ELECTRIC LAMP COMPRISING LIGHT-ABSORBING LAYER

TECHNICAL FIELD OF THE INVENTION

The invention relates to an electric lamp comprising a light-transmitting lamp vessel in which a light source is arranged, said electric lamp comprising a light- absorbing layer exhibiting a spectral transmission in the visible yellow, amber, or red wavelength range.

BACKGROUND OF THE INVENTION

Such lamps are used in automotive applications, for example as automotive headlamps, which, in operation, emit yellow light, as amber-colored light sources in indicators, or as a red-colored light sources in brake lights. Such electric lamps are also used for general illumination purposes. Furthermore, said electric lamps are used in traffic and direction signs, contour illumination, traffic lights, projection illumination, and fiber optics illumination. Alternative embodiments of such electric lamps comprise lamps whose color point is further influenced by means of a suitable combination of a light-absorbing coating and an interference filter coating.

Historically, amber lamps were made from cadmium-doped glass to produce the amber color. Cadmium, however, has become an environmentally unacceptable material. Various color-coatings for electric lamps have accordingly been developed such as, for example, paints comprising an organic dye or an inorganic pigment in a resin binder. Durability is difficult to achieve because the lamps may be exposed to temperatures from -40 to +350 ⁰ C in automotive applications, and resin- bonded paints tend to peel off or discolor before the lamp itself fails.

An electric lamp is known from US 6,570,302 comprising a light- absorbing layer covered with an optical interference film. The light-absorbing layer preferably comprises a pigment selected from Nd ₂ O ₃ , CoAl ₂ O ₄ , Fe ₂ O ₃ , ZnFe ₂ O ₄ and BiVO ₄ . A drawback of the known lamps is that pigments generally tend to absorb too much light. The level of pigment concentration required to obtain the proper coloration of the light is high, so that the resulting luminance level of the transmitted light may eventually be too low.

SUMMARY OF THE INVENTION It is an object of the invention to provide an electric lamp of the type described in the opening paragraph wherein said drawback is obviated.

The present invention provides an electric lamp comprising a light- transmitting lamp vessel in which a light source is arranged, said electric lamp comprising a light-absorbing layer, characterized in that the light-absorbing layer comprises sub-stoichiometric silicon oxide SiO _x , with 0.1 < x < 1.8 ("silicon suboxide"), as a light absorbing material.

An optically transparent, non-scattering, light-absorbing coating is obtained capable of resisting temperatures up to 400 ⁰ C when the light-absorbing layer is made of silicon suboxide. It is also durable and not susceptible to scratches or peeling. The light-absorbing layer comprising silicon suboxide is superior to layers comprising conventional yellow, amber, or red pigments owing to its excellent transparency.

A light-absorbing layer comprising silicon suboxide also has a particularly smooth and glossy surface, contrary to pigment-coated lamps. An electric lamp comprising the light-absorbing layer according to the invention may have either yellow, amber, or red optical absorption characteristics. A thin layer of silicon suboxide is sufficient already for providing a yellow colored lamp. This is due to a favorably high optical extinction coefficient.

To achieve deeper-colored, amber to red optical characteristics the electric lamp according to the invention may comprise a second light-absorbing material within the single light-absorbing layer, preferably selected from the group formed by iron

oxide, iron oxide doped with phosphorus, zinc-iron oxide, neodymium oxide, bismuth vanadate, zirconium-praseodymium silicate, or mixtures thereof. The second light- absorbing material selected from the group of red pigments can be used to adapt the color point of the electric lamp so as to meet the statutory regulations for indicator and brake lights in vehicles.

Besides comprising the first and second light-absorbing material in a mixture in one light-absorbing layer, the electric lamp may alternatively comprise the second light-absorbing material in a second light-absorbing layer.

In the electric lamp according to the invention, the light-absorbing layer may be combined with an optical interference filter formed by a series of alternating layers of a first material having a high refractive index and a second material having a low refractive index.

The aim of such a combination of a light-absorbing medium and an interference film is, in particular, to unlink the impression given by the electric lamp in the off-state from the color of the light emitted by the electric lamp in the on-state. The object is, in particular, to provide an electric lamp which emits light of a certain color in operation, for example a so-called amber-colored electric lamp, while the electric lamp has a color-neutral appearance in the off-state.

