WO2008139363A2

WO2008139363A2 - System comprising an infrared radiation source and an infrared radiation sensor

Info

Publication number: WO2008139363A2
Application number: PCT/IB2008/051746
Authority: WO
Inventors: Georg Henninger; Bruno Thoennessen
Original assignee: Philips Intellectual Property & Standards Gmbh; Koninklijke Philips Electronics N.V.
Priority date: 2007-05-09
Filing date: 2008-05-06
Publication date: 2008-11-20
Also published as: WO2008139363A3

Abstract

A system comprising an infrared radiation source, an infrared radiation sensor and means for generating and processing optical infrared signals, wherein the infrared radiation source is an electric lamp comprising a pass-band interference filter coating arranged on its inner or outer surface, which integrated pass-band interference filter coating, that is transmissive to infrared radiation in a wavelength range from 800 nm to 1200 nm, is useful for a wide variety of applications in detection, authentification, identification or remote control of objects, and is excellently suited for object detection under rough industrial conditions. Such a system provides a narrowband infrared signal in a specific frequency band that, in combination with a suitable sensor, reduces eavesdropping and is not sensitive to light sources, such as the sun and artificial light sources, that emit radiation in the visible and IR range of the electromagnetic spectrum.

Description

SYSTEM COMPRISING AN INFRARED RADIATION SOURCE AND AN INFRARED RADIATION SENSOR

BACKGROUND OF THE INVENTION

The present invention relates generally to a system comprising an infrared radiation source, an infrared radiation sensor and means for generating and processing an optical infrared signal. Such systems are useful in a wide field of applications such as analytical, imaging and diagnostic instrumentation. In addition, various remote control systems use infrared radiation as a means for controlling a main device. Such remote controls are found and used for operating and controlling many devices, especially consumer electronic devices. Most of the public is familiar with a remote control unit for controlling their television sets and VCRs. Other devices and systems, such as alarm systems, access control of persons or goods to buildings, to transportation means, to general equipment, training equipment or monitoring systems etc. may also be controlled by remote control devices.

Remote control units typically operate by emitting a sequence of infrared pulses that are received by the controlled device. The particular sequence provided reflects an encoded command (such as on, off, adjust volume, change channel) that is recognized by the controlled device. When received by the controlled device, the command is recognized and executed by the device.

Thus, a remote control unit system comprises an infrared source together with an infrared sensor that detects incident infrared radiation for decoding and processing.

The frequency ranges used by different manufacturers of remote control devices differ widely. Yet eavesdropping is a problem. Therefore, an infrared radiation source that reduces eavesdropping is needed. Systems comprising IR sources and IR sensors are also used in identification of infrared readable patterns, such as identification means. Such identification means may include IR-readable tags and barcodes, especially when comprised of IR- fluorescent ink. In optical identification systems, a potential tag or barcode is interrogated by a signal from a radiation source and, if no coded signal is returned, the tag or barcode is deemed to be legitimate.

The radiation source in identification systems typically incorporates an infrared light emitting diode (LED). Systems comprising light emitting diodes as the radiation source have the disadvantage, that, because LEDs exhibit a strong directional preference in light emission, a light diffuser must be incorporated into the light source, so as to allow identification from a range of different orientations relative to the badge.

A system for identification of primarily man-made objects, where a transponder at the object is arranged to be energized by an essentially infrared light beam at 700 - 1100 nm, and/or by visible light, which transponder sends messages to a reader via an information beam at 700 - 1100 nm, is known from WO 2004/081849. The transponder may be energized by directional light from an incandescent lamp that uses a pressurized inert gas and that preferably uses a halogen gas to avoid depletion of filament material on the inside of the lamp glass, and that has an integrated reflector that is reflective to infrared light at 700 - 1100 nm. The visible part of the energizing light beam from said lamp may be reduced by a long-pass filter in front of said lamp.

SUMMARY OF THE INVENTION

It is an object of the invention to provide a system comprising an infrared radiation source and an infrared radiation sensor of the kind for use as described above, which emits less undesirable radiation during operation and diminishes eavesdropping. Accordingly the invention provides a system comprising an infrared radiation source, an infrared radiation sensor and means for generating and processing optical infrared signals, wherein the infrared radiation source is an electric lamp comprising an integrated band-pass interference filter coating that is transmissive to infrared radiation with a wavelength range from 800 nm to 1200 nm. According to a preferred embodiment of the invention said lamp comprises an integrated band-pass interference filter coating of the narrowband type that is transmissive to infrared radiation within a wavelength range from 800 nm to 1200 nm with a pass-band wavelength width of 200 nm. The integrated narrowband interference filter coating may be transmissive to infrared radiation with a pass-band wavelength range from 800 nm to 1000 nm. Alternatively, the integrated narrowband interference filter coating may be transmissive to infrared radiation with a pass-band wavelength range from 1000 nm to 1200 nm.

Such a system provides a narrowband infrared signal in a specific frequency band that, in combination with an optoelectronic sensor, reduces eavesdropping and is not sensitive to other radiation sources, such as the sun and artificial light sources that emit radiation in the visible and IR range of the electromagnetic spectrum.

