MULTILAYER FILTER FOR LAMPS

TECHNICAL FIELD The invention relates to an electric lamp for emitting red color, comprising a light-transmitting lamp bulb in which a light source is arranged and a multilayer medium having light absorption properties exhibiting a spectral transition in the visible range, so as to filtering out colors outside said red color. Such lamps can be used in automotive applications, for example as brakes rear-lamp or a signal rear-lamp. Such electric lamps can also be used for general illumination purposes. Said electric lamps can further be used in traffic and direction signs, contour illumination, traffic lights, projection illumination and fiber optics illumination. BACKGROUND OF THE INVENTION

An electric lamp of the type mentioned in the opening paragraph is known from WO 01/97253. The disclosed multilayer medium comprises a suitable combination of a light-absorbing coating and an interference film. When the lamp of this document is on-state, the light-absorbing layer absorbs colors outside the red color and the interference layer increases this red color by preventing the transmission of the other colors while facilitating the transmission of the red color. To this purpose, the interference film is arranged for exhibiting a transmission (T) spectrum having transition around 550 nm. Moreover the interference film is designed for reflecting almost all the visible spectrum resulting in a substantially color-neutral appearance (silvery aspect) of the lamp when the lamp is in off-state - for this reason, these kinds of lamps are so-called 'SilverVision' lamps. To this purpose, the interference film has a substantial constant reflection (R) spectrum over at least substantially the entire visible region (R of the interference film is around 70% in the wavelength range from 380 < λ < 690 nm):

therefore the lamp is capable of back-reflecting most of the light coming from outside the lamp (e.g. the natural daylight), in a quite homogeneous manner over almost the whole visible spectrum, resulting in the said silverfish effect in off state. The reflectivity of the interference film must be tuned for not being too low - indeed the lamp would be too transparent and the SilverVision effect would disappear - and not too high - indeed the lumen output would become insufficient.

A drawback of the SilverVision lamp is the presence of a non negligible green and amber color components in the emitting spectrum, when the lamp is in on-state.

SUMMARY OF THE INVENTION

It is an object of the invention to provide an electric lamp having an improved red color, while outputting sufficient lumens and keeping a mirror effect in off-state. This object is achieved, in accordance with a first aspect of the invention, by providing a lamp emitting red color, comprising: a light source; a substrate; a multilayer medium comprising: ■ a color filter for filtering out colors outside the said red color;

^■ an interference filter; wherein the multilayer medium has the following spectral properties: for λ € [380-550] nm, R > 80% for a light coming from outside of the lamp; and for emitting, when the lamp is on-state, a colored light within the following colorimethc region of the CIE 1931 x,y color space: * y < 0.335; and * 0.992-y < x < 1.000-y.

A second aspect of the invention, considered as an alternated or an additional feature of the first aspect of the invention, relates to a lamp emitting red color comprising a light bulb which comprises: a light source;

a substrate; a multilayer medium comprising:

^■ a color filter for filtering out colors outside the said red color;

^■ an interference filter; wherein the multilayer medium has the following spectral properties: for λ e [380-550] nm, R > 80%; and wherein the interference filter is arranged so as to enhance the red lumen value outputting the lamp by about 40% or more in comparison with a lamp without any interference filter and comprising the same light source, the same substrate and a color filter arranged for outputting a light having the same color point, when the lamp is in on-state, e.g. 10 Im, or 20 Im or more.

A third aspect of the invention, considered as an alternated or an additional feature of the first and/or second aspects of the invention, relates to a lamp emitting red color comprising a light bulb which comprises: a substrate a multilayer medium comprising:

^■ a color filter for filtering out colors outside the said red color; ■ an interference filter; wherein the multilayer medium is arranged so as to have the following spectral properties: for λ e [380-550] nm, R > 80% and to present a transition in the transmission (T) spectrum equal to or greater than 600 nm (at T=50%). The transition is therefore shifted towards a longer wavelength comparing with prior art .

In view of prior art, the invention increases the reflexion R of the multilayer medium (and of the interference layer accordingly) within a shorter wavelength range, this range including blue and green colors.