The combination of a light-absorbing layer and an interference film achieves a synergetic effect. Firstly, the optical interference filter will further modify the emission spectrum of the electric lamp. Secondly, the presence of the interference film may increase the stability of the light-absorbing layer in that the interference film serves as an oxygen barrier for the light-absorbing layer.

A further advantage of an electric lamp comprising a combination of a light-absorbing layer and an interference film is that the spectral characteristic of the light-absorbing layer are less sensitive to variations in the thickness and/or the concentration of the light-absorbing layer.

According to one embodiment, the optical interference filter is of the bandpass type, the object being to tune the optical characteristics of the electric lamp further so as to show a desired color, such that the color point of the light emitted by the electric lamp meets statutory regulations.

Alternatively, the optical interference filter may be provided as a filter of the long wavelength pass type,

According to another embodiment, the optical interference filter is of the broadband reflective type. As silicon suboxide has a relatively high refractive index, in one embodiment of the invention at least one of the layers of a first material of the interference filter comprising a material having a high refractive index may comprises sub-stoichiometric silicon oxide SiO _x , with 0.1 < x < 1.8.

According to a further embodiment of the invention, at least one of the layers of a first material comprising a material having a high refractive index may be selected from the group formed by iron oxide, iron oxide doped with phosphorus, zinc- iron oxide, neodymium oxide, bismuth vanadate, zirconium-praseodymium silicate, or mixtures thereof.

The light-absorbing layer comprising silicon suboxide is preferably deposited as a thin- film layer by physical or chemical vapor deposition, particularly by evaporation of silicon monoxide, by sputtering of a target comprising silicon monoxide, by sputtering of a silicon-containing target in an oxygen-comprising atmosphere, or by low-pressure and plasma-enhanced chemical vapor deposition from oxygen- and silicon- containing precursors to obtain a silicon suboxide material. Fabrication techniques based on physical or chemical vapor deposition provide significant improvements in the properties of thin- film coatings, including higher density, higher refractive index, decreased sensitivity to temperature and moisture, and superior mechanical characteristics, such as durability and scratch resistance.

According to a preferred embodiment of the invention, the light-absorbing layer as well as the interference filter is deposited by physical or chemical vapor deposition.

An electric lamp according to the invention is advantageously used in automotive applications, for example as a (halogen) headlamp which emits yellow light in operation, as an amber-colored light source in indicators (also referred to as vehicle signal lamps), or as a red-colored light source in brake lights.

Such electric lamps are also useful for general illumination purposes. Said

electric lamps are further used in traffic and direction signs, contour illumination, traffic lights, projection illumination, and fiber optics illumination.

BRIEF DESCRIPTION OF THE DRAWINGS Fig. 1 is a cross-sectional view of an embodiment of the electric lamp in accordance with the invention. Fig. 2 is a side elevation of an alternative embodiment of an electric lamp in accordance with the invention.

Fig. 3 is a graph showing the absorption coefficient α of a light-absorbing layer of silicon suboxide as a function of the wavelength λ.

Fig. 4 shows, in a part of a C.I.E. 1931 color triangle diagram, the color co-ordinates of the various light-absorbing layers in accordance with the invention together with the chromaticity ranges of ECE 37 and SAE regulations for signal (indicator) color light. Fig. 5 is a graph showing the spectral reflectance and transmittance characteristics of an exemplary coating comprising a light- absorbing layer according to the invention and an interference filter coating.

DETAILED DESCRIPTION OF THE INVENTION

The invention relates to an electric lamp comprising a light-transmitting lamp vessel in which a light source is arranged. The light source of the electric lamp may be any light source. Typically, the light source is the incandescent filament of an incandescent lamp, but it may alternatively be an electrode pair in an ionizable gas of a discharge lamp, for example an inert gas comprising metal halides and possibly also mercury as buffer gas. The light source is preferably an incandescent filament in an incandescent tungsten halogen incandescent lamp for automotive and non-automotive applications.

