Patent Description:
The present invention further relates to a method of manufacturing such a lighting device.

Solid state lighting devices such as lighting devices based on light emitting diodes (LEDs) are rapidly replacing other forms of lighting devices such as incandescent lighting devices, halogen lighting devices, fluorescent lighting devices and the like due to the superior lifetime, robustness and energy efficiency of solid state lighting devices. Notwithstanding the clear advantages of solid state lighting devices over other types of lighting devices, several challenges still need to be overcome for solid state lighting devices to become widely accepted replacements of such other types of lighting devices.

For instance, it is not straightforward to produce a solid state lighting device that emulates the appearance and luminous output of traditional lighting devices, such as filament-based lighting devices. To this end, solid state lighting devices have been produced in which a plurality of solid state lighting elements, e.g. LEDs, are mounted on an elongate carrier mimicking a filament. For example, <CIT> discloses a linear LED light source comprising a plurality of LED units conductively connected to metallic support frames, wherein the metallic support frames are configured to serve as supply conductors through which electric power for driving the LED units can be fed to the LED units.

Such filament assemblies may be coated with a phosphor containing resin coating, such that the light produced by the LEDs, e.g. blue light, may be spectrally converted by the phosphor particles in the resin, e.g. to produce white light. As is well-known per se, the color temperature of the thus produced white light may be controlled by the composition and amount of phosphor in the resin. In such filament assemblies, the elongate carrier is typically made of a light transmissive material such that light can escape the filament from virtually any part of the resin surface, thereby emulating the full <NUM>° luminous distribution of a traditional filament-based incandescent lighting device. However, even though such a luminous distribution can be reasonably successfully emulated, it remains difficult to achieve homogeneity in the spectral composition of the light emitted from the surface of the resin enveloping the elongate carrier carrying the solid state lighting elements. Such differences can be quite noticeable, for instance when the light is projected onto a surface, or to an observer in motion, where the observer receives a luminous output emitted from different regions of the resin surface as the observer moves relative to the lighting device. In particular where such a luminous output is projected onto a surface such as a work surface, the spectral variations in the projected luminous output can become rather annoying to a person requiring adequate lighting to perform a particular task.

One particular solution to this problem would be to provide a diffusive coating on the light transmissive housing of the filament assembly. For instance, <CIT> discloses an LED lamp comprising a substrate, an LED light source in optical communication with its housing, a diffusive coating applied to the housing, and a luminescent coating disposed at least partially on the diffusive coating. The luminescent coating is between the LED light source and the diffusive coating. The luminescent coating has a composition selected to achieve a particular color temperature according to a characteristic of the LED light source. However, such a solution significantly reduces the luminous efficiency of the lighting device and introduces luminescent power losses. This for example is undesirable from the perspective of achieving compliance with energy efficiency regulations, which typically require a minimum amount of lumen to be produced per Watt. For instance, for a (tubular) lighting device to achieve a Class A certification in the European Union, it needs to achieve a luminous output of <NUM> lm/W or more, which is difficult to achieve especially when suffering light losses at the transmissive housing, e.g. through the presence of a diffusive layer or the like. Document <CIT> discloses an LED filament inside a transmissive housing, said LED filament comprising a plurality of LED chips which are electrically connected with each other, and a light conversion layer which covers the LED chips.

The present invention seeks to provide a lighting device comprising a transmissive housing and a filament within said transmissive housing having a transmissive substrate carrying a plurality of solid state lighting elements that addresses at least some of the aforementioned challenges.

The present invention further seeks to provide a method of manufacturing such a lighting device.

According to examples in accordance with an aspect of the invention, there is provided a lighting device as recited in claim <NUM>. The present invention is based on the insight that variations in the spectral composition of the luminous output of the filament with the solid state lighting elements are caused by variations in the thickness of the phosphor-containing resin coating enveloping the transmissive substrate carrying the solid state lighting elements, e.g. LEDs. Therefore, by providing a further phosphor layer on the housing for each region with a reduced thickness in the phosphor-containing resin coating such that such a further phosphor is optically aligned with such a region of the phosphor-containing resin coating having a reduced thickness, the spectral uniformity of the luminous output of the lighting device can be improved, whilst the remainder of the housing of the filament can remain transmissive, thereby improving the luminous efficiency of the lighting device as light losses, such as caused by a diffusive layer, are avoided due to the absence of such a diffusive layer covering an entire surface of the housing.