A first effect is a decrease of the green and amber components in the light outputting the lamp in on-state, and an increase of the red

colorimetry, while keeping an acceptable output lumen value, if compared with prior art.

Another effect is to overcome the following difficulty raised by the inventors: for increasing the red color point specification to y < 0.335 and 0.992-y < x < 1.000-y, the color filter, which typically acts as an absorber layer, has to be thickened. But an increase of the absorbing properties of such a thickened color filter brings the following drawbacks:

- the lumen output is significantly reduced; and

- the thermal stability is typically reduced, since the material of such an absorbing filter is typically limited in terms of thermal stability.

By providing an interference filter and thus a multilayer medium according to the invention, i.e. exhibiting an improvement in the red color emission, the red point and the corresponding lumens values can be sufficiently increased while having a color filter sufficiently thin for not attenuating significantly the red lumen output and for having an acceptable thermal stability in on-state.

Another effect is a bluish mirror effect of the lamp in off-state, since most of the visible spectrum is back-reflected except red components. As a result thereof, the electric lamp in accordance with the invention can very suitably be used as a red indicator lamp for automotive applications which can fulfil, if correctly tuned, the standard ECE R37 concerning both red point and/or lumen value output.

In addition, the presence of the interference film may increase the stability of the color filter in that the interference film serves as an oxygen barrier for the light-absorbing medium.

An embodiment of an electric lamp in accordance with the invention is characterized in that a wall of the lamp bulb comprises the color filter, the latter being incorporated in the wall of the lamp bulb, which is made, for example, from glass, such as quartz glass or hard glass, or from a transparent ceramic material. In this embodiment, the interference film is

preferably directly applied to a side of the wall of the lamp bulb facing away from the light source.

An alternative embodiment of the electric lamp in accordance with the invention is characterized in that the color filter is located between the lamp bulb and the interference filter.

In an alternative embodiment of the electric lamp in accordance with the invention, the color filter is located in the interference filter distributed within over one or more layers.

An embodiment of the electric lamp in accordance with the invention is characterized in that the interference filter comprises layers of, alternately, a first layer of a material having a comparatively high refractive index and a second layer of a material having a comparatively low refractive index.

A preferred embodiment of the electric lamp in accordance with the invention is characterized in that the second layer of the interference filter comprises predominantly silicon oxide, and the first layer of the interference filter comprises predominantly a material having a refractive index which is high as compared to a refractive index of silicon oxide.

Preferably, the first layer of the interference filter comprises a material chosen from the group formed by titanium oxide, tantalum oxide, zirconium oxide, niobium oxide, hafnium oxide, silicon nitride and combinations of said materials. Preferably, the material of the first layer of the interference filter predominantly comprises titanium oxide or niobium oxide. Preferably, the interference filter is TiO2/SiO2 type-films or

Nb2O5/SiO2-type films.

The light source of the lamp may be an incandescent body, for example in a halogen-containing gas or it may be an electrode pair in an ionizable gas, for example an inert gas with metal halides or mercury or a LED or any other kind of light source. An innermost gastight envelope may

surround the light source. It is alternatively possible, that the outermost envelope surrounds the lamp vessel.

The interference filter and possibly the color filter may be provided in a customary manner by means of physical vapor deposition (PVD) - e. g. reactive magnetron sputtering or evaporation, or chemical vapor deposition (CVD) - e. g. low pressure CVD (LPCVD), or plasma enhanced CVD (PECVD), or plasma impulse CVD (PICVD), wet chemical depostion techniques - e. g. sol gel coating by spraying and dipping.

These and other aspects of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Fig. 1A is a cross-sectional view of an embodiment of the electric lamp in accordance with the invention;

Fig. 1 B is a side view of an alternative embodiment of the electric lamp in accordance with the invention;

Fig. 2 shows the emission spectrum of a P21W lamp coated with a color filter containing red pigments, deposited using a sol-gel process;

Fig. 3 shows the reflection (a) and transmission (b) spectrum as a function of the wavelength of a Nb2O5/SiO2 interference filter to be deposited onto the color filter of Fig.2;

Fig. 4 shows the emission spectrum of a Philips HiPer 24W lamp with a color filter made of a 500nm sputtered Fe2O3.