Figure 1 is a cross-sectional view of an embodiment of an electric automotive headlight lamp in accordance with the invention. Said electric lamp has a light-transmitting lamp vessel 1 , for example made of glass, which is sealed in a gastight

manner and which accommodates two light source elements 2, 3, helical tungsten incandescent filaments which are connected to current conductors 4, 5, 6 which extend from the lamp vessel 1 to the exterior. The current conductors are connected to contact tags 8, 9 of the socket 10. Electric element 3 provides the main-beam headlight, while electric element 2 together with reflector 7 provides the low-beam headlight. The lamp vessel 1 is filled with a gas containing a halogen, for example hydrogen bromide. Part of the lamp vessel 1 is covered with a light-absorbing layer 12 in the form of a light- absorbing thin-film coating, which is formed by silicon suboxide. The lamp also comprises an anti-glare cap 11. Figure 2 is a side elevation of an alternative embodiment of the electric incandescent lamp in accordance with the invention. Said electric lamp comprises a lamp vessel 1 accommodating a helical coiled-coil filament 2 that is stretched between pole wires 13 so as to be substantially perpendicular to the lamp axis. In the embodiment of the electric lamp shown in Figure 2, the light- absorbing layer 12 is provided in the form of a light-absorbing coating on the entire outer surface of the lamp vessel 1.

The light-absorbing layer comprises sub-stoichiometric silicon oxide SiO _x as a light-absorbing material, wherein 0.1 < x < 1.8 ("silicon suboxide"). A composition range between x =0.3 and x = 1.4 is preferred.

For manufacturing the light-absorbing layer comprising sub- stoichiometric silicon oxide, a thin- film technology, such as physical or chemical vapor deposition, is preferably used.

Physical vapor deposition (PVD) processes are particularly well suited to the mass production of coatings for electric lamps because this type of process is compatible with thickness monitoring and automated control techniques. Physical vapor deposition is a process in which material vaporized from a solid source is transported in the gas phase through a vacuum or low-pressure gaseous or plasma environment and subsequently condensed onto a substrate. Vacuum deposition by thermal resistive or arc evaporation, by sputtering, and by ion plating techniques are known PVD processes. According to a preferred embodiment, a sputtering deposition process is used to produce the light-absorbing layer comprising silicon suboxide. A target

comprising silicon is electrically negatively biased in a typical range of 200 to 1000 V DC to attract positive ions of the working gas (e.g. argon) toward the target. Ions generated in a process region can bombard the target surface with sufficient energy in order to dislodge atoms from the target. The process of biasing a target to generate a plasma that causes ions to bombard the target and remove silicon atoms from the target surface is commonly called sputtering. The sputtered atoms travel generally towards the lamp vessel substrate that is to be sputter-coated, and the sputtered atoms are deposited on the lamp vessel.

The silicon atoms react with oxygen gas in the plasma to reactively deposit the silicon suboxide compound on the lamp vessel. This variant of the process is generally known as "reactive sputtering". Reactive sputtering is preferably used to form thin layers of silicon suboxide on complex lamp shapes.

DC magnetron sputtering is the most widely practiced commercial form of sputtering. A magnetron having a pair of opposed magnetic poles is disposed near the back of the target to generate a magnetic field close to and parallel to the front face of the target. The induced magnetic field traps electrons and extends their lifetime before they are lost to an anodic surface or recombine with gas atoms in the plasma. As a result of the extended lifetime and of the need to maintain charge neutrality in the plasma, additional argon ions are attracted into the region adjacent to the magnetron to form a high-density plasma there. The sputtering rate is increased thereby.

A layer of sub-stoichiometric silicon oxide manufactured by physical vapor deposition, particularly by reactive electron-beam sputtering, forms a very dense, compact layer with improved longevity at reduced cost.

The refractive index of such layers ranges between 4.1 and 1.7, depending on the specific composition and method of preparation. Typically, the refractive index is between 3.8 and 2.3.

The light-absorbing layer is applied to at least a portion of the outer surface of the lamp vessel, as shown, and may be applied to the entire exterior, as desired. The light-absorbing layer comprising silicon suboxide of the general formula SiO _x , with 0.1 < x < 1.8, preferably 0.3 < x < 1.4, has yellow, amber, or red

color characteristics. The removal of oxygen in the composition shifts the absorption towards the longer wavelength range.

The film thickness influences the color characteristics of the light- absorbing layer. In a coating comprising no further color correctors, the film thickness is preferably in a range from 20 nm to 2000 nm. If the film thickness is less than 20 nm, absorption hardly takes place, and a chromaticity satisfying various standards cannot be achieved. If the thickness of the layer exceeds 2000 nm, then too much light is absorbed, which adversely affects the lumen output of the electric lamp. It should be noted that even thick layers do not tend to crack under thermal loads. Preferably, the uniform thin layer has a layer thickness from 500 nm to

1000 nm, which thickness is sufficient to allow light to exit from the lamp with a yellow color that meets the ECE specification for vehicle amber signal lamps.