For example, the transmissive substrate may comprise a mounting surface carrying at least some of said solid state lighting elements and a pair of side surfaces at opposite sides of the mounting surface, each of said side surfaces being covered by one of said regions of the resin coating. The phosphor-containing resin coating may have a reduced thickness over the side surfaces of the transmissive substrate, such that light emitted through the regions of the phosphor-containing resin coating over these side surfaces will have a different spectral composition than light emitted through other regions of the phosphor-containing resin coating, e.g. regions covering the mounting surface of the transmissive substrate, which differences is spectral composition are compensated for by the additional phosphor layer(s) on the housing as explained above.

The transmissive substrate may contain a single mounting surface, that is, the solid state lighting elements may be mounted on a single surface of the transmissive surface only. In such a scenario, the solid state lighting elements may be dual-emitting such that the solid state lighting elements are arranged to emit light towards the transmissive substrate as well as away from it. This has the advantage that heat management of the filament can be achieved in a reasonably straightforward manner whilst achieving a luminous distribution that closely emulates a full <NUM>° luminous distribution. Alternatively, the transmissive substrate may further comprise a further mounting surface opposing said mounting surface, said side surfaces extending between the mounting surface and the further mounting surface, wherein the solid state lighting elements may be distributed across the mounting surface and the further mounting surface.

The transmissive substrate may be made of any suitable material. In a particular example, the transmissive substrate is a sapphire substrate, but alternative materials such as a transmissive ceramic material or a flexible polymer material such as polyimide are equally feasible.

Each further phosphor layer preferably is arranged on an inner surface of the transmissive housing such as to protect the further phosphor layer from accidental damage, e.g. through scratching or the like. Alternatively, each further phosphor layer may be arranged on an outer surface of the transmissive housing, which has the advantage that the further phosphor layer(s) can be applied to the housing in a particularly straightforward manner.

The transmissive housing may be made of any suitable transmissive material. For example, the transmissive housing may be a transparent housing made of glass or plastic. In a particular set of examples, the transmissive housing is shaped as a tubular body, e.g. the lighting device may be a tubular LED (TLED) device producing a luminous output of at least <NUM> lm/W. In this particular set of examples, the transmissive housing may comprise a pair of said further phosphor layers each extending along said tubular body and having a radial width in a range of <NUM>-<NUM>° such that <NUM>-<NUM>% of the surface of the transmissive housing remains uncovered by the further phosphor layers, which ensures that the lighting device can achieve excellent luminous output efficiency.

The lighting device may comprise a plurality of said filaments extending within said tubular body; and a pair of electrode rails extending within said tubular body, wherein each filament is conductively connected to each of said electrode rails by respective support arms extending between one of said electrode rails and said filament. This allows for the filaments to be connected in parallel, which limits the resistance along the chain of filaments and therefore ensures that the filaments can be driven by a driver that is compact enough such that it can fit within the lighting device, e.g. in an end cap of the lighting device.

Where the tubular body is a plastic body, the plastic body may comprise a pair of channels in its inner surface, each of said channels extending along said tubular body and housing one of said electrode rails. This ensures that the filament assembly can be securely mounted within the tubular body, thus providing a particularly robust lighting device.

Alternatively, the filament may extend along said tubular body, i.e. substantially along its entire length, in which case the lighting device may further comprise a driver at a first end of said tubular body, said driver having a first connection to a first terminal portion of said filament proximal to said first end and a second connection to a second terminal portion of said filament proximal to a second end of said tubular body opposing said first end in order to electrically connect the filament to the driver. In this case, the lighting device may further comprise a support structure extending from said second terminal portion of the filament to said second end of the tubular body in order to secure the filament within said tubular body.

According to examples in accordance with another aspect of the invention, there is provided a method of manufacturing a lighting device as recited in claim <NUM>.

