BACKGROUND OF THE INVENTION

The invention relates to an LED device with a substantial emission in the UV- B wavelength range, i.e. radiation in a wavelength range of 280-320 nm, during operation. Since long it is known that the Vitamin-D response curve for generation of Vitamin-D in humans peaks in the UV-B wavelength range. Unfortunately, said response curve largely overlaps with the harmful actinic and erythema response curves. Hence, administering UV-B radiation to humans for generation of Vitamin D involves the risk that too much harmful radiation is simultaneously administered to the human body that might cause actinic and erythema reactions.

W02010016009A1 discloses an UV-B emitting device comprising an LED having at least part of its emission in the UV-B region of 280-320 nm during operation and a filter in optical communication with the LED and being configured to block part of said UV- B radiation at a cut-off wavelength (= CW). In the known device the UV-B emitter could have an emission peak wavelength (= PWL) in the range of 305-315 nm and the filter typically blocks radiation at wavelengths shorter than 305 nm and/or longer than 315 nm.

The cut-off or blocking wavelength is defined as the wavelength where the transmission/blocking of the filter is 50%. In the known UV-B emitting device the position of the blocking wavelength range and emission of the LED is chosen such that the lamp has relatively high efficacy, and where the emitted radiation by the known UV-B emitting device has a relatively large ratio of "Response Vitamin D'V'Response erythema". Yet, the known UV-B emitting device has the disadvantage that its performance is unsatisfactory in different aspects.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an UV-B emitting, LED device in which the disadvantage of at least one aspect of the known prior art UV-B emitting device is counteracted. Thereto the LED device of the type as described in the opening paragraph has the features of:

LED device comprising: - an LED being configured to emit, during operation, an emission spectrum having a highest emission peak, PWL, and having a majority, i.e. more than 50%, of its emission spectrum in a UV-B wavelength range of 280-320 nm, and

- a filter arrangement having an edge steepness, ES, at a cut-off wavelength, CW, and being in optical communication with the LED and being configured to block radiation below said CW, wherein said PWL is in the wavelength range of 300-315 nm, and wherein said CW is in the range of 305-315 nm.

The disadvantageous different aspects of the prior art UV-B emitting device typically relate to its form-factor, i.e. the device being relatively large and non-robust, use of hazardous materials, and to the efficiency of its use of its UV-B emission spectrum for generating Vitamin D in humans.

The Photo Biological safety standard demands that the UVB radiation level stays below a certain level, but to produce as much as possible Vitamin D, in other words to make an effective product, one would like high levels of UVB radiation. To meet these demands, in the determination of the emission spectrum of the LED device according to the invention, not only the erythema response curve, but in particular the actinic response curve is considered. Furthermore, instead of considering the peak value of the ratio between the Vitamin D response/actinic response or Vitamin D response/erythema response, the inventors had the new approach to consider the integrated value of the product of the filtered emission spectrum of the UV-B emitting LED and the vitamin D response curve, primarily irrespective of the energy balance of the LED device. In other words, the inventors searched for a favorable wavelength range where the largest amount of Vitamin D generating radiation that can be administered while limiting the maximum actinic irradiance to lmWa/m2, yet without primarily considering the efficacy of the LED device. Because the absolute value of the first derivative of the slope of the actinic curve in the area 305-315nm is less than the absolute value of the first derivative of the slope of the Vitamin D action curve, it is desired and beneficial to have the LED radiation, as specified by its PWL and FWHM, enhanced in that area. This desired enhancement is attained by applying the filter as specified by its ES and CW. I.e., compared to the unfiltered LED spectrum, it is attained that the overlap of a filtered LED spectrum with the actinic curve is more reduced than the overlap of the filtered spectrum with the Vitamin D curve. For example, of a LED with a PWL of 308 nm and a filter with a CW at 307 nm filter, the radiometric power reduces to 63%, the actinic power reduces to 9% and the Vitamin D power reduces to 25%. So at the cost of 37% of total flux, the actinic hazard is reduced by a factor of about ten and the Vitamin D only by a factor of about four. The loss in efficacy of the device, because in the cut of the total flux of the device by application of the filter, is taken into account only as a secondary consideration.