If the electric lamp having the aforementioned structure is lit, the light radiated from the filament coil or the discharge area is transmitted through the film comprising silicon suboxide formed on the surface of the lamp vessel. The violet, blue, and green parts of the radiation are abated by the layer comprising silicon suboxide. As a result, the color of the emitted light is shifted into the yellow to amber or red range, the specific color being dependent on the value of the parameter x representing the amount of oxygen in the silicon suboxide, on the density of the material, and on the thickness of the layer.

Fig. 3 shows the measured absorption properties of a thin- film layer sample of SiO _x , with O.l ≤ x ≤ l.8. It is a spectral graph in which the absorption coefficient α of the thin-film layer comprising SiO _x , with 0.1 < x < 1.8, in units [nm ¹ ] is plotted as a function of the wavelength λ in nm of the radiation. Thus an electric lamp comprising a light-absorbing layer comprising silicon suboxide can provide inter alia a yellow to amber colored emission that meets both colorimetry and luminance requirements under the applicable SAE and ECE requirements for exterior vehicle lamps.

Fig. 4 shows a variety of color points of yellow to amber colored coatings comprising silicon suboxide for use in warm-white headlights or in signal lamps, with

respect to their x-and y-coordinates in the CIE chromaticity diagram. It should be noted that the color point is not only influenced by the exact composition of the sub- stoichiometric silicon oxide, but also by the deposition parameters and by the thickness of the thin-film layer. In Europe, an amber color point for automotive indicator lamps is defined by the ECE 37 regulation known to those skilled in the art. Allowable colors correspond to the narrow area in Fig. 4 defined by the color points (0.571, 0,429), (0.564, 0.429), (0.595, 0.398), and (0.602, 0.398). The larger area in Fig. 4 corresponds to the SAE (Society of Automotive Engineers) requirements. The color coordinates of the amber SAE region are defined by (0.560, 0.440), (0.545, 0.425), (0.597, 0.390), and (0.610, 0.390).

The yellow to red colored, transparent layer comprising silicon suboxide may be provided after further layers of a second material that influences the emission spectrum of the electric lamp have been applied. A co-operation of two or more colored layers applied one upon the other can result in a desired subtractive color as appropriate for a particular application.

According to one embodiment of the invention, a yellow sub- stoichiometric silicon oxide layer may be combined with a second light-absorbing material, e.g. a red material. The second light-absorbing, red material may comprise any suitable inorganic or organic pigment capable of filtering out a substantial amount of light in the wavelength range below 600 nm. Inorganic pigments that can be deposited by physical vapor deposition, such as iron oxide, may be preferably used for this purpose. Alternatively, iron oxide doped with phosphorus, zinc-iron oxide, neodymium oxide, bismuth vanadate, zirconium-praseodymium silicate, or mixtures thereof may be used.

The combined color of the yellow and the red material provides amber to red colored light. In accordance with this embodiment of the invention, a red incandescent lamp is provided having a sealed lamp envelope, a filament located within the lamp envelope, and a coating applied to an external surface of the envelope, wherein the coating contains silicon suboxide and a red pigment. The amber light-absorbing layer comprising silicon suboxide provides an initial filtering of the shorter wavelengths of

visible light, so that the red pigment has to filter out less light and therefore can be applied in a manner that does not significantly impact the overall opaqueness of the lamp. As a result, the lamp produces the desired red colored light at a higher total light output than conventional red painted or coated bulbs. According to a further embodiment of the invention, the electric lamp may additionally comprise an optical interference filter coating formed of alternating layers of a first material having a high refractive index and a second material having a low refractive index.

The transmission or the reflection of specific wavelengths of light can be closely controlled by means of an optical interference filter. An interference filter may thus be designed to transmit specific wavelengths and reflect those which are undesirable. There is minimal absorption of light by the interference filter.

Examples of materials of which the layers of a low refractive index may be composed are magnesium fluoride MgF ₂ , AI2O3, silicon dioxide SiO ₂ (refractive index 1.46), and mixtures thereof. Examples of materials with a high refractive index are titanium oxide (refractive index 2.4-2.8), niobium oxide (refractive index 2.34), tantalum oxide (refractive index 2.18), zirconium oxide (refractive index 2.16), hafnium oxide, silicon nitride, and combinations of said materials.