Fig. 5 shows the reflection spectrum as a function of the wavelength of a multilayer medium composed of a Nb2O5/SiO2 interference filter on a 500 nm Fe2O3 color filter;

Fig. 6 shows, in a part of a C. I. E. 1931 color triangle diagram, the color co-ordinates of different Philips HiPer 24W electric lamps measured at 13.5 V, comprising either Nb2O5/SiO2 interference + Fe2O3 color filters or only Fe2O3 color filters; Fig. 7 shows lumen values measured from Philips HiPer 24W electric lamps at 13.5 V as a function of the color (according to the corresponding

y-coordinate of the C. I. E. 1931 color space), showing a lumen comparison between bulbs coated with single Fe2O3 color filters (curve (a)) and bulbs coated with Fe2O3 color filters and interference filters of Nb2O5/SiO2 (curve (b)). DETAILED DESCRIPTION OF THE INVENTION

It must be noted that as used in this specification and the appended claims, the singular forms "a", "an", and "the" include plural referents unless the context clearly dictates otherwise. Unless defined otherwise, all technical and scientific terms herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

The foregoing description of preferred embodiments of the invention is not intended to be exhaustive or to limit the invention to the disclosed embodiment. Various changes within the scope of the invention will become apparent to those skilled in the art and may be acquired from practice of the invention.

In the various drawings, the same reference numerals designate identical or similar elements.

The Figs, are purely schematic and not drawn to scale. Particularly for clarity, some dimensions are exaggerated strongly. In the Figures, like reference numerals refer to like parts whenever possible.

Fig. 1A is a cross-sectional view of an embodiment of the electric lamp in accordance with the invention. Said electric lamp has a light- transmitting light bulb 1 , for example of glass, which is sealed in a gastight manner and which accommodates an electric element 2, in the Figure a (spiral-shaped) tungsten incandescent body, which is connected to current conductors 3 which issue from the light bulb 1 to the exterior. The lamp shown, which is alternatively referred to as P21 W (12 volt, 21 watt) is filled with an inert gas, for example an Ar/N2 mixture, having a filling pressure of approximately 1 bar.

In the embodiment of the electric lamp shown in Fig. 1A, the color filter is provided, in the form of a light-absorbing coating 6, organic or inorganic, on an outside of the light bulb 1 (on a wall of the light bulb), and

an interference filter 5 is provided on said light absorbing coating (also see fig. 1 B).

The light-absorbing coating 6 may include a mix of an organic and an inorganic dye (in the MTMS matrix), such as for example iron oxide, can be combined with an organic perylene dye with Cl 71137 also referred to as "pigment red 149". with a layer thickness of, for example, about 2 μm. The light-absorbing coating 6 may be realized before forming the interference filter by wet chemical technology (sol gel processes via dipping, spraying or other known methods). An example of the red emission spectrum of a P21 bulb coated with a sol gel based red absorbing layer is shown in Fig.2.

Alternatively or in combination, the light-absorbing coating 6 is of Fe2O3 layer or substoichiometric silicon oxide (SiOx) or a combination thereof of about 1 μm or less, or of another red-absorbing material or mixture. These materials might be preferred due to their better thermal stability than red-pigmented color filter 6. The light-absorbing coating 6 can be realized at the same time of the interference filter 5 by using well- known vacuum deposition techniques (such as those aforementioned). An example of the red emission spectrum of a P21 bulb coated with a 500 nm sputtered Fe2O3 absorbing layer is shown in Fig.4: these red absorbing layers 6 starts to stop to absorb at a wavelength of about 550- 560 nm.

In Fig. 1A, an interference filter 5 is applied onto the color filter 6, which interference filter 5 comprises layers of alternately a first layer of a material having a comparatively high refractive index, for example titanium oxide (average refractive index of TiO2 approximately 2.4-2.8), niobium oxide (average refractive index of Nb2O5 approximately 2.34), tantalum oxide (average refractive index of Ta2O5 approximately 2.18) or zirconium oxide (average refractive index of ZrO2 approximately 2.16), and a second layer of, predominantly, silicon oxide (average refractive index approximately 1.46). The TiO2/SiO2, Nb2O5/SiO2, Ta2O5/SiO2

interference filters are preferably arranged for exhibiting a minimized number of layers.

