FIELD OF THE INVENTION

The invention relates to a luminescent lamp that is dimmable.

BACKGROUND OF THE INVENTION It is known that fluorescent lamps, herein given the more general term luminescent lamps, are not (easily) dimmable. Whereas GLS (global light source) or incandescent lamps, such as halogen lamps, are relatively easily tuneable, thereby substantially following the black body line / black body locus (BBL), luminescent lamps do not have this ability. US5592052 for instance describes a fluorescent lamp having at least two phosphor coatings on the surface of the sealed lamp bulb, typically an inner surface. There is a variable driving means which preferentially activates one phosphor and not the other phosphors, at one arrangement or setting or configuration of the driving means, while at another arrangement or setting or configuration of the driving means it activates in addition a different or several different phosphors. Each phosphor may be a blend of phosphors and the phosphors and/or blends may be overcoated upon one another forming multiple layers or all mixed together and applied as a one layer coated on the lamp surface. The known lamp uses standard fabricating techniques and materials, yet allows the user to change the colour temperature of the lamp by controlling parameters of the electrical driving signal, that is the spectrum and quantity of light emitted are changed in response to the changed driving signal so that the user can arrange the light output to be more or less blue or red or to balance the longer wavelengths perceived against the shorter wavelengths perceived. For instance, the first phosphor is excited by the 254 nanometer radiation from mercury and the second phosphor is substantially unexcited by the 254 nanometer radiation, and the second phosphor is excited by the 330 to 440 nanometer radiation from mercury and the first phosphor is substantially unexcited by the 330 to 440 radiation. In another embodiment, there are three layers of phosphors, the first layer responsive to 185 nm radiation, the second layer responsive to 365 nm radiation, and the third layer responsive to 254 nm radiation. Further, WO0031207 for instance describes that in a luminescent screen of a fluorescent lamp an organic europium compound is incorporated for converting mercury radiation between 350 nm and 450 nm into red light. The organic europium compound is diluted in an organic polymer. When the fluorescent lamp is dimmed its colour point shifts into the red similar to incandescent lamps and not into the blue.

SUMMARY OF THE INVENTION

Hence there has been an interest in generating luminescent lamps that are dimmable, preferably dimmable in such a waythat, when the lamp is dimmed, a "natural" colour change may be perceived from cool light to warm light (i.e. from light with a higher correlated colour temperature (T _c ) to light with a lower correlated colour temperature).

A disadvantage of prior art solutions may be that less efficient or less stable luminescent materials have to be applied. Other disadvantages may be that the luminescent lamp cannot be operated as efficientlyas possible. Hence an aspect of the invention is to provide an alternative luminescent lamp, which preferably further obviates one or more of above-described drawbacks.

The invention uses in an embodiment the property of "capacity- induced coupling" of power into, and in fact also out of, the discharge tube of a luminescent lamp, such as a compact fluorescent lamp (CFL). Amongst others, by this capacity-induced incoupling and/or capacityinduced outcoupling one may activate different segments of the tube. By assigning different attributes (such as for instance spectral distribution or colour point of the luminescence of the luminescent material) to the different segments, the properties of the "bulb" can be changed depending on activation of the segments and/or during dimming. By using for instance different luminescent materials (coatings) for the segments, like one particular luminescent material/coating for one segment (creating for instance cool light / high T _c ) and another luminescent material/coating for the other segment of the tube (creating warm light / low T _c ) it is possible to follow the BBL (Black Body Line or Black Body Locus) when dimming the lamp. In this way it is possible to follow the Black Body Line when dimming the light and to create the same dimming behaviour as a GLS bulb. When using traditional technology it is not possible to dim a single-tube CFL bulb and follow the BBL at the same time. Other attributes or states can be created also, such as two or more colours, or light that does / does not contain a spectrum to suppress melatonin. Therefore, the invention provides a luminescent lamp comprising a tubular discharge vessel (herein also indicated as "tube" or "discharge vessel" or "discharge tube") enclosing, in a gastight manner, a discharge space containing mercury and an inert gas; a first electrode and a second electrode, arranged to receive, during operation of the luminescent lamp, power from a power source and maintaining a discharge within the discharge vessel; a controller, arranged to control, during operation of the luminescent lamp, the power provided from the power source to the electrodes of the tubular discharge vessel (such as between a maximum value, a minimum value, and at least an intermediate value (including a plurality of intermediate values)); a luminescent material coating, comprising luminescent material, having a coating surface with a coating surface area, the luminescent material coating being attached to the discharge vessel, wherein the luminescent material of the luminescent material coating is able to emit light with a certain colour upon excitation by UV light from the discharge; an optional optical filter, arranged downstream from the tubular discharge vessel and the said luminescent material coating, having a transmission for light emitted by luminescent material, and having an optical filter area; wherein one or more of (a) the luminescent material is arranged to show different luminescent colours over the coating surface area and (b) the optional optical filter is arranged to show different wavelength dependencies for the transmission over the optical filter area, and wherein the luminescent lamp is arranged to generate light, during its operation, downstream of the tubular discharge vessel, the luminescent material coating, and the optional optical filter. By varying the power coupled into the discharge tube, the extent and/or location of the discharge varies within the discharge tube. Since different types of luminescent material (including luminescent material mixtures) and/or different types of filter are used (i.e. different attributes) along the discharge vessel, by varying the power coupled into the discharge tube, the different luminescent materials are addressed (with concomitant different luminescences), respectively, and/or the different optical filters are addressed, respectively.