A lighting device manufactured in this manner benefits from having an improved luminous efficiency owing to the use of a transmissive housing, whilst variations in the spectral output of the luminous device are reduced due to the presence of one or more further phosphor layers on the transmissive housing to convert light produced by the solid state lighting elements that is not converted by the phosphor-containing resin layer enveloping the filament.

Preferably, the transmissive housing is shaped as a tubular body and each further phosphor layer extends along said tubular body, the method further comprising forming a filament assembly by mounting a plurality of said filaments to a pair of electrode rails such that the corresponding regions of the respective filament leaking light produced by said solid state lighting elements that is unconverted by said phosphor are aligned with each other, each filament being conductively connected to each of said electrode rails by respective support arms extending between one of said electrode rails and said filament; and wherein mounting the filament in said transmissive housing comprises mounting said filament assembly in the tubular body such that said electrode rails extend along said tubular body. This has the advantage that the respective filaments can be driven in parallel, which makes it easier to achieve a desirable luminous output, e.g. a homogeneous luminous output, along the tubular housing, whilst securing the electrode rails in or against the tubular housing furthermore provides a particularly robust filament assembly within the tubular body.

Alternatively, the transmissive housing is shaped as a tubular body and each further phosphor layer extends along said tubular body, the method further comprising forming a first connection between a driver and a first terminal portion of said filament; forming a second connection between the driver and a second terminal portion of said filament opposing said first terminal portion; and wherein mounting the filament in said transmissive housing comprises mounting said filament such that said driver is located at a first end of said tubular body and said second terminal portion of said filament is located proximal to a second end of said tubular body opposing said first end, the method further comprising mounting a support structure between said second terminal portion of said filament and said second end of the tubular body. In this manner, a robust assembly of a single, elongate, filament within the tubular housing can be achieved.

<FIG> schematically depicts an example of a filament <NUM> that may be used in various embodiments of the lighting device of the present invention. The filament <NUM> comprises a transmissive substrate <NUM> for carrying one or more solid state lighting elements <NUM>. The transmissive substrate <NUM> for example may be a transparent sapphire substrate or a transmissive ceramic substrate where a relatively rigid filament <NUM> is required, or may be a polymer substrate such as a polyimide substrate if the filament <NUM> is required to be more flexible. The solid state lighting elements <NUM> may take any suitable shape or form, e.g. one or more LEDs such as blue light emitting LEDs. The solid state lighting elements <NUM> may be mounted on a single major surface <NUM> of the transmissive substrate <NUM>, in which case the solid state lighting elements <NUM> preferably emit light from at least two emission surfaces, e.g. towards the mounting surface <NUM> of the transmissive substrate <NUM> and away from this surface such as to provide a filament <NUM> that exhibits a luminous distribution emulating that of the filament of an incandescent light bulb. Alternatively, the further major surface <NUM> of the transmissive substrate <NUM> opposing the major surface <NUM> may also carry one or more solid state lighting elements <NUM> (not shown) in order to achieve the desired luminous distribution of the filament <NUM>. In this case, the solid state lighting elements <NUM> may emit light from a single surface only, e.g. away from the surface on which they are mounted.

The transmissive carrier <NUM> carrying the solid state lighting elements <NUM> is typically enveloped in a resin <NUM> containing a phosphor, e.g. in the form of particles suspended in the resin, which phosphor converts the light emitted by the solid state lighting elements <NUM> into light having a different spectral composition. For example, the phosphor may convert the light emitted by the solid state lighting elements <NUM> into white light having a particular color temperature, which color temperature may be controlled by the chemical composition of the phosphor. Any suitable phosphor may be used for this purpose. For instance, one or more phosphors that convert blue light into yellow/green light, e.g. garnets such as YAGaG:Ce, LuAGaG:Ce, or A<NUM>B<NUM>O<NUM>:Ce in which A represents a chemical element such as Y, Lu or La and B represents a chemical element such as Al, Ga, or Fe, and phosphors that convert blue light into orange/red light, e.g. alumonitridosilicate or calsin phosphors such as CaAlSiN<NUM>:Eu phosphors for example, or nitridosilicates, may be used for this purpose, although it should be understood that the invention is not limited to these example phosphors and phosphors with any suitable composition may be used. The resin may be any suitable transmissive resin, preferably a transparent resin. Examples of suitable resins include ethyl type and phenyl type silicones. Other suitable resins will be immediately apparent to the skilled person.

