuterium lamp, and excimers lamps having a gas filling of a rare gas, a mixture of rare gases, or a mixture of at least one halogen and at least one rare gas. The far UV emitting device could have the feature that the discharge gas filling is chosen from a low pressure Xe, Ne/Xe, or Ne/Ar/Xe filling, and from a medium pressure Xe2 filling, wherein low pressure is in the range of 100 Pa to 3500 Pa, and medium pressure is in the range of 3500 Pa to 80000 Pa. The far UV emitting device could have the feature that it further comprises a deep UVC converting phosphor arranged downstream of the far UVC phosphor and being configured during operation to be excited by deep UVC radiation in a wavelength range of 230-280 nm optionally emitted by the discharge gas filling and/or the far UVC phosphor, and to convert said deep UVC radiation into longer wavelengths of >= 290 nm. The far UV emitting device could have the feature that the deep UVC converting phosphor comprises an activator from the group Ce³⁺, Gd³⁺, Tm³⁺, Bi³⁺, or Pb2+, preferably Ce³⁺ as an activator. The far UV emitting device could have the feature that the deep UVC converting phosphor comprises at least one of Lu(1-x)AG:Bi³⁺Gdx³⁺, Sc(1-x)PO4:Cex³⁺; Lu(1- _x)PO₄:Ce_x ³⁺; Y_(1-x)PO₄:Ce_x ³⁺; Gd_(1-x)PO₄:Ce_x ³⁺; and Sr_(1-x)SiO₃:Pb_x2+, wherein 0.01 <= x <= 0.2. Use of the far UV emitting device according to the invention for disinfection. 20 BRIEF DESCRIPTION OF THE DRAWINGS The invention will now be further elucidated by means of the schematic drawings, which are by no means intended to limit the scope of the invention but rather to exemplify the ample possibilities of the invention. In the drawings Figure 1 shows 4fn energy levels for trivalent rare-earth ions; Figure 2 shows potential lanthanides for far UVC 200-230nm emission via 4fn- 15d¹ → 4fn in LiYF4 host lattice; Figure 3 shows potential lanthanides for far UVC 200-230nm emission via 4fn- 15d¹ → 4fn in YPO₄ host lattice; Figure 4A-B shows potential suitable host lattice groups with suitable shifts for Pr³⁺, Nd³⁺ and/or Tm³⁺ dopants for far UVC 200-230 nm emission; Figure 5A-C shows spectral properties of an example of a suitable far UVC 200-230 nm emitting phosphor based on the CaSO₄:Pr³⁺,Na+ emission spectrum; Figure 6A-C shows spectral properties of an example of a suitable far UVC 200-230 nm emitting phosphor based on the YPO₄:Nd³⁺ emission spectrum; Figure 7A-D shows spectral properties of an example of a suitable far UVC 200-230 nm emitting phosphor based on the LiY₄:Tm³⁺ emission spectrum, high-spin 4f^n-1 5d¹ → 4fn emission only (high spin); Figure 8A-B shows spectral properties of an example of a suitable deep UVC 230-280 nm converting phosphor derived from YPO₄:Ce³⁺; Figure 9 shows the potential of materials with Gd³⁺ and/or Tm³⁺ dopant as a suitable deep UVC 230-280 nm converting phosphor; Figure 10 shows energy level transitions of Bi³⁺ and/or Pb2+ potentially suitable for conversion of deep UVC 230-280 nm into longer wavelengths; Figure 11A-B shows spectral properties of examples of a Bi³⁺ sensitized Gd³⁺ phosphor and a Pb2+ sensitized phosphor for conversion of deep UVC 230-280 nm into longer wavelengths; Figure 12 shows a first embodiment of a far UV emitting device according to the invention; Figure 13 shows a second embodiment of a far UV emitting device according to the invention; and Figure 14 shows a third embodiment of a far UV emitting device according to the invention. 