On the base of this new approach, the inventors found that the maximum amount of radiation for Vitamin D generation, i.e. the Vitamin D dose, that could be administered to a human, while limiting the actinic irradiance radiation to maximally lmWa/m2, is mainly determined by the following parameters, i.e.:

- the peak wavelength, PWL, of the UV-B emitting LED;

- the Full Width at Half Maximum, FWHM, of the emission spectrum of the UV-B emitting LED;

- the cut-off wavelength, CW, of the filter; and

- the edge steepness/width, ES, of the filter edge of the (long pass) filter.

In the description the expression a filter with "cut-off wavelength, CW" is intended to mean a long wave pass filter designed to be >= 50% transmissive for wavelengths longer than the cut-off wavelength, CW. The edge steepness, ES, is the width of the wavelength range of the transition from the filter of being blocking for at least 90%, to the filter becoming transmissive, i.e. being blocking for less than 20%. The degree of blocking/transmissive function of the filter is determined for UV/light incident at a perpendicular angle on said filter. The expression "blocking" can alternatively be expressed as “non-transmissive”, “opaque” or “opaqueness” (via reflection and/or absorption), and is defined as the degree to which something reduces the passage of light, and here means that the filter being, for example, <=5% transmissive for radiation at perpendicular incidence of said radiation on said filter. An example of a relatively cheap, potentially suitable filter is the N- WG305nm glass filter of Schott having a cut-off wavelength, CW, of 305 nm and an edge steepness, ES, of 30 nm. Filters with a relatively steep ES, i.e. with ES <= 10 nm, for example <= 5 nm or ES <= 3 nm, such as ES <= 1 nm, in approximately the desired CW range of 305-315 nm, are available by Semrock, see https://wmv.semrock.com/filters.aspx. see for example https://www.semrock.com/FilterDetails. aspx?id=FF01-300/LP-25 having a CW at 306 nm and an ES of about 5 nm . An example of a suitable LED is DOWA UVB LED, model 308-FD-01-U04-SIG, based on aluminum-Gallium Nitride (Al, Ga)N, and having a peak wavelength of 308 nm and a FWHM of 15 nm, see for example https://fh- muenster.de/ciw/downloads/personal/iuestel/iuestel/2017-06-0 7 UV-Strahlung - The Good the Bad and the Uglv-l.pdf ) Table 1 gives, as a starting point, an example of the maximum Vitamin D dose, also referred to as SDD, of a LED having a PWL in the 300-315 nm wavelength range, a FWHM of 15 nm of the emission spectrum of the LED, with no filter applied, and with a maximum actinic irradiance of lmWa/m2 at an irradiation distance of 20 cm. SDD is related to the Vitamin D irradiance, wherein 1 SDD = 100 Vitamin D weighted Joule/m2 (or 100 J.d/m2) as received in 30000 seconds, and mWa is the actinic irradiance For this starting point, the maximum SDD goes up to about 0.51. Table 2 gives examples of the LED device according to the invention using the same LED, but wherein a long wave pass filter having a relatively steep edge steepness of ES <= 3 nm, in the Table 2 ES = 1 nm, is applied at various cut-off wavelengths CW between 298-308 nm, for a maximum actinic-irradiance- of lmWa/m2 for an irradiation distance of 20 cm. A shown in Table 2, for the same maximum acceptable actinic irradiance of lmWa/m2, the maximum SDD goes up to 0.98, almost twice as much as in the unfiltered case.

Table 1. Maximum Vitamin D dose (SDD) as a function of PWL UV-B LED with FWHM of 15 nm, no filter applied.

Table 2. Maximum Vitamin D dose (SDD) as a function of PWL UV-B LED with FWHM of 15 nm, with filter applied at varying CW, the ES of the filter being 1 nm at the CW.