A problem associated with the enveloping of the transmissive substrate <NUM> with the resin coating <NUM> is that the thickness of the resin coating <NUM> around the transmissive substrate <NUM> is not constant. As shown in <FIG>, the resin coating <NUM> typically has a thickness r<NUM> and r<NUM> over the major surfaces <NUM> and <NUM> respectively, whereas the resin coating <NUM> has a significantly smaller thickness r<NUM> over the side surfaces <NUM> of the transmissive substrate <NUM> that extend between the opposing major surfaces <NUM> and <NUM>. This reduced thickness r<NUM>, which typically is a function of the thickness d of the transmissive substrate <NUM>, is caused by the fact that when coating the transmissive substrate <NUM> carrying the solid state lighting elements <NUM>, it is difficult to achieve the same thickness of the resin coating <NUM> over the side surfaces <NUM> compared to the major surfaces <NUM>, <NUM>. Consequently, light emitted by the solid state lighting elements <NUM> travelling through the transmissive substrate <NUM> and exiting this substrate through the side surfaces <NUM> will typically exit the filament <NUM> through a window <NUM> defined by the dashed lines extending from the side surfaces <NUM>. As this light passes through a relatively thin layer of the resin coating <NUM>, there is insufficient phosphor in the optical path of the light rays through this part of the resin coating <NUM>, such that at least some of the light rays escaping from the filament <NUM> in this manner will be unconverted, i.e. have its original spectral composition as produced by the solid state lighting elements <NUM>. This therefore can cause spatial variations in the spectral composition of the luminous output produced by the filament <NUM>.

Such spatial variations may be compensated by diffusing the luminous output, e.g. by fitting the filament <NUM> in a diffusive housing. However, this causes light losses and therefore compromises the optical efficiency of the lighting device including the filament <NUM>. On the other hand, when using a transmissive housing with the filament <NUM>, the optical efficiency of the lighting device is improved at the cost of producing a luminous output having spatial variations in its spectral composition. In accordance with the teachings of the present invention, a lighting device <NUM> is provided, such as the lighting device <NUM> schematically depicted in <FIG>, in which the housing <NUM> is made of an optically transparent material such as glass or an optically transparent polymer, which housing <NUM> carries a further phosphor layer <NUM> on one of its surfaces, i.e. on an inner surface or an outer surface of the housing <NUM>, which further phosphor layer <NUM> is optically aligned with the emission windows <NUM> such that unconverted light escaping the filament <NUM> fitted in the fitting or end cap <NUM> of the lighting device <NUM> is converted by the further phosphor layer <NUM> to improve the homogeneity in the spectral composition of the light emitted from the lighting device <NUM> and to improve the perceived optical efficiency of the lighting device <NUM> as the human eye is less sensitive to blue light, e.g. light having a wavelength of <NUM>-<NUM> than to yellow-green light, e.g. light having a wavelength of <NUM>-<NUM> such that the luminous flux as perceived by the human eye sensitivity is increased by the further phosphor layer <NUM>.

The further phosphor layer <NUM> preferably has a minimal width W to ensure that large areas of the housing <NUM> remain transparent such that the optical efficiency of the lighting device <NUM> is not overly compromised by the addition of the further phosphor layer <NUM>. The width W preferably is chosen such that only light emitted from the windows <NUM> is incident on the further phosphor layer <NUM> to ensure its width remains minimal. The further phosphor layer <NUM> may have any suitable chemical composition. In a particular example, the phosphor in the further phosphor layer <NUM> has the same chemical composition as the phosphor in the resin coating <NUM>, although it is also feasible for the phosphor in the further phosphor layer <NUM> to have a different chemical composition to the phosphor in the resin coating <NUM>. As will be readily understood by the skilled person, the thickness of the further phosphor layer <NUM> preferably should be chosen such that all unconverted light emitted by the filament <NUM> is converted when passing through the further phosphor layer <NUM>. The further phosphor layer <NUM> may be a resin layer in which phosphor particles are suspended. The resin layer may have the same composition or a different composition as the resin coating <NUM>. Of course, other carrier materials for the phosphor in the further phosphor layer <NUM> may also be contemplated.