21 DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Figure 1 shows 4fn energy levels for trivalent rare-earth ions. Several lanthanides can never give far-UVC emission. Sm³⁺, Eu³⁺, Tb³⁺, Dy³⁺ have a too dense energy level scheme, hence excitation in the VUV or far-UVC will lead to non-radiative decay to a lower state, yielding visible emission. Furthermore, for Ce³⁺, Eu2+: 4f^n-15d¹ levels are at too low energy. For Yb³⁺ charge-transfer state is at too low energy. Pm³⁺ is radio- active. Based on this scheme, suitable lanthanides for far UVC emission 200-230 nm range, seem Pr³⁺, Nd³⁺, Gd³⁺, Ho³⁺, Er³⁺, Tm³⁺. Figure 2 shows potential lanthanides for far UVC 200-230 nm emission via 4f^n-15d¹ → 4fn in LiYF host lattice. Se ^{n-1 1} n 4 veral lanthanides can give efficient 4f 5d → 4f emission.4f^n-15d¹ → 4fn emission occurs when there is a significant energy gap from the lowest 4f^n-15d¹ excited state to the next 4f level. This is always the case for Ce³⁺; but emission is in the range UV-B to orange. This is the case for Pr³⁺, Nd³⁺, Er³⁺, Tm³⁺ in certain host lattices e.g. LiYF4. When 4f^n-15d¹ → 4fⁿ emission occurs, the emission to the ground state is often the strongest. The energy of the lowest 4f^n-15d¹ excited state is strongly dependent on the host lattice. For Pr³⁺, Nd³⁺ and Tm³⁺ it is possible by proper host lattice choice to get the lowest 4f^n-15d¹ excited state in the range of interest and get 4f^n-15d¹ → 4fn emission. Figure 3 shows potential lanthanides for far UVC 200-230 nm emission via 4f^n-15d¹ → 4fⁿ in YPO₄ host lattice. The lowest 4f^n-15d¹ excited state shifts down in energy with respect to LiYF4 for all lanthanides. The energy shift is in essence constant for all lanthanides, and is given by the difference in depression D(A) of the host lattice between LiYF4 and YPO4. In this way, the lowest 4f^n-15d¹ excited state shifts closer towards the target range for e.g. Nd³⁺ and Tm³⁺, but not enough. For Nd³⁺+ and Tm³⁺ a host lattice is needed with a larger D(A) than YPO4, whereas for Pr³⁺ a host lattice is needed with a smaller D(A) than YPO₄ to get the lowest 4f^n-15d¹ level in the energy range of interest. Figure 4A-B shows potential suitable host lattice groups with suitable D(A) for Pr³⁺, Nd³⁺ and/or Tm³⁺ dopants for far UVC 200-230 nm emission (taken from P. Dorenbos, J. Lumin.91 (2000) 155-176). Pr³⁺, Nd³⁺, Tm³⁺ can all give a very good performance with a filter efficiency (i.e. the fraction of the emission in the 200-230 nm range relative to the total emission spectrum) similar or better than the KrCl discharge, when the emission spectrum is shifted to the optimum spectral position. 22 Figure 5A-C shows spectral properties of an example of a suitable far UVC 200-230 nm emitting phosphor (derived from) CaSO₄:Pr³⁺,Na+. As shown in figure 5A the original phosphor has its major emission in the desired 200-230 nm far UVC range and having its highest emission peak at about 222 nm, yet still a significant emission in the undesired 230-280 nm deep UVC range. This spectrum can be shifted towards shorter wavelengths by choosing the proper host lattice. Ideally, the shift is about +4500 cm-1 resulting in a shift of about 20 nm to shorter wavelengths, i.e. the emission now has its highest emission peak at about 202 nm, yielding an improved emission spectrum as shown in figure 5B, having substantially all of its emission in the desired far UVC wavelength range of 200-230 nm. Said shifted spectrum is obtained under the assumption that the spectral shape of the emission spectrum of CaSO₄:Pr³⁺,Na+ does not change and that it can be shifted while keeping the spectral shape. These assumptions are reasonable as it is well-known that the position of the lowest 4f →5d level can be shifted over a wide range by tuning the host matrix whereas the 4f levels remain at the same energy, and the four 4f →5d emission bands with decreasing intensity are observed in many Pr³⁺ materials. Figure 5C shows the percentage of the emission spectrum in the 200-230nm wavelength range and 230-280nm wavelength range as a function of the position of the highest emission peak of the emission spectrum. As shown, the optimum range for the highest emission peak is 200-212 nm, rendering > 70% emission in 200-230 nm range and limiting emission in the deep UVC range 230-300 nm to < 30%. More ideally, the range for the highest emission peak is 200-206 nm, rendering > 80% emission in the 200-230 nm range and limiting emission in the 230-300 nm range to < 20%. Figure 