Preferably, the material of the first layer of the interference film predominantly comprises silicon dioxide. Preferably, the material of the second layer of the interference film predominantly comprises titanium oxide, niobium oxide, or tantalum oxide.

The Ti(VSiO ₂ , Nb ₂ O ₅ /SiO ₂ , or Ta ₂ O ₅ /SiO ₂ interference films preferably comprise only a small number of layers. Preferably, the interference filter film stack comprises at least three and at most approximately ten layers. As a result of the relatively small number of layers, the reflection spectrum of the interference film is relatively uniform. In addition, the manufacturing cost of such an interference film is relatively low.

A particularly favorable effect of such an interference film is that its influence on the color rendering of the electric lamp is also relatively weak. The interference filter and the light-absorbing layer(s) can be positioned in different ways relative to the lamp envelope. However, since the light-absorbing layer

absorbs short-wave radiation, the interference filter, which reflects said radiation, should be disposed at the side of the light-absorbing layer facing away from the filament body. The layer comprising silicon suboxide is preferably positioned between the lamp vessel and the interference filter stack. Alternatively, the layer comprising silicon suboxide may be positioned within the interference filter stack, preferably in proximity to the lamp vessel to ensure effective absorption. Thus, at least one of the layers of a first material comprising a material having a high refractive index may comprise sub-stoichiometric silicon oxide SiO _x , with 0.02 < x < 1.8, preferably 0.05 < x < 1, as the refractive index of silicon suboxide typically is 2.6 to 2.7.

Alternatively or additionally, at least one of the layers of a first material comprising a material having a high refractive index may comprise a material selected from the group formed by iron oxide, iron oxide doped with phosphorus, zinc-iron oxide, neodymium oxide, bismuth vanadate, zirconium-praseodymium silicate, or mixtures thereof. Like silicon suboxide, these materials can be deposited by thin- film techniques and help to decrease the transmission of the interference filter in the violet, blue, and green wavelength ranges. These measures ensure that the incandescent lamp according to the invention emits essentially red light and is suitable for use as a stop light lamp or tail light lamp of an automobile. The interference filter coating may be provided in a customary manner by various deposition techniques, for example a dip-coating or spraying process or chemical vapor deposition, such as low-pressure chemical vapor deposition, plasma-enhanced or plasma impulse chemical vapor deposition. For reasons of economy and improved coating characteristics, both the light-absorbing layer and the interference filter are provided by means of physical vapor deposition, particularly by magnetron sputtering. Experiments have shown that the adhesion of the combination of light- absorbing layer and interference film on the lamp vessel of the electric lamp is excellent and not subject to change during lamp life. No visible delamination of the applied coatings was observed during the service life of the electric lamp in accordance with the invention.

According to one embodiment, the interference filter may be a bandpass

filter which transmits part of the visible light and reflects other parts of the spectrum. In particular, the optical interference filter stack reflects essentially all light except for a selected amount of transmission in a selected yellow, amber, or red wavelength range.

Bandpass filters are the most common interference filters and are designed as very accurate color filters having a high transmittance for a preselected portion of the visible light spectrum and a high reflectance for adjacent spectral ranges. Used in front of the light-absorbing layer, the bandpass filter provides light that is perceived by humans to be highly saturated (intense) in color. The wavelength widths of the passband of the interference filter can be tuned through the control of the thickness and number of layers and can thus be made as wide or narrow as desired.

In addition to the fact that the bandpass interference filter contributes to the desired shift of the color temperature and or color point position, the combination has a number of further advantages. In the first place, light reflected by the interference filter passes through the light-absorbing layer twice, which further improves the effectiveness of the absorption process. Furthermore, light reflected to and between the interference filters on either side of the lamp vessel passes through the light-absorbing layer twice at each reflection. Simultaneously the color rendering index of the electric lamp remains relatively high.

In order to bring about a desired color shift, a bandpass filter with a center wavelength preferably in the range around 590 nm may be used. A combination of an amber-colored coating comprising silicon suboxide and a bandpass interference filter comprises, for example, a 100 nm silicon suboxide layer and a ten- layer TiCVSiO ₂ filter, which, in accordance with a notation known to those skilled in the art, is referred to as: glassjSiO !(LH) ⁵ [air, wherein the geometrical layer thicknesses of the materials having a low and a high refractive index are, respectively, L=50 nm SiC>2 and H=50 nm Tiθ2. Such filters are known from US6570302.