A particular class of lighting devices that may benefit from the teachings of the present invention is tubular light devices. <FIG> schematically depicts such lighting device <NUM> in which the housing <NUM> has a tubular body <NUM>, with the filament <NUM> (not shown) extending in the elongation direction of the tubular body <NUM>. In such a scenario, the further phosphor <NUM> typically also extends in the elongation direction of the tubular body <NUM>, e.g. on an inner surface or an outer surface of the tubular body <NUM>, which tubular body <NUM> may carry a plurality of such further phosphor layers <NUM>, typically one further phosphor layer <NUM> for each window <NUM> of the filament <NUM>. For example, where the filament <NUM> comprises two such windows <NUM>, as for instance is the case when using a rectangular transmissive substrate <NUM>, two further phosphor layers <NUM> will extend along the elongation direction of the tubular body <NUM> such that each window <NUM> is optically aligned with one of the further phosphor layers <NUM>. The use of a filament <NUM> in such a tubular lighting device <NUM> ensures that the lighting device can produce a <NUM>° luminous distribution, contrary to tubular lighting devices deploying solid state lighting elements where the solid state lighting elements are mounted on a carrier that itself is mounted on part of the surface of the tubular body <NUM> such that this part of the tubular body <NUM> is prevented from transmitting light generated by the solid state lighting elements. In order to make such a luminous distribution more omnidirectional, the tubular body <NUM> typically carries a diffusive layer or the like, but this reduces the optical efficiency of such a tubular lighting device as previously explained.

<FIG> schematically depicts an example of how the filament <NUM> may be mounted in the transmissive housing <NUM>. The transmissive housing <NUM> may comprise a pair of channels <NUM> into which electrode rails <NUM>, <NUM> may be mounted, which electrode rails are conductively coupled to the filament <NUM> through support arms <NUM>, <NUM>, e.g. support wires or the like, that suspend the filament <NUM> within the transmissive housing <NUM>. For example, the respective electrode rails <NUM>, <NUM> may be slid into the channels <NUM> when mounting the filament <NUM> within the tubular body <NUM>, i.e. within the transmissive housing <NUM>. This is more clearly shown in <FIG>, which shows the channels <NUM> to comprise a slot <NUM> through which the support arms <NUM>, <NUM> can extend from the electrode rails <NUM>, <NUM> embedded in the channels <NUM> to the filament <NUM>. The channels <NUM> may be formed in the transmissive housing <NUM> in any suitable manner. For example, where the transmissive housing <NUM> comprises a plastic tubular body <NUM>, the channels <NUM> may be formed in the plastic tubular body <NUM> when molding the plastic tubular body <NUM>.

Of course, the electrode rails <NUM>, <NUM> may be mounted within the tubular body <NUM> in any suitable manner. For example, where such channels <NUM> are more difficult to achieve, e.g. in case of a glass tubular body <NUM>, the electrode rails <NUM>, <NUM> may be adhered to the inner surface of the tubular body <NUM> instead. This for instance maybe achieved using a UV-activated adhesive or any other suitable type of adhesive. In case of a UV-activated adhesive, the electrode rails <NUM>, <NUM> may be coated or with such an adhesive and positioned within the tubular body <NUM> after which the tubular body <NUM> is exposed to UV radiation in order to activate the adhesive and secure the electrode rails <NUM>, <NUM> against the tubular body <NUM>. Alternatively, at least part of the inner surface of the tubular body <NUM> may be coated with such an adhesive and activated with UV light once the electrode rails <NUM>, <NUM> have been positioned within the tubular body <NUM>. Other suitable ways of securing the electrode rails <NUM>, <NUM> against the tubular body <NUM> will be apparent to the skilled person.