In a specific embodiment, the controller is arranged to control one or more of a voltage amplitude and power (i.e. voltage) frequency of the power provided to the electrodes by the power source. A way of dimming is frequency dimming, i.e. the frequency of the voltage over the electrodes is varied (or more precisely, increased). By increasing the frequency (relative to a minimum value), more capacity outcoupling to the environment of the discharge tube will take place. In this way, the extent of the discharge within the discharge tube reduces, and in part of the discharge tube the discharge may extinguish. When the frequency is reduced (relative to a maximum value), the outcoupling to the environment may be reduced, and the extent of the discharge may increase. Therefore, in an embodiment, the spectral distribution of the light generated by the luminescent lamp may be varied by varying the frequency of the voltage coupled in. As will be clear to a person skilled in the art, the lamp is driven AC. Preferably, the controller is arranged to control the power frequency in a range of 50 kHz - 500 kHz, especially in a range of about 100 kHz - 300 kHz.

Alternatively or additionally, the amplitude of the signal may be reduced or increased. A smaller voltage (relative to a maximum value) may reduce the extent of the discharge in the discharge tube and a larger voltage (relative to a minimum value), may increase the extent of the discharge. Therefore, in an embodiment, the spectral distribution of the light generated by the luminescent lamp may be varied by varying the amplitude of the voltage coupled in. Preferably, the controller is arranged to control the voltage amplitude in a range of 0.1 kV - 10 kV, especially in a range of 0.5 kV - 5 kV. This may especially apply for external electrodes (see also below); for internal electrodes, the controller may be arranged to control the voltage amplitude in a range of about 50 V - 200 V. As mentioned above, in an embodiment, the luminescent material is arranged to show different luminescent colours over the coating surface area. This implies that in an embodiment over the length of the discharge tube, luminescent light generated at a first position may show a different luminescent colour, or more precisely a different spectral distribution of the luminescence, than luminescent light generated at a second position, etc. (i.e. there are different segments). Hence, in this way, the luminescent colour may vary along the discharge vessel (especially in a direction along an longitudinal axis).

The term "luminescent material" is known to persons skilled in the art, and especially relates to inorganic materials, and more especially relates to di-band or triband luminescent materials. The term "luminescent material" may also relate to a mixture of luminescent materials.

Different mixtures may provide different emissions with different spectral distributions of the emission. However, also substantially "identical" luminescent materials may show emissions with different spectral distributions, for instance dependent upon the activator or co-dopant concentration. A person skilled in the art is able to select, depending upon the activator type, activator concentration, optional co-dopant and its concentration, and the inorganic host material, optionally the ratio of luminescent material components, and optionally also the layer thickness of the luminescent coating to be applied, the desired emission of a luminescent material (mixture). By varying the luminescent material properties over the discharge tube area, especially in a direction along the longitudinal axis, the spectral distribution of the emission may vary over the length of the discharge tube.

The term "spectral distribution of the emission" relates to the emission spectrum as obtained upon excitation of the luminescent material (i.e. including mixtures of luminescent materials) (by UV light from the discharge). The emission colour or luminescence colour is in general indicated with x,y coordinates, according to the CIE colour diagram, known to persons skilled in the art. When the spectral distribution changes, the x and/or y coordinate may change: this may result in another colour and (correlated) colour temperature. The term "spectral distribution of the emission" is for the sake of simplicity herein also indicated as "luminescent colour". The luminescent material is arranged to absorb at least part of the UV light generated by the discharge, especially 254 nm (and optionally one or more of 248 and 365 nm). Upon absorption of this light, the luminescent material emits light with a specific spectral distribution. The light emitted by the luminescent material is especially in the visible range, i.e. especially in the range of about 380-780 nm. Hence, the term "spectral distribution of the emission" especially relates to the spectral distribution of the visible emission.

The term white light herein is known to persons skilled in the art. It especially relates to light having a correlated colour temperature between about 2000 and 20000 K, especially 2700-20000 K, for general lighting especially in the range of about 2700 K and 6500 K, and for backlighting purposes especially in the range of about 7000 K and 20000 K, and especially within about 15 SDCM (standard deviation of colour matching) from the BBL (black body locus), especially within about 10 SDCM from the BBL, even more especially within about 5 SDCM from the BBL. The term "predetermined colour" may relate to any colour within the colour triangle, but may especially refer to white light.