6A-C shows spectral properties of an example of a suitable far UVC 200-230 nm emitting phosphor (derived from) YPO4:Nd3+. Figures 6A-C are similar to figures 5A-C, however the emission spectrum in Fig 6B has a shift of about -3500 cm-1 with respect to fig 6A. As shown in figure 6C, the optimum range for the highest emission peak is 204-217 nm, rendering > 80% emission in 200-230 nm range and limiting emission in the 230-300 nm range to < 10%. Considering that emission from the competing 2G9/24f level should be prevented i.e. the lowest 4f^n-15d¹ level should not be positioned above the 2G9/24f level, it is preferred to choose the host lattice such that the 4f^n-15d¹ emission maximum is in the range 210-217 nm. This means that the target shift with respect to YPO4 is around -5500 cm^-1. Figure 7A-D shows spectral properties of an example of a suitable far UVC 200-230 nm emitting phosphor (derived from the) LiY₄:Tm³⁺ 4f^n-15d¹ emission high-spin emission only (HS). Tm³⁺ can give high-spin and low-spin 4f^n-15d¹ emission in certain host 23 lattices, e.g. in LiYF4. The high-spin 4f^n-15d¹ states is the lowest in energy so gives the strongest emission. In most lattices and at room temperature only emission from the high- spin 4f^n-15d¹ state will be observed. The emission spectrum in both Figures 7A and 7B has a shift of about -10500 cm-1 with respect to the high-spin 4f^n-15d¹ emission spectrum of LiY4:Tm³⁺, and in addition in Figure 7B the emission bands are broadened to reflect the performance at room temperature i.e. the target operation temperature of the invention. Figure 7C-D are similar to figure 5C, yet for two different situations., i.e. at a temperature of about 10K (figure 7C) and at room temperature (figure 7D) leading to a broadened spectrum. As shown in figure 7C-D, the optimum range for the highest emission peak is 202-222 nm, rendering > 70% up to even > 80% emission in 200-230 nm range and limiting emission in the 230-300 nm range to < 20%. In several garnet materials the Tm³⁺ 4f →5d emission should be at the right position, see table III. An overview of Figures 5C, 6C and 7C-D in numbers related to specific spectral wavelength ranges, is given in table IV below. It shows that in particular Nd³⁺ and Tm³⁺ have a better filter efficiency than KrCl discharge when the emission spectrum is at the optimum position. Table IV. Figure 8A-B shows spectral properties of an example of a suitable deep UVC 230-280 nm converting phosphor (derived from) YPO4:Ce³⁺. Lanthanide phosphors can be used to convert VUV light from e.g. Xe excimer discharge into 200-230 nm light. Candidates are Pr³⁺, Nd³⁺, Tm³⁺ with 4f^n-15d¹ → 4fn emission. However, there will always be emission at longer wavelengths. The emission in the range 250-290 nm should be removed for safety. This can be done by a dichroic short-pass filter, but that is very expensive. This can alternatively be done using a second phosphor layer. This phosphor needs to: 24 - Have strong absorption in about the 250-290 nm range and essentially no absorption in the 200-230 nm range. The exact absorption range depends on the emission spectrum of the first phosphor emitting the far-UVC radiation. - Emission in the visible or in UV-A is a bonus. Using lanthanide 4fn → 4f^n-15d¹ absorption, Ce³⁺ is the only possible candidate as it has only five 5d energy levels, which can be significantly spaced apart. Generally there is a significant energy gap between the first two 5d levels; but they are always at too low energy and cannot be brought to 200-230 nm range. There can be a gap between the higher levels in cases, e.g. in orthophosphates. In ScPO₄ the 5d levels are expected at somewhat lower energy than in YPO4, such that the energy gap between the C and E absorption bands could be right where the main far-UVC emission from CaSO₄:Pr³⁺ is (Fig 8B). Some broadening and extra bands due to impurities could occur in practice. All other lanthanides have too dense 4f^n-15d¹ energy level