The filament <NUM> in some examples may consist of a plurality of filaments <NUM> as schematically depicted in <FIG> and <FIG>, in which each filament <NUM> is connected to the electrode rails <NUM>, <NUM> through respective support arms <NUM>, <NUM>. For example, each filament <NUM> may extend in between a pair of support arms <NUM>, <NUM>. This has a number of advantages. Firstly, it allows for the respective filaments <NUM> to be connected to the electrode rails <NUM>, <NUM> in parallel, thus lowering the resistance encountered when driving the solid state lighting elements <NUM> along the respective filaments <NUM>. Consequently, the requirements for the driver of the solid state lighting elements <NUM> are less demanding, for instance because of a smaller voltage drop across the solid state lighting elements <NUM> extending along the tubular body <NUM> due to this parallel connection of the filaments <NUM>. This means that the driver can be smaller, which makes it easier to fit the driver in an end cap of the tubular body <NUM>. Secondly, using multiple smaller filaments <NUM> rather than a single filament <NUM> extending along the entire tubular body <NUM> makes it easier to achieve a robust lighting device <NUM>.

The further phosphor layers <NUM> are optically aligned with the windows <NUM> at the side surfaces of the transmissive carrier <NUM> within the filament <NUM>. Although the further phosphor layers <NUM> are shown on the outer surface of the transmissive housing <NUM>, it is reiterated that the further phosphor layers <NUM> may also be located on the inner surface of the transmissive housing <NUM>, where they are less prone to accidental damage, e.g. through scratching of the outer surface of the transmissive housing <NUM> although where such accidental damage is of limited concern, the positioning of the further phosphor layers <NUM> on the outer surface of the transmissive housing <NUM> may be more cost-effective. In a particular example, each further phosphor layer <NUM> covers a portion of the tubular body <NUM> having a radial width in a range of <NUM>-<NUM>°. For example, <FIG> schematically depicts a cross-sectional view of a tubular lighting device <NUM> in which the radial width of the further phosphor layer <NUM> is <NUM>°, as indicated by the pair of <NUM>° sections on either side of the horizontal plane dissecting the further phosphor layer <NUM>, whereas in <FIG> each further phosphor layer <NUM> has a radial width of <NUM>°. As previously explained, the (radial) width W of each further phosphor layer <NUM> is typically chosen such that all unconverted light emitted through the windows <NUM> facing the side surfaces of the transmissive substrate <NUM> within the filament <NUM> is incident on such a further phosphor layer <NUM>, with the actual width W being a function of the dimensions and positioning of the filament <NUM> within the transmissive housing <NUM>, such as the thickness d of the transmissive substrate <NUM>.

<FIG> schematically depicts an exploded view of an example tubular lighting device <NUM> according to the present invention. The one or more filaments <NUM> typically extend between opposing end caps <NUM> that fit onto the opposing ends <NUM>, <NUM> of the transmissive, or more preferable transparent, tubular body <NUM>, with at least one of the end caps <NUM> housing a driver arrangement <NUM> for the one or more filaments <NUM>. Preferably, as previously explained, where multiple filaments <NUM> are present within the transparent tubular housing <NUM>, the filaments <NUM> are connected in parallel to electrodes rails <NUM>, <NUM> via support arms <NUM>, <NUM>, which electrode rails <NUM>, <NUM> may be fitted within channels <NUM> inside the tubular body <NUM>. This reduces the voltage that the driver arrangement <NUM> has to provide owing to a smaller voltage drop across the solid state lighting elements <NUM> on the filaments <NUM> along the tubular body <NUM>, such that the driver arrangement <NUM> can be kept compact, making it easier to fit the driver arrangement <NUM> into an end cap <NUM>. It is furthermore reiterated that although the further phosphor layer <NUM> is shown on the outer surface of the tubular body <NUM>, it is equally feasible for the further phosphor layer <NUM> to be situated on the inner surface of the tubular body <NUM>.