As such the disclosed solution is counterintuitive as the most effective part of the emission spectrum of the UV-B LED is cut off, i.e. the CW is where the LED has a significant part of its emission and where the Vitamin D response curve is relatively high. Furthermore, surprisingly a relatively effective spectrum is obtained for the combinations of CW and PWL where the CW is being positioned practically at the PWL of the UV-B emitting LED, i.e. CW = 307 nm for the PWL range of 305-309 nm. The downside of this is that a higher LED flux is needed to get the Vitamin D dose, but this downside is overruled by the great benefit it brings, i.e., by cutting away the short wavelength side of the LED spectra the ratio between Vitamin D dose and the Actinic dose/irradiance improves impressively. This means that a much more effective Vitamin D device can be realized, i.e. at the same

Actinic dose/irradiance, a higher Vitamin D dose can be administered. The improvement can be very substantial, the inventors found an improvement up to a factor 2.5 of the case with use of an appropriate filter compared to cases without use of a filter. The ratio between Vitamin D and actinic exposure can even be increased to levels above that what a conventional phosphor, for example Gadolinium Lanthanum Borate:Bi (GLBB), can do. Calculations with realistic LED spectra and filters show that filtered LED spectra can be at least a factor of 1.5 better than the GLBB phosphor.

As a comparison and as a kind of reference value, simulations reveal that the GLBB phosphor as mentioned in the cited prior art and applied as a phosphor in a low- pressure mercury discharge lamp in unfiltered condition or with a filter with a CW < 305 nm and/or CW > 315 nm, has a SDD of about 0.61. In the cited prior art lamp, the GLBB phosphor is excited by 254 nm of the Hg-discharge, and its emission spectrum shows peak line emissions at about 311 nm and 313 nm, and a significantly lower peak emission at 305 nm. As shown in the table 2, with the UV-B emitting LED device according to the invention, significant improvements in the amount of maximum SDD is obtained with the use of an appropriate filter. Yet it is clear that the GLBB performs better than the combinations of UV- B LED and filter represented by the white cells of Table 2, with SDD values of lower than 0.61. The combinations represented by the light grey cells of Table 2 are already an improvement over the GLBB, while the dark grey cells or Table 2 represent the most interesting area of combination for significant improvement over GLBB, i.e. being about 1.5 times better than GLBB. Furthermore, related to the form-factor of the prior art device, the use of an UV-B LED has various advantages over the use of GLBB in a low-pressure mercury discharge lamp, as for example in that it is more robust, more compact, and free of mercury.

The LED device could have the feature that 0 nm < ES <= 30 nm, preferably ES <=15 nm, more preferably ES <= 5 nm, most preferably ES <= 2 nm. The steeper the ES, the better fine tuning of the desired UV-B emission spectrum is attainable, the higher SDD values are attainable for a maximum actinic irradiance of lmWa/m2.

The LED device could have the feature that the CW is in the wavelength range of 305-312 nm, preferably 305-309 nm, most preferably 305-307 nm.

GW: Depending on small differences in the PWL and/or FWHM of the LED emission spectrum, we see that the optimum CW shifts, and also that the most preferred CW can also be at lower CW that is indicated in our tables. For example, it is seen in table 2 that a CW in the range of 306-308 nm, preferably with an ES <= 2 nm, is a sweet point for some LEDs.

The LED device could have the feature that the PWL is in the wavelength range of 300-315 nm, preferably 300-311 nm. The shorter wavelength range of 300-311 nm, such as 300-307 nm, in the wavelength range of 300-315 nm is more beneficial from the secondary consideration of efficacy, and also the filter gain is slightly larger here.

The LED device could have the feature that the emission spectrum of the LED has a FWHM, wherein 10 nm <= FWHM <= 30 nm, preferably 10 nm <= FWHM <= 18 nm. Available UV-B emitting LEDs typically have a FWHM in the range of 10 nm to 30 nm. Yet, the smaller the FWHM of said UV-B emitting LEDs the better the effect on the maximum attainable SDD values for a maximum actinic irradiance of lmWa/m2.