scheme, as there are many 4f^n-15d¹ energy levels. Figure 9 shows the potential of materials with Gd³⁺ and/or Tm³⁺ dopant as a suitable deep UVC 230-280 nm converting phosphor when using lanthanide 4fn → 4fn absorption, though the 4fn → 4fn absorption is weak so Gd³⁺ and Tm³⁺ should be part of the host lattice. Gd³⁺ and Tm³⁺ are suitable as they have no 4f energy levels in the 43500-50000 cm-1 range, and they do have (some) 4f energy levels in the 34500-41000 cm-1 range. The 4fn- 15d¹ energy levels should be > 50000 cm-1, which is the case for many regular host lattices e.g. ortho-phosphates and fluorides. There is only absorption in parts of the 250-290 nm range, in narrow lines, so the suitability depends on where in the 230-300 nm range emission by the far UVC phosphor occurs. The emission could be inefficient due to concentration quenching, yet this can be counteracted by addition of a co-dopant, for example with a suitable activator ion as Eu³⁺. Figure 10 shows energy level transitions of Bi³⁺ and/or Pb2+ as s2-ions. These ions are suitable for conversion of deep UVC 230-280 nm into longer wavelengths because they have limited energy levels, often lying in the UV, giving strong absorption bands. Position of energy levels can easily be tuned by host lattice Figure 11A-B shows spectral properties of potentially suitable examples of Bi³⁺ and Pb2+ phosphors for conversion of deep UVC 230-280 nm into longer wavelengths. The examples shown in figures 11A-B have some gaps in the 200-230 nm range and give some emission < 300 nm, however, said emission in the < 300nm range can be counteracted by addition of Gd³⁺ as a co-dopant, as shown in figure 11A for Lu_(1-x)AG:Bi³⁺Gd_x ³⁺ ,. LuAG is an abbreviation for Lutetium Aluminum Garnet, i.e. Lu3Al5O12. In figure 11B the 25 significant effect of the host lattice on the excitation and emission properties of phosphors having Pb2+ as a sensitizer is shown. As shown only SrSiO₃:Pb2+ appears suitable as a deep UVC converting phosphor being transmissive for a large sub-range of the far UVC range, and having its excitation spectrum both in the VUV and deep UVC range while emitting at longer wavelengths, such as in the UVB range. Figure 12 shows a first embodiment of a far UV emitting device according to the invention. More specifically, figure 12 shows a coaxial dielectric barrier discharge lamp, also referred to as DBD-lamp, with an annular shaped gastight discharge space 1. The double walled lamp is made of VUV and UVC transmissive Y₃Al₅O₁₂, referred to as YAG = Yttrium Aluminate Garnet, and has an outer wall 3 and an inner wall 2 which are spaced apart by a gap 9 through which an air flow 7 can stream. The inner wall forms the annular gastight discharge space 1 which is filled with a discharge gas of pure Xenon at a medium pressure of about 10000 Pa. On an outer surface 2c of the inner wall the electrodes 4 are provided as a mesh structure for generating a discharge and enabling generated VUV light of about 172 nm to pass through. On top of the electrodes, downstream of the discharge, a first, far UVC generating phosphor 10, a Tm³⁺ phosphor with a highest peak emission at about 215-220 nm is provided that converts the VUV for about 80% into a far UVC subrange of the far UVC wavelength range and which converts also for about 20% of the VUV into a deep UVC sub- range of the deep UVC wavelength range. Said combined radiation of far UVC and deep UVC is radiated into the gap 9 through which air 7 flows and both the far UV and deep UV is used for disinfection of said air. On an inner surface 3a of the outer wall, downstream of the far UVC phosphor, a second deep UVC converting phosphor 5, Sc_(1-x)PO₄:Ce_x ³⁺, is provided which is almost completely transmissive for the sub-range of the far UVC wavelength range generated by the first phosphor, i.e. with the exception of an minor absorption band at about 210 nm, but which converts the deep UVC sub-range as generated by the first phosphor into