<FIG> schematically depicts an alternative example of a tubular lighting device <NUM> of the present invention, in which a single filament <NUM> extends along the tubular body <NUM> such that a first end <NUM> of the single filament <NUM> is arranged proximal to the end portion <NUM> of the tubular body <NUM> and the opposite second end <NUM> of the single filament <NUM> is arranged proximal to the opposite end portion <NUM> of the tubular body <NUM>. The filament <NUM> may be supported within the tubular body <NUM> in any suitable manner. For example, the filament <NUM> may be wedged in between the opposing end caps <NUM>, or the filament <NUM> may be supported by the driver arrangement <NUM> at the first end <NUM> of the tubular body <NUM> and by a support member <NUM> extending from the end portion <NUM> of the filament and the end cap <NUM> at the opposing end <NUM> of the tubular body <NUM>.

The driver arrangement <NUM> may be mounted in any suitable location, e.g. within the end cap <NUM> at the end portion <NUM> of the tubular body <NUM>. In this example, the driver arrangement <NUM> is electrically connected to the filament <NUM> through a first connection <NUM> at the first end <NUM> of the filament <NUM> and through a second connection <NUM> that extends from the driver arrangement <NUM> to the second end <NUM> of the filament <NUM>. The second connection <NUM> may be a separate wire or the like or may be a conductive track running over the transmissive substrate <NUM> of the filament <NUM>, e.g. over a side surface <NUM> or the major surface <NUM> opposing the mounting surface <NUM> carrying the solid state lighting elements <NUM>. Alternatively, the conductive track may run over the inner surface of the tubular body <NUM>, in which case the conductive track preferably is made of an optically transparent conductive material such as Indium Tin Oxide such that the conductive track does not interfere with the luminous distribution of the tubular lighting device <NUM>.

The lighting device <NUM> according to the embodiments of the present invention may be manufactured in accordance with a manufacturing method <NUM>, a flow chart of which is shown in <FIG>. The method <NUM> starts in operation <NUM> with the provision of a transmissive substrate <NUM>, e.g. a transparent sapphire substrate, a transmissive white ceramic substrate, a polymer substrate such as a polyimide substrate or the like, onto which a plurality of solid state lighting elements <NUM> are mounted in operation <NUM> to produce a filament <NUM>. The solid state lighting elements <NUM> may be mounted onto the transmissive substrate <NUM> in any suitable manner, and as such mounting techniques are well-known per se, this will not be explained in further detail for the sake of brevity only.

Next, the filament <NUM> is enveloped with a phosphor containing resin coating <NUM> in operation <NUM>, e.g. by dipping the filament <NUM> in a bath containing said coating or in any other suitable manner. As explained in more detail previously, the resin coating <NUM> will typically include at least one region <NUM>, e.g. two regions or windows <NUM> facing the side walls <NUM> of the transmissive substrate <NUM>, that will leak light produced by the solid state lighting elements <NUM> that is unconverted by the phosphor in the resin coating <NUM> when the filament <NUM> is in use. A transmissive housing <NUM>, e.g. a glass housing or plastic housing, which may take the shape of a tubular body <NUM> in preferred examples, is provided in operation <NUM> into which the filament <NUM> is to be mounted.

A further phosphor layer <NUM> is formed on part of the transmissive housing <NUM> in operation <NUM> such that each of the regions or windows <NUM> of the resin coating <NUM> can be optically aligned with a further phosphor layer <NUM>. For instance, in case of a bulbous transmissive housing <NUM>, a single further phosphor layer <NUM> may be formed extending across the bulbous transmissive housing <NUM> such that an opposing pair of regions or windows <NUM> of the resin coating <NUM> may be optically aligned with different section of the single further phosphor layer <NUM>. Alternatively, in case of the tubular transmissive housing <NUM>, a pair of further phosphor layers <NUM> may be formed extending along the elongation direction of the tubular transmissive housing <NUM> such that each of the regions or windows <NUM> of the resin coating <NUM> may be optically aligned with one of the further phosphor layers <NUM>. As will be understood from the foregoing, the one or more further phosphor layers <NUM> are dimensioned based on the dimensions of the filament <NUM>, e.g. the thickness d of the transmissive substrate <NUM>, and the positioning of the filament <NUM> within the transmissive housing <NUM>. The further phosphor layer <NUM> may be formed on a section of an inner or outer surface of the transmissive housing <NUM> in any suitable manner, e.g. using conventional masking techniques. For example, the tubular body <NUM> may be partially filled with a D-shaped plug leaving exposed the section on which the further phosphor layer <NUM> is to be formed. The exposed section is subsequently filled with a liquid resin containing the further phosphor(s) until this liquid reaches the plug, after which the resin is cured (e.g. using a hot air stream or UV light, depending on the type of resin), and the plug is removed from the tubular body <NUM>, such as disclosed in <CIT> for example.