The terms "blue light" or "blue emission" especially relates to light having a wavelength in the range of about 410-490 nm. The term "green light" especially relates to light having a wavelength in the range of about 500-570 nm. The term "red light" especially relates to light having a wavelength in the range of about 590-650 nm. The term "yellow light" especially relates to light having a wavelength in the range of about 560-590 nm. The term "light" herein especially relates to visible light, i.e. light having a wavelength selected from the range of about 380-780 nm.

These terms do not exclude the fact that especially the luminescent material may have a broad band emission having emission with wavelength(s) outside the range of for instance about 500-570 nm, about 590-650 nm, and about 560-590 nm, respectively.

However, the dominant wavelength of emissions of such luminescent materials (or of the LED, respectively) will be found within the herein given ranges, respectively. Hence, the phrase "with a wavelength in the range of especially indicates that the emission may have a dominant emission wavelength within the specified range. Characteristic luminescent materials may be selected from the group consisting of BaMgAlioOi7:Eu ²⁺ (BAM), and analogous materials (for instance wherein one or more of Ba, Mg, Al (and O) are at least partly substituted with other cations (or anions, respectively)), LaPO4:Ce ³⁺ ,Tb ³⁺ (LAP), and analogous materials (for instance wherein especially La may be at least partly substituted with other cations), Y ₂ OsIEu ³⁺ , and analogous materials (for instance wherein Y is at least partly substituted with other cations), halophosphates, in general indicated with the formula Cas(PO4)3(Cl,F):Sb ³⁺ , Mn ²⁺ , and YsAIsOi ₂ )Ce ³⁺ , and analogous materials (for instance wherein one or more of Y, Al (and O) are at least partly substituted with other cations (or anions, respectively)). For the sake of clarity, possible (further) codopants have not been included in this general description of characteristic luminescent materials. Analogous materials are known to persons skilled in the art. Such materials have different spectral distributions of the emission, since the spectral position and/or spectral distribution of the emissions are different and/or the relative intensities of the emissions are different, respectively.

As mentioned above, the type of luminescent material, or especially the spectral distribution of its luminescence, may vary over the surface of the discharge vessel. In an embodiment, the luminescent material comprises a first luminescent material and a second luminescent material having different luminescent colours. The terms "first" and "second" are here only used to indicate that the luminescent materials may have different emission properties (i.e. different spectral distributions of the emission, or different colour points or different correlated colour temperatures), herein also indicated as "spectral properties" of the luminescent materials. In yet another embodiment, the luminescent material comprises a plurality of different luminescent materials (including mixtures of luminescent materials), such as 2-4 (i.e. a "first, a second, a third and a fourth luminescent material"), with two or more different spectral distributions of the luminescences, respectively. The luminescent material is in general applied to the internal surface of the discharge vessel as coating ("luminescent material coating") but may also be applied as external coating. In principle, the coatings may also be applied to both the internal surface and the external surface. Methods to provide luminescent materials to tubes are known in the art. By coating ("attaching"), a coating or layer is provided on the (internal) discharge vessel surface. The term "layer" may comprise one or more layers. Thus the term layer may also be interpreted in an embodiment as a plurality of layers. Layers may for instance be arranged adjacent, non-adjacent or on top of each other. Hence, the discharge tube may comprise a plurality of layers on top of each other but also adjacent to each other. In the former, the composition of the layer stack may in an embodiment vary over the discharge tube; in the latter the type of individual layers may vary over the discharge tube. By coating the luminescent material on the internal (and/)or external surface, the luminescent material is attached to the internal (and/)or external surface, respectively.

In principle, two main embodiments how the emission can be varied over the discharge vessel are presented. The first option has been described above, and includes variation of the luminescent material, i.e. variation in the spectral distribution of the luminescence of the luminescent material. This includes variations in luminescent materials over the discharge tube. Alternatively (thus when not varying the luminescent material over the discharge tube) or additionally, optical filters may be used to obtain the desired effect. Therefore, in an embodiment of the luminescent lamp, this lamp does comprise the optical filter, wherein the optical filter comprises a first optical filter and a second optical filter, wherein the transmissions of the first optical filter and the second optical filter have different wavelength dependencies. As mentioned above, the optical filter is arranged "downstream" of the tubular discharge vessel and also arranged downstream of the luminescent material coating. In embodiments wherein the luminescent material coating is provided on the internal surface, the arrangement of the optical filter downstream of the discharge vessel implies also an arrangement of the optical filter downstream of the luminescent material coating. For embodiments wherein the luminescent material is provided on the external surface, the optical filter will be provided downstream of the luminescent material coating, either adjacent or remote thereof.

The terms "upstream" and "downstream" relate to an arrangement of items or features relative to the propagation of the light from the light source (here especially the discharge), wherein, relative to a first position within a beam of light from the light source, a second position in the beam of light closer to the light source is "upstream", and a third position within the beam of light further away from the light source is "downstream". Thus the luminescent material is downstream of the discharge, but may be upstream of the internal surface (when using an internal surface coating) or may be downstream of the external surface (when using an external surface coating); the external surface (see below) of the discharge vessel is downstream of the internal surface, but in general upstream of the optional optical filter.