longer wavelength radiation range of 300-380 nm. Said second phosphor prohibits that deep UV is issued to the exterior. Figure 13 shows partly worked open perspective view of a second embodiment of a far UV emitting device according to the invention. More specifically, Fig. 13 shows a low-pressure discharge lamp 300 with an elongated discharge vessel 303, having a wall 302, and is made of quartz glass transmissive for at least far UVC, i.e. Suprasil. The lamp comprises an electrode 305 at each end, which electrode is a coil formed by a triple coiled tungsten wire 306 supported by conducting lead wires 307, 309 which extend through a quartz glass pinch 311 of a seal 310. The double or triple coiled tungsten wire 306 is 26 provided with an emitter material such as oxides of barium, calcium, and strontium for reducing the work function of the electrode. The seal 310 hermetically seals off the discharge vessel 303. The lead wires 307, 309 are connected to pin-type contacts 313 in the respective end caps 312 which are provided at either end of the lamp 300. The discharge vessel 303 has a discharge space 317 which is filled with a low pressure rare gas filling, with a pressure in-between 100 Pa and 4000 Pa, typically chosen from Xe, a mixture of Xe and Ne, and a mixture of Ne, Ar and Xe, in the figure the filling is with Xenon to create a Xe discharge and emission thereof during operation of the lamp, yielding a discharge emission that is mainly concentrated around the Xe 147 nm. No mercury is dosed. A first phosphor coating 304 of Ca(1-x-y) SO4:Prx³⁺,Na + y , as a far UVC emitting source, is provided on the wall 302 downstream of the discharge, and faces the discharge space 317. Said first phosphor coating 304 converts the 147 nm vacuum UV into far UVC radiation peaking in the far UVC sub-range of 220-230 nm, but also generates some deep UVC radiation in the deep UVC sub-range around 250nm as an undesired side-product. A second phosphor coating 308, as a deep UVC converting phosphor, in this case a Sc_(1- x)PO4:Cex³⁺ phosphor, is provided downstream of the first phosphor layer 304, i.e. in between the first phosphor layer 304 and the wall 302. Said second phosphor 308 is at least transmissive for the radiation of the first phosphor 304 emitted in the far UVC sub-range, i.e. for the emission in the range of 220-230 nm, but is excited by the radiation emitted by the first phosphor 304 in the deep UVC sub-range , i.e. for the emission around the 250 nm wavelength range, and converts said deep UVC radiation into longer, non-harmful radiation wavelengths. Figure 14 shows a schematic diagram of a cathodeluminescent lamp (CLL) as a third embodiment of a far UV emitting device according to the invention. The CLL comprises a lamp envelope 1401 in which a cathode 1407 is arranged. The lamp envelope is evacuated through exhaust tube 1411. The lamp envelope comprises a lamp wall 1403 on which a stacked coating is provided facing the cathode and being spaced from the cathode. The stacked coating comprises a far UVC layer 1406, a deep UVC converting layer 1413, and an anode 1404. A voltage is applied between the cathode and anode through contacting elements 1405 causing field emission of electrons from the cathode which are accelerated towards the anode while bridging the spacing between cathode and anode. The accelerated electrons pass through the anode layer 1404 deposited on top of the phosphor layers, hit the far UVC phosphor layer 1406 which subsequently emits radiation 1415 comprising far UVC radiation. In the case that the far UVC phosphor also emits some deep UVC radiation, a deep 27 UVC converting phosphor layer could be provided, which converts said deep UVC radiation into radiation of longer wavelengths than deep UVC. In the CLL Ca_(1-x-y)SO₄:Pr_x ³⁺,Na_y+ can be suitably applied as far UVC emitting phosphor and Sc(1-z)PO4:Cez³⁺ as deep UVC converting phosphor, wherein 0.002 <= x <= 0.1, 0 < y <= 0.1, and 0.002 <= z <= 0.1.