The LED device could have the feature that 0.1 nm <= ES <= 5 nm and the PWL is in the range of 300-308 nm. From experiments it followed that a combination of relatively small ES values with a relatively short PWL is favorable over a combination of relatively small ES values with relatively long PWL. On the other hand, from said experiments it followed that a combination of relatively large ES values with a relatively long PWL is favorable over a combination of relatively large ES values with relatively short PWL. Hence, the LED device could have the feature that 10 nm <= ES <= 20 nm and PWL is in the range of 310-315 nm.

The LED device could have the feature that the LED has an emission spectrum with a majority in the UVB wavelength range, such as at least more than 50% or at least 60%

, or which comprises at least 75% UVB radiation, such as 80% UVB radiation, for example 90% or more, in the range of 280-320 nm. The LED device could have the feature that the LED has an emission spectrum comprising at the most 20% UVC radiation, such as at the most 10% UVC radiation, or at the most 5% UVC radiation, such as 3% or less, in the range of 100-280 nm, and/or the LED device could have the feature that the LED has an emission spectrum comprising at the most 40% UVA radiation, such as at the most 30% UVA radiation, or at the most 20% UVA radiation, such as 10% or less, in the range of 320-380 nm. Quite often UV-B emitters also have some undesired emission outside the UV-B range, which is acceptable to some degree, but, in the context of this invention, preferably this undesired emission should be limited as much as possible, with the sum of the amount of UV-A and UV-C being at the most 30%, such as at the most 20%, such as 10% or less.

The LED device could have the feature that the filter is a dichroic filter and 0.1 nm <= ES <= 15 nm. In the context of this invention it is favorable to have an ES that is a small as possible and which ES should not be larger than 30 nm. With dichroic filters a relatively steep ES is attainable, for example down to about ES ~ 0.7 nm, yet such dichroic filters are relatively expensive. Dichroic filters with a relatively steep edge steepness ES, i.e. with ES <= 3 nm, such as ES <= 1 nm, at approximately the desired cut-off wavelength range of 305-315 nm, are available by Semrock, see https://www.semrock.com/filters.aspx.

The LED device could have the feature that the filter is a glass filter and 15 nm < ES <= 30 nm. Typically (colored) glass filters are relatively cheap, yet have an ES that is less steep than the ES that is attainable by dichroic filters. Typically, the attainable ES for glass filters often cannot be smaller than 15 nm. Hence, in the context of this invention, if a relatively cheap filter is desired, it is preferred to have an ES that is a small as possible and which ES should not be larger than 30 nm. An example of a relatively cheap, potentially suitable filter is the N- WGD305nm glass filter of Schott having a cut-off wavelength, CW, of 305 nm and an edge steepness, ES, of 30 nm.

Alternatively to the filter being either only a dichroic filter or only a colored glass filter, a combination of a dichroic filter with a glass filter, either a stacked combination of two separate filters or integrated into a single filter, can be beneficial, for example being relatively cheap. Typically a dichroic filter is much more expensive than a glass filter. By combining the dichroic filter with a glass filter, the dichroic filter can then be made with a relatively low number of layers for being blocking for wavelengths in the CW range. Yet, the relatively low number of layers typically render the dichroic filter to be less blocking, i.e. leaking, in other (UV) wavelength ranges remote from the CW range. As such, such transmission in said other wavelength ranges can be undesired, and then the glass filter can be chosen and used to filter said undesired radiation passing through the dichroic filter.

The LED device could have the feature that the LED is provided with an optical element configured to collimate and/or redirect the UV-B radiation towards a target area. This reduces the risk of unintended exposure to said UV-B radiation of humans and/or materials etc. adjacent or in the neighborhood of said target area, and renders a more efficient use of said radiation for its intended purpose.