The optical filter is arranged to have a transmission for light emitted by the luminescent material. This transmission will in general be above about 20%, more preferably above about 50%. However, this may vary over the (visible) spectral part and may also vary over the surface of the optical filter. Parts of the filter, i.e. part of the total optical filter area, may be arranged to have other transmission characteristics (i.e. wavelength dependence of the transmission) than another part (other parts) of the optical filter. In this way, dependent upon the position (area) at the optical filter, certain parts of the luminescent emission may be reduced relative to other parts of the luminescent spectrum, whereas at another part (area) of the optical filter, this may be the other way around (i.e. segments are provided). For instance, a first part may reduce transmission of yellow light, thereby providing light with a higher correlated colour temperature, and a second part may not substantially reduce transmission of yellow light, thereby providing light with a lower correlated colour temperature.

The terms "first" and "second" with respect to the optical filter are used in comparable way to the use of these terms with respect to luminescent materials, and so the optical filter may comprise a plurality of optical filters, with two or more different spectral distributions of the transmissions, respectively.

As will be clear to a person skilled in the art, the embodiments described above may be combined. By varying the power coupled into the discharge vessel, such as by varying the frequency or the amplitude of the AC voltage applied to the electrodes, the spectral properties of the light of the luminescent lamp vary. These spectral properties may amongst others be selected from the group consisting of intensity, colour point, correlated colour temperature and spectral distribution of the light, more especially colour point, correlated colour temperature and spectral distribution of the light, even more especially colour point and correlated colour temperature.

In a preferred embodiment, the controller is arranged to provide, during use of the luminescent lamp, light with different correlated colour temperatures, depending on the power provided to the tubular discharge vessel. For instance, at high power, the lamp may provide relatively cool white light (relatively high correlated colour temperature), whereas at lower power, the lamp may provide relatively warm white light (relatively low correlated colour temperature).

In a further embodiment, the controller is arranged to provide, during use of the luminescent lamp, light with different impacts on the melatonin level of a person receiving the light of the luminescent lamp, depending on the power provided to the tubular discharge vessel. Depending upon the power incoupling, the spectral distribution of the lamp may change. For instance, at high power incoupling, the spectral distribution may be selected to reduce melatonin production, and at a lower power incoupling (and outcoupling), the spectral distribution of the light may be selected to minimize reduction of melatonin production.

The discharge tube can be driven in a number of ways. The discharge may be generated in a symmetric way and in an asymmetric way. A way to generate the discharge in an asymmetric way is to ground, or otherwise maintain a constant potential (at), one of the electrodes. When reducing the power coupled into the discharge tube of an asymmetrically driven discharge tube, the extent of the discharge will diminish and in parts of the discharge tube there will be no discharge; the discharge withdraws (partly) from the tube (in the direction of the other electrode). In symmetrically driven discharge tubes, the discharge starts to vanish from a position between the electrodes when reducing the power coupled into the discharge tube. This implies that in the former embodiment, light production at one end starts to decrease when decreasing the power coupled into the discharge tube, whereas in the latter embodiment, light production at a position between the electrodes starts to decrease when decreasing the power coupled into the discharge tube.

Therefore, in an embodiment, the tubular discharge vessel has a tube length, wherein the first electrode and the second electrode are arranged to (be able to) provide, during use of the luminescent lamp, an asymmetric discharge flux over the tube length at an intermediate value between a maximum value and a minimum value of the power provided to the discharge vessel.

In such an embodiment, it may be in a first variant advantageous that the different luminescent colours over the coating surface area of the discharge vessel are distributed asymmetrically along the tube length. In a second variant, which may be combined with the first, wherein the luminescent lamp comprises the optional optical filter, and wherein the optical filter encloses at least part of the tubular discharge vessel, it may be advantageous that the different wavelength dependencies for the transmission over the optical filter area are distributed asymmetrically along the tube length.

However, in another embodiment, the tubular discharge vessel has a tube length, wherein the first electrode and the second electrode are arranged to (be able to) provide, during use of the luminescent lamp, a symmetric discharge flux over the tube length at an intermediate value between a maximum value and a minimum value of the power provided to the discharge vessel.