Claims

28 CLAIMS: 1. A far UV emitting device having a photon emission, during operation, for a majority in the far UVC range, the far UV emitting device comprising: - at least one of a vacuum ultraviolet, VUV, source configured to generate during operation mainly VUV radiation in a wavelength range of 100-200 nm and a cathode luminescent lamp, configured to generate during operation accelerated electrons having an energy in the range of 2-30 keV; - a far UVC phosphor being configured to be excited by said VUV radiation and/or said accelerated electrons and to exhibit said photon emission; - the far UVC phosphor having a majority, preferably at least 70%, of its photon emission during operation at least in a far UVC sub-range of a far UVC wavelength range of 200-230 nm. 2. The far UV emitting device as claimed in claim 1 further comprising a lamp envelope comprising a UV-transmissive wall and enclosing a space in a gastight manner, wherein either - said space comprises a discharge gas filling configured to generate during operation said VUV radiation in a wavelength range of 100-200 nm, or - said space is evacuated and comprises a cathode on one side of said space and an anode with a far UVC generating phosphor as the far UVC source on another, opposite side of said space, wherein, during operation, electrons are emitted from the cathode and accelerated towards the anode, which electrons upon hitting the far UVC phosphor release energy which is converted into far UVC radiation by the far UVC phosphor. 3. The far UV emitting device as claimed in claim 1 or 2, wherein the far UVC phosphor has an activator chosen from Pr³⁺, Nd³⁺ and Tm³⁺. 4. The far UV emitting device as claimed in claim 1, 2 or 3, wherein the far UVC phosphor comprises a host lattice or a mixture of host lattices, said host lattice being at least 29 one of the group consisting of fluorides, chlorides, bromides, iodides, halo-oxides, oxides, oxy-sulfides, halo-sulfides, selenides, and oxy-nitrides. 5. The far UV emitting device as claimed in claim 1, 2 or 3, wherein the far UVC phosphor comprises an oxide host lattice or a mixture of oxide host lattices, said oxide host lattice being at least one of the group consisting of nitrates, sulfates, carbonates, phosphates, hydrates, hydroxides, borates, silicates, germanates, aluminates, gallates, and transition metal complexes. 6. The far UV emitting device as claimed in any one of the preceding claims 1 to 5, wherein the activator is Pr³⁺ and has a highest peak emission in the wavelength range of 200-210 nm, i.e. wherein the host lattice is 7. The far UV emitting device as claimed in any one of the preceding claims 1 to 5, wherein the activator is Nd³⁺ and has a highest peak emission in the wavelength range of 210-217 nm, i.e. wherein the host lattice is chosen from

30 8. The far UV emitting device as claimed in any one of the preceding claims 1 to 5, wherein the activator is Tm³⁺ and has a highest peak emission in the wavelength range of 202-222 nm, i.e. wherein the host lattice is chosen from 31

32 9. The far UV emitting device as claimed in any preceding claim, wherein the far UVC phosphor is provided on an inner surface of the UV-transmissive wall. 10. The far UV emitting device as claimed in any preceding claim, wherein the discharge gas filling is comprised in a lamp, wherein the lamp is chosen from the group consisting of a deuterium lamp, and excimers lamps having a gas filling of a rare gas, a mixture of rare gases, or a mixture of at least one halogen and at least one rare gas. 11. The far UV emitting device as claimed in claim 10, wherein the discharge gas filling is chosen from a low pressure Xe, Ne/Xe, or Ne/Ar/Xe filling, and from a medium pressure Xe₂ filling, wherein low pressure is in the range of 100 Pa to 3500 Pa, and medium pressure is in the range of 3500 Pa to 80000 Pa. 12. The far UV emitting device as claimed in any preceding claim, further comprising a deep UVC converting phosphor absorbing and converting deep UVC radiation, arranged downstream of the far UVC phosphor and being configured during operation to be excited by deep UVC radiation in a wavelength range of 230-280 nm optionally emitted by the far UVC phosphor, and to convert said deep UVC radiation into longer wavelengths of >= 290 nm. wherein the deep UVC converting phosphor is substantially transmissive for the emission of the far UVC source in the far UVC range. 13. The far UV emitting device as claimed in claim 12, wherein the deep UVC converting phosphor comprises an activator from the group Ce3+, Gd³⁺, Tm³⁺, Bi³⁺, or Pb2+, preferably Ce³⁺ as an activator. 14. The far UV emitting device as claimed in claim 13, wherein the deep UVC converting phosphor comprises at least one of Lu_(1-x)AG:Bi³⁺Gd_x ³⁺, Sc_(1-x)PO₄:Ce_x ³⁺; Lu_(1- 33 PO:Ce³⁺; Y PO:Ce³⁺; Gd PO:Ce³⁺; and Sr )SiO3: 2+ x) 4 x (1-x) 4 x (1-x) 4 x (1-x Pbx , wherein 0.01 <= x <= 0.2. 15. Use of the far UV emitting device as claimed in any preceding claim for disinfection.