The interference filter 5 has one or several of the following spectral properties: • for λ e [380-550] nm, R > 80%, and preferably R > 90% for a light coming from outside the lamp;

• the transition (T=50%) in the transmission spectrum of the interference filter 5 or of the whole multilayer medium is at about 600 nm or greater for a light coming from the light source.

It is to be noticed that R value of the interference filter 5 is similar as the R value of the whole multilayer medium, since the color filter 6 has neglictable impact on R.

Additionally, it is preferable that this interference filter 5 presents also the following spectral properties: For λ e [650-800] nm: R < 20%, preferably R < 10%. Additionally or alternatively to the preceding, it is preferable that this interference filter 5 presents also the following spectral properties: For λ e [650-800] nm: T > 80% or preferably T > 85%, or preferably T > 90%, or preferably T > 95%. The interference filter 5 configuration can be tuned according to the optical results to be reached.

The reflectivity is therefore increased, comparing with prior art, for the wavelengths inferior to those of red color and is decreased in the red color. This increases the red point. Moreover, even if the number of layers of the interference filter 5 is high, the lumen output remains acceptable since the interference filter 5 acts like a longpass filter (i.e. which transmits the large wavelengths of the visible spectrum) and the color filter 6 is not too thick. Furthermore, the interference filter 5 is a good mirror for a large part of the visible spectrum: the mirror vision of the lamp in off-state is therefore obtained. The interference filter 5 may be realized by using well-known vacuum deposition techniques (such as those aforementioned).

Fig. 1 B is a side view of an alternative embodiment of the electric incandescent lamp in accordance with the invention. Said electric lamp comprises a quartz glass light bulb 11 accommodating an incandescent body as the light source 12. Current conductors 13 are connected to said light source and issue from the light bulb 11 to the exterior. The light bulb 11 is filled with a halogen-containing gas, for example hydrogen bromide. At least a part of the light bulb 11 is covered with a color filter 16 in the form of a light-absorbing coating.

In the example shown in Fig. 1 B, an interference filter 15 is applied onto the color filter 16.

The light bulb 11 in Fig. 1 B is mounted within an outer bulb 14, which is supported by a lamp cap 17 to which the current conductors 13 are electrically connected.

Example 1 : Technical specifications:

- Philips P21 Wbulb

- color filter 6: iron oxyde combined with an organic perylene dye with Cl 71137 also refered to as "pigment red 149" with a layer thickness of about 2 μm deposited by sol-gel process on the bulb - interference filter 5: deposited onto the color filter 6 by reactive magnetron sputtering:

Table 1 : Interference filter

Fig.2 depicts the absorbing properties of the color filter 6, for which most of the absorbed light has wavelengths smaller than 560 nm.

Fig.3 shows the reflection spectrum (a) and the transmission spectrum (b) of the interference filter 5. The reflectance is above 80% for λ < 550 nm (including blue and green) and above 95% for λ < 500 nm. This improved interference filter 5 according to the invention allows therefore an increasing of the red transmission from the light source and a decreasing of the transmission of the other colors from the light source: the interference filter shifts the emission color more to the red comparing with the same lamp but without the interference filter. This interference

filter further reflects most of the spectrum except the long wavelengths: accordingly, the interference filter 5 gives a bluish mirror effect in off-state. Therefore the interference filter is designed in a way that it highly transmits the red (650nm to 800nm) and highly reflects for wavelengths between 400 and 550nm. Thus for a lamp lighted-on, the colorimetry is shifted into the red SAE zone and has a light output which may be conformed with ECE R37 regulations. Indeed, this lamp of Example 2 emits a red light corresponding to the following color point measured in the CIE 1931 x,y color space: x = 0.677 y = 0.323

The lumen value of this lamp meets also the ECE R37 regulations for lumen value (which is 88.0 Im for PR21W - corresponding to Philips P21W) since its outputs about 99.7 Im for a color temperature of about 3000 K.