According to an alternative embodiment, the optical interference filter is of the long wavelength pass filter type. Long wavelength pass filters act as their name implies, allowing transmission of radiation of a long wavelength range and reflecting the unwanted radiation in the short wavelength range.

The combination of a light-absorbing layer and a long wavelength pass

interference filter enables the appearance of the electric lamp to be changed. It particularly enables a distinction to be made between the appearance of the electric lamp in the off state and the color of the light emitted by the electric lamp during operation. The aim is, in particular, to provide an electric lamp that emits a certain yellow, amber, or red color in operation, whereas in the off state the electric lamp has an at least substantially color-neutral appearance.

Such lamps are especially useful as indicator lamps or brake lights of vehicles. For reasons of safety, it is important that, apart from a color-neutral appearance, such indicator lamps or brake lights are at least substantially free of color in the reflection of light that is (accidentally) incident on the electric lamp. If, for example, sunlight or light originating from oncoming traffic is incident on a headlamp of a vehicle comprising an indicator lamp, the appearance of said headlamp, in reflection, should be at least substantially colorless or, in reflection, said lamp should emit at least substantially no color. Otherwise, this might confuse other road users and give rise to unsafe and/or undesirable situations.

A favorable embodiment of the electric lamp in accordance with the invention is characterized in that the interference film reflects predominantly in a wavelength range in which the light-absorbing silicon suboxide absorbs. As silicon suboxide absorbs in the violet to blue range of the visible light, a long wavelength pass filter with a transition wavelength in the range of 550 to 590 nm is advantageously combined with the light-absorbing layer comprising silicon suboxide for further suppressing light from the violet, blue, and green spectral regions. Designs of optical interference filters matching the spectral transition of an amber-colored light-absorbing layer in general are disclosed in WO03/071583. The adaptation of the thicknesses of the layers of low optical refraction and high optical refraction in these filters is optimized such that the edge of the interference filter is situated in the wavelength region from 550 nm to 590 nm. An interference filter with comparatively few layers can thus be produced which has a steep transition from the spectral region of low transmission to the spectral region of high transmission in the wavelength region from 550 nm to 590 nm.

The spectral characteristic of the electric lamp in accordance with the

invention in reflection differ from the spectral characteristic in transmission. In transmission, the light emitted by the electric lamp meets statutory regulations with respect to the color point, whereas in reflection the electric lamp is color-neutral, the appearance of the electric lamp being, for example, silvery or grayish. According to a further embodiment of the invention, the optical interference filter is a reflective broadband filter that reflects visible light in the wavelength range from 400 to 690 nm with an average reflectance R between 50 and 90%, as known from WO2005/052986. Such an electric lamp emits also colored light when in operation and has a substantially color neutral appearance in the off state. The appearance of the electric lamp can be silvery.

Example

According to a specific example, a combination of a layer comprising silicon suboxide and a broadband reflective interference filter is provided, wherein the silicon suboxide layer is part of the interference filter stack. Table I shows the number of layers, the materials used, and the physical thicknesses of each layer of this example.

Table 1: Filter design of the specific example

The color co-ordinates (x,y) of an electric lamp provided with said combination are x=0.557, y=0.421 in the transmission mode and x=0.310, y=0.316 in the reflection mode, measured as reflection of a standard light source for daylight. Fig. 5 is a graph showing the spectral reflectance and transmittance characteristics of the exemplary lamp, the reflectance R (black) and transmittance T (grey) being given in percentages as a function of the wavelength λ.

This combination meets the ECER37 standard for amber-colored indicators in the transmission mode. Such a lamp has a color-neutral silvery appearance in the off-state with no significant color shift of any reflected light.

It will be clear to those skilled in the art that many variations are possible within the scope of the invention. The scope of protection of the invention is not limited to the examples given above. The invention is embodied in each novel characteristic and each combination of characteristics. Reference numerals in the claims do not limit the scope of the protection thereof. The use of the word "comprising" does not exclude the presence of elements other than those mentioned in the claims. The use of the word "a" or "an" in front of an element does not exclude the presence of a plurality of such elements.