The LED device could have the feature that the filter is a phosphor with an excitation in the UVC-UVB range with, excitation absorption edge in the range of 305-315 nm, and an emission for at least 80% in the visible wavelength range. The blocking function of the filter typically is either via reflection and/or absorption of radiation. Alternatively said blocking function could be attained by conversion via a phosphor which has an excitation spectrum in the UVC-UVB range with an absorption edge in the range of 305-315 nm and an emission for at least 80% in the visible wavelength range and less than 1% emission at shorter wavelengths than 350 nm, and preferably essentially only emission, i.e. over 90%, at wavelengths longer than 420 nm to avoid blue hazard. Said absorption edge of the phosphor relates to a relatively high absorption of shorter wavelengths and relatively low absorption (or relatively high transmission) of longer wavelengths, i.e. comparable to the filter characteristics of a long-pass filter. Examples of (potentially) suitable phosphors are GdMgB5O10:Bi,Gd,Tb; Y202S:Tb; Y2W06:Eu; and Srl.95ZnW06:Eu0.05 (both with Eu3+). (see: https://dspace.hbrary.uu.nl/bitstream/handle/1874/16173/blas se_study_gadohnium.pdf/seque nce=l ; https://www.ijitee.org/wp-content/uploads/papers/v9i4/D14120 29420.pdf ; https://www.researchgate.net/figure/Emission-l-ex-14-300-nm- and-excitation-spectra-l-em- 14-450-nm-of-Y-2-WO-6-The-inset_fig3_314266347 ; and https://onlinelibrary.wiley.com/doi/epdf/10.1002/bio.30097sa ml_referrer .)

The LED device could have the feature that the LED comprises a phosphor having an emission of >85% in the wavelength range of 310-314 nm, preferably the phosphor comprises Gadolinium as an activator, more preferably is La3B309: Bi, Gd (= GLBB) or (potentially) GdMgB5O10:Bi,Gd (see https://dspace.library.uu.nl/bitstream/handle/1874/16173/bla sse_study_gadolinium.pdi7seque nce=l). As well-known Gadolinium activated phosphors, like the GLBB phosphor, have a major emission in the 310-314 wavelength range, yet also a small emission in the 304-307 nm wavelength range. When the typical Gadolinium emission spectrum, such as the GLBB spectrum, is combined with, for example a steep dichroic filter with an ES <= 3nm, such as ES = lnm, and a CW at about 308 nm, an attractive combination of emission spectrum and filter characteristics is obtained, as the undesired range of 304-307 nm is essentially filtered out, while the favorable 310-314 nm range is essentially fully transmitted, an SDD of about 0.99 is then attained.

The LED device could have the feature that it comprises a plurality of LEDs for emitting said UV-B radiation. Thus, to reach a maximum allowable intensity for administering UV-B radiation for Vitamin D generating is enabled. Furthermore, it is relatively simply enabled to irradiate a relatively wide target area.

The LED device could have the feature that it comprises at least one further LED emitting visible light during its operation. Thus, operation of the device, i.e. the device being in the on-state, is made visible. Preferably, the visible light is a spot that emits visible light in the same direction as the UV-B radiation, thus facilitating aiming of the UV-B emitting device at a target area. The invention further relates to a method for irradiating a target substance by the LED device according to the invention. The method comprising the steps of:

- setting a Vitamin D dose amount to be dosed to the target substance;

- determining an exposure time based on the Vitamin D dose amount and a specified maximum allowable actinic irradiance; and

- irradiate the target substance for said exposure time.

The method according to the invention has the advantage of enabling administering relatively high amounts of Vitamin D generating radiation to humans while limiting the maximum actinic irradiance to lmWa/m2.

The invention will be further illustrated with reference to the attached drawings, which schematically will show embodiments according to the invention. It will be understood that the invention is not in any way restricted to these specific embodiments.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1 A-C depict action spectra response curves for actinic and vitamin D synthesis, the response ratio Vit D: actinic, and emission curves of resp. an unfiltered UV-B LED and an unfiltered UV-B LED;

Fig. 2A-C depict various contour plots of lamp-filter parameters and lamp- lamp parameters;

Fig. 3 depicts the mutual position of the GLBB emission curve and the curve for the Vit D - Actinic ratio; and

Fig. 4 depicts an embodiment of LED device according to the invention.