When the lamp is switched off, the lamp provides a bluish reflectance effect.

Example 2:

Technical specifications: - Philips HiPer 24W lamps

- color filter 6: made of a sputtered Fe2O3 (500 nm thickness) on the bulb

- interference filter 5 deposited on the color filter 6 by reactive magnetron sputtering:

Table 2: Interference filter

Figure 4 depicts the absorbing properties of the color filter 6, for which the absorbed light has wavelengths smaller than 550 nm.

Figure 5 shows the theoretical reflection spectrum of the multilayer medium (i.e. color filter 6 + interference filter 5). It can clearly be seen that the multilayer medium is reflective below approximately 600 nm where the light source emits light which is not needed to create red. Furthermore this is the wavelength range in which the underlying absorber 6 absorb(s).

Figure 6 and 7 shows Results obtained from measurements performed on lamps Philips HiPer 24W being on-state with the same color temperature (about 3040 K) but having different layers configurations:

Figure 6 shows the color coordinates x,y of these lamps in the CIE 1931 x,y color space, while Figure 7 shows the corresponding lumen values for these lamps.

- Results 21 , 22 correspond to lamps having a sputtered Fe2O3 color filter and a Nb2O5/SiO2 interference filter according to the invention;

- Results 23, 24 corresponds to lamps only with a single Fe2O3 color filter (i.e. without any interference filter).

Result 21 corresponds to the lamp of Example 2. The color filter of Result 23 is thinner than the color filter of Result 24. Some configurations (e.g. results 21 , 24) meet the ECE R37 standard

(zone 10 of the CIE 1931 ).

Firstly, it can be deduced from Results 23 and 24 of Figure 6 and 7, that thicker the Fe2O3, greater the color point, and lower the lumen value. If Result 24 meets the ECE R37 for the color, neither Result 24 nor Result 23 meets this ECE R37 for the lumen value (which is of 86.25 Im or greater for PR24W - i.e. corresponding to Philips HiPer 24W 12V).

Now, Results 22 and 23 have a similar color point (about x=0.6575 and about y=0.339) but have a significant difference in the outputting lumen value (Result 22 has about 110 Im while Result 23 has about 76 Im). Therefore, the lamp with an interferential filter and a color filter according to Result 22 increases the output lumen value by about 45 % by comparison with the lamp of Result 23 with no interference layer, while having a similar color point.

From the previous analysis and from extrapolation of the graph of Figure 7, the lamp with an interferential filter and a color filter according to Result 21 , outputting a determinate color point (x=0.332; y=0.665), increases the output lumen value by about 40% if we compare with a lamp having no interference layer and having a color filter outputting the similar determinate color point. These results show in particular that the multilayer medium according to the invention allows increasing significantly the lumen value so as to meet the ECE R37 regulation for lumen value, while keeping a similar color point so as to meet the ECE R37 regulation for the color point.

It is to be noticed that lamp of Result 21 (Example 2) has the following light features: color point: x=0.665; y=0.332

lumen value: 90.5 Im

Therefore, the lamp of Result 21 (i.e. the lamp according to Example 2) meets the ECE R37 regulation for both the color point and the lumen value.

The light emitting device according to the invention can be used notably for other lighting applications than automotive lighting, i.e. any application which need the lighting effect according to the invention. The light emitting device according to the invention does not necessary comprise an incandescent or halogen lamp, but may also comprise alternative light sources (e.g. HID, LED) and might be Lambertian or not.

The aforementioned substrate (1 ) is not necessary a bulb but can be any other type of substrate, preferably transparent, on and/or in which a multilayer medium may be formed. For example, this substrate can be a flat or curved surface placed at the outlet of a luminaire in which light source(s) is (are) located.

The light emitting device may comprise only one light source, preferably placed at the optical centre of the light emitting device, or a plurality of light sources (e.g. a plurality of LEDs, several filaments), aligned on a straight or curved axis, or placed according to a straight or curved matrix.