In such an embodiment, it may be in a first variant advantageous that the different luminescent colours over the coating surface area of the discharge vessel are distributed symmetrically along the tube length. In a second variant, which may be combined with the first, wherein the luminescent lamp comprises the optional optical filter, and wherein the optical filter encloses at least part of the tubular discharge vessel, it may be advantageous that the different wavelength dependencies for the transmission over the optical filter area are distributed symmetrically along the tube length. Since the light generated by the luminescent lamp may be composed of contributions of different spectral distributions, i.e. over the discharge vessel and optionally over the filter area the optical properties may vary, it may be advantageous to enclose at least partly the discharge vessel and optional optical filter in a transmissive diffuser, providing diffuse light. Such diffuse light may provide a better mixing of the different contributions. Therefore, in an embodiment, the luminescent lamp further comprises a transmissive diffuser, arranged downstream of the discharge vessel and the optional optical filter. The phrase, "arranged downstream of the discharge vessel and the optional optical filter" indicates that the diffuser is arranged downstream of the discharge vessel; if the luminescent lamp also comprises an optical filter (which is arranged downstream of the discharge vessel), the diffuser is arranged downstream of this optical filter. In this way, light may be well mixed.

In yet another embodiment, the luminescent lamp further comprises a light mixing chamber, comprising a window to allow, during use of the luminescent lamp, escape of light to the exterior of the light mixing chamber, wherein the light mixing chamber at least partly encloses the discharge vessel.

In yet a further embodiment, the first electrode and the second electrode are arranged external to the discharge vessel. This may be advantageous in view of the dimensions of the discharge tube and the production of the luminescent material coated discharge tube. Preferably, the electrodes are arranged external to the discharge vessel, since in such embodiment, incoupling and outcoupling of the power into and out from the discharge vessel, respectively, may be more easily tuneable. For instance, electrodes may be used that circumferentially enclose extreme parts of the discharge tube.

Note that the terms "first electrode" and "second electrode" may also refer to a plurality of electrodes, respectively.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of example only, with reference to the accompanying schematic drawings in which corresponding reference symbols indicate corresponding parts, and in which:

Figure Ia-Ih schematically depict some embodiments of the invention and some principles of the invention;

Figures 2a-2d schematically depict a number of embodiments in more detail;

Figures 3a-3b schematically depict embodiments of the luminescent lamp, wherein the electrodes are arranged external to the discharge vessel;

Figures 4a-4c schematically depict a number of embodiments of the luminescent lamp, wherein the lamp further comprises a diffuser; and

Figures 5a-5d schematically depict further embodiments of the lamp according to the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

Figure Ia schematically depicts an embodiment of the invention; figure Ib schematically depicts in more detail part of the in figure Ia schematically depicted embodiment. Figure Ia shows a luminescent lamp 1 comprising a tubular discharge vessel

100 enclosing, in a gastight manner, a discharge space 101 containing mercury and an inert gas, such as argon etc. The discharge vessel has an internal surface 120. The luminescent lamp 1 further comprises a first electrode 111 and a second electrode 112, arranged to receive, during use of the luminescent lamp 1, power from a power source 200 and arranged to generate a discharge 104 within the discharge vessel 100. By way of example, the discharge 104 is drawn as a "cloud". The power source 200 is not necessarily part of the luminescent lamp 1 , and may for instance be power from a socket or power point (not shown). The luminescent lamp 1 further comprises a controller 300, arranged to control, during use of the luminescent lamp 1, the power provided to the tubular discharge vessel 100, especially between a maximum value, a minimum value, and at least an intermediate value. The phrase "between a maximum value, a minimum value, and at least an intermediate value" is used to indicate that the controller 300 is not a mere on-off switch, but a controller that can choose at least one, but preferably more than one value between a maximum value and a minimum value. The minimum value may be the off state. The phrases "the power provided to the tubular discharge vessel 100" or "the power coupled into the tubular discharge vessel 100" either indicate the power that may be capactively coupled into the discharge vessel 100 via external electrodes (see below), or relates to power provided to the discharge vessel 100 when using electrodes 111,112 that are arranged internally. Though figures Ia-Ic and 2a-2d depict internal electrodes 111,112, these embodiments may also be employed with external electrodes 111,112 (see figures Ih, 3a-3b). Variation of the power provided to the discharge vessel may especially be done by varying one or more of the frequency and the amplitude of the voltage.

The discharge vessel 100 further comprises a luminescent material coating 121, comprising luminescent material 122, here attached to the internal surface 120 of the discharge vessel 100. The luminescent material coating 121 (here internal coating) has a coating area 121a on the surface. The luminescent material 122 of the luminescent material coating 121 is able to emit luminescent material light 150 (fig. Ib) with a certain luminescent colour upon excitation by UV light 50 from the discharge 104 (see figure Ib). In this way, the luminescent lamp 1 is arranged to generate light 5 (based on luminescent material light, optionally transmitted by optional optical filter 400 (fig Ic), see below), during use of the luminescent lamp 1, downstream of the tubular discharge vessel 100 and the optional optical filter 400 (see below).

The external surface of the discharge vessel 100 is indicated with the reference 140; its area, the external surface area of the discharge vessel 100, is indicated with the reference 140a. The length of the discharge vessel 100 is indicated with the reference L and the longitudinal axis of the discharge vessel 100 is indicated with the reference 130. The internal and external diameter of the discharge vessel 100 are indicated with the references dl and d2, respectively. Light generated by the luminescent lamp 1 is indicated withthe reference 5. Light generated by the luminescent material is indicated with the reference 150. Light 5 may be perceived by an external observer (not indicated), whereas luminescent material light 150 may have to pass through one or more of the discharge tube 100, the optional filter 400 (fig Ic), an optional diffuser (see below), or an optional window (see also below).