Finally, the filament <NUM> is mounted in the transmissive housing <NUM> in operation <NUM> before the method <NUM> terminates in operation <NUM>. In operation <NUM>, the filament <NUM> may be mounted in the transmissive housing <NUM> by aligning each of the regions <NUM> of the resin coating with a further phosphor layer <NUM> on the transmissive housing <NUM> such that each further phosphor layer <NUM> is optically aligned with one or more of the regions <NUM> of the resin coating <NUM> such that the further phosphor layer <NUM> is arranged to receive said unconverted light leaking from the region <NUM> of the resin coating <NUM>. The filament <NUM> may be supported by the end cap <NUM> and extend from this end cap <NUM> into the transmissive housing <NUM> in case of a bulbous transmissive housing <NUM>.

Where the transmissive housing <NUM> is shaped as a tubular body <NUM> such that each further phosphor layer <NUM> extends along the tubular body, the method <NUM> may further comprise forming a filament assembly by mounting a plurality of the filaments <NUM> onto a pair of electrode rails <NUM>, <NUM> such that the corresponding regions <NUM> of the respective filament leaking light produced by the solid state lighting elements <NUM> that is unconverted by said phosphor are aligned with each other. Each filament <NUM> in this filament arrangement may be conductively connected to each of the electrode rails <NUM>, <NUM> by respective support arms <NUM>, <NUM> extending between the electrode rails <NUM>, <NUM> and the filament <NUM>. In this case, operation <NUM> further comprises mounting the filament assembly in the tubular body <NUM> such that the electrode rails <NUM>, <NUM> extend along the tubular body, e.g. by sliding the electrode rails <NUM>, <NUM> in the channels <NUM> or by adhering the electrode rails <NUM>, <NUM> to the inner surface of the tubular body <NUM> once the electrode rails <NUM>, <NUM> are positioned within the tubular body <NUM>.

Alternatively, where a single filament <NUM> is to be mounted in such a tubular body <NUM>, e.g. a filament <NUM> substantially extending along the length of the tubular body <NUM>, operation <NUM> of the method <NUM> may further comprise forming a first connection <NUM> between a driver <NUM> and a first terminal portion <NUM> of the filament <NUM> and forming a second connection <NUM> between the driver <NUM> and a second terminal portion <NUM> of the filament <NUM> opposing its first terminal portion <NUM>. In this case, mounting the filament <NUM> in the tubular body <NUM> may further comprise mounting the filament <NUM> such that the driver <NUM> is located at a first end <NUM> of the tubular body <NUM> and the second terminal portion <NUM> of the filament <NUM> is located proximal to a second end <NUM> of the tubular body <NUM> opposing its first end <NUM> and mounting a support structure <NUM> between the second terminal portion <NUM> of the filament <NUM> and the second end <NUM> of the tubular body <NUM>, e.g. between the second terminal portion <NUM> of the filament <NUM> and an end cap <NUM> at the second end <NUM> of the tubular body <NUM> such as to stabilize the filament <NUM> within the tubular body <NUM>.

Claim 1:
A lighting device (<NUM>) comprising:
a transmissive housing (<NUM>);
a filament (<NUM>) within said transmissive housing, said filament comprising a transmissive substrate (<NUM>) carrying a plurality of solid state lighting elements (<NUM>); and
a phosphor containing resin coating (<NUM>) enveloping said filament, said resin coating including at least one region (<NUM>), with a reduced thickness, leaking light produced by said solid state lighting elements that is unconverted by said phosphor;
characterised in that
the transmissive housing carries a further phosphor layer (<NUM>) covering a part of the transmissive housing and being optically aligned with each of said regions of the resin coating with the reduced thickness, each of said further phosphor layers being arranged to receive said unconverted light leaking from at least one of said regions, whilst the other part of the housing remains transmissive.