DETAILED DESCRIPTION

Fig. 1 A depicts the action spectra response curves for actinic 10 and vitamin D 12 synthesis in the wavelength range of 290-330 nm. Furthermore, Fig. 1 A depicts the ratio curve 14 in the response of Vitamin D:actinic in said wavelength range of 290-330 nm. As shown in Fig. 1A, the response curves of Vitamin D 12 and actinic 10 have a significant overlap. Hence, administering UV-B radiation to humans for generation of Vitamin D involves the risk that too much harmful radiation is simultaneously administered to the human body that might cause actinic reactions. To assess the possibility to administer as much as possible UV-B radiation for Vitamin D generation, while limiting the administering radiation that might cause actinic reaction as much as possible, therefore also the curve 14 of the ratio between Vitamin D response and actinic response is given. This ratio has its highest peak 16, i.e. has a relative value of at least 90% of the top value of 100%, in the wavelength range of about 307-313 nm. It is further shown in Fig. 1 A that the response level of the curve 10 for actinic reaction is relatively low in the wavelength range of 307-313 nm, i.e. ranges from about 4% to < 1% (of maximally 100%), while the response level of Vitamin D in said 307-313 nm range is still substantial, i.e. ranges from about 50% to about 10% (of maximally 100%). This means that an UV-B emitting light source, e.g. UV-B emitting LED, emitting in the UV-B range of about 307-313 nm can be suitably applied with acceptable efficacy for irradiation of humans to stimulate the generation of Vitamin D.

In relation with what is depicted in Fig. 1A, Fig. IB shows an emission curve 18 of an unfiltered, commercially available UV-B emitting LED, having an emission peak 20 at about 308 nm and a FWHM of about 15 nm. To put the UV-B emission of the LED into perspective, Fig. 2B further also shows the response curves for Vitamin D 12 and actinic reaction 10. It is clear from Fig. 2B that the unfiltered emission curve 18 of the unfiltered UV-B emitting LED has a substantial overlap with the actinic response curve 10, in particular for UV-B wavelengths of 304 nm and shorter, for example down to 295 nm. To avoid said substantial overlap, the emission curve 18 of the UV-B emitting LED could be shifted as a whole by about 10 nm to larger wavelengths, hence the UV-B emission would then peak at about 318 nm. This, however, would involve two disadvantages, i.e. the LED device for Vitamin D generation would become too inefficient, and the ratio between Vitamin D: actinic would become less favorable. Therefore, the inventors has the insight to apply a filter instead, which filter would aim to block the undesired part of the mission curve of the UV-B emitting LED, i.e. UV-B wavelengths below 305 nm. Fig. 1C shows the filtered emission curve 22, also referred to as LED device emission curve 22, of the same UV-B emitting LED applied in Fig. IB, but with application of a specified filter having a CW of about 307 nm and an ES of about 2 nm. As visible, the emission curve 22 of the LED device comprises hardly any radiation that causes actinic reactions, but yet still has an LED device emission peak 19 at about 308 nm comprises a substantial portion of radiation that stimulates Vitamin D generation when administered to humans.

Fig. 2A-C depict various contour plots of lamp-filter parameters and lamp- lamp parameters. In Fig. 2A the effect of the emission properties of the UV-B emitting LED, i.e. LED Peak WaveLength, PWL, and LED Full Width at Half Maximum, FWHM, is given on the Vitamin D SDD values. The contours are given for LEDs in combination with a long- pass filter having a Cut-off Wavelength, CW, at 307 nm, and an Edge Steepness, ES, of 1 nm. A long-pass filter blocks shorter wavelengths and transmits longer wavelengths. As shown in Fig. 2A for the specified type of filter the higher SDD values are obtained for LEDs having an emission spectrum with a relatively short PWL, i.e. between 300-311 nm, and a relatively small FWHM, i.e. 10-16 nm. Aa FWHM of 10 nm being taken as a lower limit as it is about the smallest FWHM for commercially available LEDs.

Fig. 2B shows the contour plots of obtainable SDD values by the combined effect of the LED PWL and the filter CW position in the wavelength range of 300-315 nm. The other parameters of the LED and the filter are fixed, i.e. the FWHM of the LED is 15 nm and the ES of the filter is 1 nm. These contour plots show that there is hardly any effect of the PWL position in the 300-315 nm range on the SDD value, but that the effect of the CW position is dominant. Relatively high values of SDD, i.e. > 0.9, are obtained for the filter having a CW in the range of 305-310 nm, yet even an extended range for the CW of 303-313 nm is interesting and provides an interesting increase in SDD values, i.e. SDD > 0.7, over the reference value of SDD = 0.61 of GLBB (as applied without filter).