As schematically depicted in figure Ic, the luminescent lamp 1 may further comprise an optional optical filter 400 (especially for visible light), arranged downstream from the tubular discharge vessel 100 and having a transmission for luminescent material light 150. This optical filter 400 may have an optical filter area 400a. The optical filter 400 may be attached to the discharge vessel 100. In such an embodiment the external surface area 140a of the discharge vessel 100 and the optical filter area 400a may substantially be the same. However, the optical filter 400 may also be arranged at a non-zero distance from the discharge vessel 100.

Figures Id-Ig schematically depict some principles of the invention. Figure Id schematically depicts the discharge tube 100, and below it schematically the intensity (flux) of the discharge 104 (not depicted) over the length L of the discharge tube 100. In state 1 the flux in the tube is more or less constant along the tube 100; in state 2 the tube 100 is dimmed partly and the flux at the right is lower than at the left; in state 3 the tube 100 is dimmed further and the right of the tube 100 has (almost) no flux coming from the tube 100. Using these characteristics of the tube 100 during dimming it is possible to make the tube 100 change from one state to another state using different attributes. As shown in figure Ie, more light from the luminescent material (not depicted) will escape from the tube 100 at the high flux side (left) and less (or substantially) no light will escape from the tube at the low flux side (right). Figures Id and Ie provide an example of an asymmetrically driven discharge tube 100, wherein the flux vanishes only in the direction of one of the electrodes 111,112 (not depicted).

Also Figure If schematically depicts the discharge tube 100, and below it schematically the intensity (flux) of the discharge over the length L of the discharge tube 100. In state 1 the flux in the tube 100 is more or less constant along the tube 100; in state 2 the tube 100 is dimmed partly and the flux in the centre is lower than at the sides; in state 3 the tube 100 is dimmed further and the centre of the tube 100 has (almost) no flux coming from the tube 100. Again, using these characteristics of the tube during dimming it is possible to make the tube 100 change from one state to another state using different attributes. As shown in figure Ig, more light from the luminescent material (not depicted) will escape from the tube 100 at the high flux sides (left and right) and less (or substantially) no light will escape from the tube 100 at the low flux side (centre). Figures If and Ig provide an example of a symmetrically driven discharge tube 100, wherein the flux vanishes in the direction of both the electrodes 111,112 (not depicted).

Figure Ih by way of example schematically depicts an embodiment wherein the luminescent material 122 is attached to the discharge vessel 100 as external coating, i.e. luminescent material coating 121 (here external coating) is attached to the external surface 140. Optionally, downstream thereof, also the optical filter 400 may be arranged, attached to the luminescent material coating 121 or remote therefrom. In this embodiment, by way of example, the electrodes 111,112 are arranged external to the discharge vessel 100. As mentioned herein, all embodiments schematically depicted with internal electrodes 111,112 may also have variants wherein, instead of internal electrodes, external electrodes 111,112 are applied. The references 111 and 112 refer to electrodes in general, irrespective of their arrangement internal or external to the discharge tube. Internal electrodes may also have an external part, as known to persons skilled in the art, whereas external electrodes may be arranged completely external to the discharge tube 100, as depicted herein. This effect (see especially figures Id-Ig), that the discharge 104, or more precisely, the extent of the discharge 104, may decrease when reducing the power coupled into the discharge vessel 100, may be used to tune the spectral properties of the lamp light 5. Therefore especially one or more of (a) the luminescent material 122 is arranged to show different luminescent colours over the coating surface area 121a of the discharge vessel 100 and (b) the optional optical filter 400 is arranged to show different wavelength dependencies for the transmission over the optical filter area 400a. Embodiments thereof are schematically depicted in figures 2a-2d.

Figure 2a schematically depicts an embodiment with an asymmetric distribution of the luminescent material coating 121, i.e. the luminescent material 122 is arranged to show different luminescent colours over the coating surface area 121a of the discharge vessel 100. Here, the luminescent material 122 comprises a first luminescent material 122' (including luminescent material mixtures) and a second luminescent material 122" (including luminescent material mixtures) having different luminescent colours. Part of the internal surface 120 is coated with the first luminescent material 122' and part of the internal surface 120 is coated with the second luminescent material 122". Of course, more than two different luminescent materials 122 may be applied than the schematically depicted two luminescent materials 122' and 122" (see also figure 2c). Note that when the discharge tube 100 is driven asymmetrically, and the discharge 104 (not depicted for the sake of clarity) vanishes from one side of the discharge vessel 100, depending upon which side the discharge 104 vanishes (see also figures Id-Ig), the first luminescent material 122' or the second luminescent material 122" will provide less luminescent material emission 150' or 150", respectively, compared with the situation in which the discharge 104 has not vanished (partly). As mentioned above, due to reduction of the power or outcoupling to the external of the discharge tube 100, the extent of the discharge 104 within the discharge tube 100 reduces. In this way, the spectral properties of the light 5 may vary over the discharge tube 100 along the longitudinal axis 130. Thus the luminescent material coating area 121a includes regions (here 2 regions (or segments)) with different spectral distribution of the luminescence of the luminescent materials. Figure 2b schematically depicts an embodiment with an asymmetric distribution of the optical filter 400, i.e. the optical filter 400 is arranged to show different wavelength dependencies for the transmission over the optical filter area 400a. Here, the optical filter 400 comprises a first optical filter 400' and a second optical filter 400", wherein the transmission of the first optical filter 400' and the second optical filter 400" have different wavelength dependencies of the transmissions. In this way, the transmission may vary over the optical filter area 400a. Note that the optical filter 400 (fig 2a) may be adjacent to the external surface 140 of the discharge vessel 100 (drawn), but may also be remote therefrom (not drawn). Note that when the discharge tube 100 is driven asymmetrically, and the discharge 104 (not depicted for the sake of clarity) vanishes from one side of the discharge vessel 100, depending upon which side the discharge 104 vanishes (see also figures Id-Ig), the first luminescent material 122' or the second luminescent material 122" will provide less luminescent material emission 150' or 150", respectively, compared with the situation in which the discharge 104 has not vanished (partly). As mentioned above, due to reduction of the power or outcoupling to the outside of the discharge tube 100, the extent of the discharge 104 within the discharge tube 100 reduces. In this way, the spectral properties of the light 5 may vary over the discharge tube 100 along the longitudinal axis 130. Thus, the optical filter area 400a includes regions (here two regions (or segments)) with different wavelength dependencies of the transmission.

Figures 2c and 2d schematically depict variations on figures 2a and 2b. In figure 2c, the luminescent material coating 120 comprises a first luminescent material 122' (including luminescent material mixtures), a second luminescent material 122" (including luminescent material mixtures) and a third luminescent material 122"' (including luminescent material mixtures). When the first luminescent material 122' and the third luminescent material 122"' are substantially identical (i.e. providing substantially identical spectral properties such as one or more of colour point, spectral distribution of the emission, and correlated colour temperature), the distribution of the different luminescent materials may substantially be symmetrical, thereby allowing a symmetrically driven discharge tube 100. When they are substantially different, an asymmetrically driven discharge tube may be applied. Note that the spectral properties of the plurality of luminescent materials 122 (be it three as schematically depicted here, or being more than three), may gradually change from one side to another side of the discharge tube 100. For instance, luminescent material may be applied with gradually increasing correlated colour temperatures over the coating surface area 120a (along the longitudinal axis). In figure 2d, the optical filter 400 comprises a first optical filter 400', a second optical filter 400" and a third optical filter 400'".

When the first optical filter 400' and the third optical filter 400'" are substantially identical with respect to the wavelength dependence of the transmission, the distribution of the optical filters may substantially be symmetrical, thereby allowing a symmetrically driven discharge tube 100. When they are substantially different, a asymmetrically driven discharge tube 100 may be applied. Note that the wavelength dependence of the transmission of the plurality of optical filters 400', 400", ... (be it three as schematically depicted here, or being more than three), may gradually change from one side to another side of the discharge tube 100. For instance, the optical filter 400 may be applied with gradually changing wavelength dependency of the transmission over the optical filter area 400a (along the longitudinal axis 130).

The phrase "along the longitudinal axis" indicates a direction parallel to the longitudinal axis 130. Though it is not excluded that there is variation in luminescent material 122 or optical filter 400 in a direction perpendicular to the longitudinal axis 130 (thus a circumferential variation), the advantages of the invention may especially be obtained when there is a variation in a direction from the first electrode 111 (in general one extremity of the discharge tube) to the other electrode 112 (in general the other extremity of the discharge tube), i.e. in a direction along the longitudinal axis 130.

Figures 3a and 3b schematically depict preferred embodiments, wherein the electrodes 111,112 are arranged external from the discharge tube 100. In a preferred embodiment, the electrodes are circumferentially enclosing extremities (or end parts) of the discharge tube 100.

Light 5 (fig 4a-4c) may be composed of different contributions having different spectral distributions. Since these contributions may be provided at different locations (segments) at the luminescent lamp (i.e. at the discharge vessel and the optional optical filter 400), it may be advantageous to mix the light 5. Figures 4a-4c schematically depict a non- limiting number of embodiments.

In figures 4a (side view cross section) and 4b (top view cross section), the luminescent lamp 1 further comprises a transmissive diffuser 500, arranged downstream of the discharge vessel 100 and the optional optical filter 400 (not depicted). The diffuser 500 may be adjacent to the discharge vessel 100 or the optional optical filter 400, if present, but may preferably also be arranged at a non-zero distance therefrom, respectively.