Fig. 2C shows the contour plots of obtainable SDD values by the combined effect of the LED PWL and the edge steepness, ES, for the LED PWL in the 300-315 nm wavelength range and the ES being in the range of 1-20 nm. The other parameters of the LED and the filter are fixed, i.e. the FWHM of the LED is 15 nm and the CW of the filter is at 307 nm, essentially being (about) the most favorable value following from the contour plot of Fig. 2B. The contour plots in Fig. 2C show that in general the smaller the ES, the higher SDD values are obtained. Typically this effect is attained for ES <= 6 nm. Also, this effect is particular clear for the shorter PWL, i.e. <= 307 nm, in the wavelength range for PWL of 300-315 nm. For PWL >=310 nm, the effect of the ES on the obtainable SDD value is less strong.

Fig. 3 depicts the mutual position of the GLBB emission curve 24 and the curve 14 for the Vit D - Actinic ratio. As shown the Gadolinium Lanthanum Borate: Bismuth phosphor, GLBB, has a main part of its emission spectrum and highest peak 26 in the 310- 313 nm wavelength range, practically coinciding with the top 16 of the Vitamin D: actinic ratio curve 14, i.e. where said ratio is >= 90%. Furthermore, in the 310-313 nm range the Vitamin D response is still substantial, i.e. about 15% (see Fig. 1A-C). Also shown is that the GLBB emission spectrum has some, less favorable, secondary emission 26 in the 304-307 nm wavelength range, yet in said range the ratio is >= 60%. With a filter having the right CW at about 308 nm and an ES of 2nm or less, the GLBB spectrum can be favorably modified, to render the LED device to have an improved emission spectrum with SDD values of over 90. In Fig. 4 an embodiment of LED device according to the invention is depicted. The LED device 100 comprises two UV-B emitting LED as a light source 110 arranged to generate UV-B LED light 122. Although Fig. 4 shows two LEDs to form the light source 110, a single or a plurality of more than two LEDs or LED packages may also be provided. The LED device 100 further comprises an envelope 120 which at least partially encloses the light source 110. In Fig. 4, a thread 125 of the envelope 120 is arranged on the outside of the envelope 120 for connection to a corresponding circumferential thread 131 of a carrier 130 of the LED device 100, which connection provides an efficient, easy and fast assemblage of the LED device 100. Alternatively, the thread 125 may be arranged on an inside of the envelope 120. The carrier 130 is arranged to support the light source 110, and may, for example, be a substrate for the LED light source 110, which typically is mounted on a printed circuit board, PCB 127. The envelop 120 and the PCB together enclose a cavity 121 in which the light source 110 is accommodated. The envelope 120 in Fig. 4 is bulb-shaped from a base portion of the envelope 120 up to a light exit window 128 of the envelope 120, from which light exit window 128 the light from the light source 110 exits the LED device 100. The envelope 120 of the illumination device 100 comprises glass, and also the thread 125 of the envelope 120 comprises glass. The envelope 120 further comprises a coating 126 on the inside of the envelope 120, which coating 126 may be highly specular reflective. Alternatively, the coating 126 may be provided on the outside of the envelope 120, as the transparency of the glass of the envelope 120 allows for the coating 126 to be applied to the exterior of the envelope 120. The envelope thus functions as a reflector by which aiming of the UV-beam is facilitated. At the light exit window 128 a filter 132 is provided, in Fig. 4 the filter 132 is a dichroic filter, but this could alternatively be a colored glass filter. The filter 132 has the correct CW and ES for partly blocking and partly transmitting the desired part of the UV-B spectrum range of the UV radiation generated and emitted by the LEDs 110. By said filter 132 the UV-B LED light 122 is modified into LED device light 124.