In yet another embodiment, schematically depicted in figure 4c, the luminescent lamp 1 further comprises a light mixing chamber 550, comprising a window 555 (sometimes also indicated as exit window) to allow, during use of the luminescent lamp 1, escape of light 5 to the exterior of the light mixing chamber 550, wherein the light mixing chamber 550 at least partly encloses the discharge vessel 100. Wall(s) 553 and bottom 552 are preferably reflective, and may for instance comprise a reflective coating. For instance MCPET (microcellular polyethylene terephthalate) may be applied as reflective material. In this way, good mixing of the light 5 may be obtained. The discharge vessel 100 is arranged in a space provided by the mixing chamber 550, which space is indicated as chamber space 551.

Figures 5a-5d schematically depict a number of embodiments wherein the discharge tube 100 is not straight but curved. Other curved embodiments are known in the art. Figures 5a and 5c schematically depict the luminescent lamp 100 at full power, or at least settings wherein the discharge 104 (not depicted) substantially extends along the discharge tube 100, and substantially all luminescent material 122 (not depicted) is excited. Figures 5b and 5d schematically depict the same embodiments, but now the discharge 104 has vanished from the central part of the discharge tube 100 (these examples show non-limitingly a symmetrically driven discharge tube). Less luminescent light is produced at the centre, which is indicated with the light colour (the dark colour of the discharge tube 100 represent more luminescence).

The luminescent lamp 1 may further comprise a user interface (not depicted). The user interface may electrically be connected to the controller 300. The user interface or user input device ("local" or "remote") controls the light 5 generated by the luminescent lamp 1 as selected by the user. The interface or input device may comprise control action buttons shown in an intuitive way, how the end user can navigate along the available settings. A microprocessor may allow a user to generate dynamic light effects via an algorithm. The user interface may comprise a remote control unit. EXAMPLE

A luminescent lamp with a discharge tube of 1 m, internal diameter of 4 mm and glass thickness of 0.6 mm was provided. Copper electrodes were provided to the external surface of the discharge tube at both extremities. The frequency could be varied between about 80 kHz-300 kHz and the voltage could be varied between about 1 kV and 2 kV. The lamp current was about 0.1 mA to 20 mA. The discharge tube was coated with luminescent material, wherein a first part was coated with 1700 K luminescent material and a second part was coated with 4000 K luminescent material.

The following data were obtained:

J P lamp (W) Tc(K) X y I flux (Im) I efficacy (lm/W) ! ^'" Ϊ8JO3 ^" 2733 0.4476 0.3925 1961 ! ^" 53JO

I 16.00 2712 0.4484 0.3915 J 858 J 53.63 1Ϊ00 ^" 2688 04494 ^" 6Ϊ3903 ^{"" "} 738 ^{' "} 52Jl

I 12.00 2646 0.4514 0.3889 I 601 I 50.08

I 9.99 2561 0.4568 0.3879 J 455 J 45.55

I 8.01 2409 0.4691 0.3890 ; 314 ; 39.20

I 7.00 2303 0.4794 0.3910 I 248 I 35.43 loo ^" 2Ϊ73 0.4946 0:3955 j ^" Ϊ94 [ ^" 32.33

I 5.00 2081 0.5070 0.3992 J 147 J 29.40

J 4.01 2026 0.5145 0.4011 I 109 I 27.18

I 3.00 1978 0.5222 0.4038 ! ⁷⁴ J 24.67

I 1.99 1939 0.5269 0.4037 ! ⁴² J 21.11 LOO 1935 ^" 0Ϊ53Ϊ9 ^" ό:4095 \ ^" \ϊ 17!OO

From this data, it appears that the lamp substantially follows the BBL when dimming, like incandescent lamps, unlike prior art luminescent lamps.

The term "substantially" herein, such as in "substantially all emission" or in "substantially consists", will be understood by a person skilled in the art. The term

"substantially" may also include embodiments with "entirely", "completely", "all", etc. Hence in embodiments the adjective substantially may also be removed. Where applicable, the terms "substantially" or "about" may also relate to 90% or higher, such as 95% or higher, especially 99% or higher, even more especially 99.5% or higher, including 100%. The term "comprise" includes also embodiments wherein the term "comprises" means "consists of.

Furthermore, the terms first, second, third and the like in the description and in the claims are used for distinguishing between similar elements. It is to be understood that the terms so used are interchangeable under appropriate circumstances and that the embodiments of the invention described herein are capable of operation in other sequences than described or illustrated herein.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference signs placed between parentheses shall not be construed as limiting the claim. Use of the verb "to comprise" and its conjugations does not exclude the presence of elements or steps other than those stated in a claim. The article "a" or "an" preceding an element does not exclude the presence of a plurality of such elements. The invention may be implemented by means of hardware comprising several distinct elements, and by means of a suitably programmed computer. In the device claim enumerating several means, several of these means may be embodied by one and the same item of hardware. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.