Accordingly, the system is useful for a wide variety of applications in detection, authentification, identification and remote control of objects, and is excellently suited for object detection under rough industrial conditions.

It is a characteristic of the system that variants with wide-angle optical systems are possible.

According to a preferred embodiment of the invention, the integrated narrowband interference filter coating is a thin film optical interference filter coating comprising a plurality of alternating high and low refractive index layers (H, L).

In one embodiment of the invention the thin film optical interference filter coating comprises at least three multiperiod, spectrally adjacent stacks comprising a plurality of alternating high and low refractive index layers (H, L).

Preferably, the optical interference filter coating has the following structure: a(H/2LH/2)^x b(H/2LH/2)^y c(L/2HL/2)^z , with 0.6<a<1.6; 0.8<b<1.9; 1.6<c<3.6; 3<x<8, 3<y<8 and 3<z<12 and a reference wavelength between 450 and 550 nm. The optical interference coating of such a structure produces maximum infrared reflection in a portion of the electromagnetic spectrum where infrared emission of the electric lamp is the highest.

As the material of the first layer (L) having the first refractive index, a material predominantly comprising silicon oxide or aluminum oxide is preferred.

As the material of the second layer (H) having the second refractive index, a material chosen from the group formed by titanium oxide, zirconium oxide, hafnium oxide, niobium oxide, tantalum oxide and physical mixtures or layered composites thereof is preferred.

In a preferred embodiment of the invention the system comprises an infrared sensor, which is selected from the group of optoelectronic sensors. In such a system the wavelength (or spectral distribution) of the radiation source is selected in a way such that the source only (or essentially) energizes the sensor.

If said band-pass interference filter coating is arranged on the inner or outer surface of the lamp vessel, the lamp emits IR radiation in a 360° distribution - contrary to an arrangement wherein the filter coating is deposited on a separate plate. Typically, in the system according to the invention the electric lamp is an incandescent halogen lamp containing halogen gas or a halogen compound in the lamp vessel.

If the shape of said lamp vessel is elliptical or spheroidal, radiation in the visible and in the FIR-range is reflected back to the filament and improves the performance of the lamp.

If the shape of said lamp vessel is cylindrical, subsequent processing of the infrared signal emitted by the lamp is facilitated in a range of different orientations.

According to one embodiment of the invention the electric lamp comprises a reflector comprising an IR-reflective coating. Especially if an interference filter coating is chosen, the focus of the IR signal is improved.

The invention also relates to an electric lamp comprising a lamp vessel, said lamp comprising an integrated band-pass interference filter coating that is transmissive to infrared radiation with a wavelength range from 800 nm to 1200 nm.

DETAILED DESCRIPTION

The invention relates to a system for, inter alia, identification or remote control of objects comprising an infrared radiation source and an infrared radiation sensor, wherein the infrared radiation source is an electric lamp comprising an integrated band-pass interference filter. Typically, the infrared radiation source is adapted by operating means to produce a beam of infrared radiation that extends to the infrared radiation sensor. Said operating means may comprise pulse-generating means adapted to produce a sequence of signal pulses, means coupled to said pulse-generating means for operating said radiation source in said predetermined sequence in response to signal pulses from said pulse-generating means, and control means for varying the output of said pulse- generating means. Also optical means for collecting and/or reflecting the beam may be incorporated into the system. Optical means typically include a projection portion and a collection portion. The projection portion directs the infrared radiation along a projection path extending from the infrared radiation source to the target region of an object. The collection portion collects the light reflected from an object when the object occupies the target region and directs the collected light along a collection path extending from the target region to the sensor.

The infrared radiation sensor, such as a photodiode sensor, receives the collected beam and converts the beam into an electrical signal representative of the received radiation. Preferably, for the sensor use is made of optoelectronic sensors, such as photocells and photodiodes, which are based on the photoelectric effect and are dependent on the wavelength range of the radiation that energizes them. Otherwise, temperature sensors, based on the thermoelectric effect, such as pyroelectric detectors, which work independently of the specific wavelength range, are used. The above-mentioned object of the invention is achieved primarily by providing an infrared radiation source that comprises an electric lamp comprising a lamp vessel having an interference band-pass filter arranged on its inner or outer surface to transmit radiation in the near infrared range from 800 to 1200 nm and to reflect visible light and radiation in the far infrared range. An electric lamp to be used for the purpose of the present invention may be any electric lamp, such as a discharge lamp or an incandescent lamp. Preferably, it is an incandescent halogen lamp comprising a filament in a lamp vessel, filled in a known way with an inert gas mixture comprising a halogen additive. Preferably, the lamp vessel consists of IR- transmissive glass, quartz glass or fused quartz. Incandescent halogen lamps are conventionally used for illumination of commodities as well as for other applications that require a high degree of color rendering. Such incandescent lamps emit not only visible light but also a considerable amount of infrared radiation.

For use in the present invention, the incandescent halogen lamp is preferably operated at mains voltage and therefore at an elevated operating temperature, above 2500° K and in particular above 2900° K, so that the radiation it emits has its major component in the near-infrared region, specifically in a wavelength region between 800 nm and 1200 nm.

According to a first variant of the invention, the electric lamp geometry is designed so that infrared light emitted from the filament and transmitted by an interference coating arranged on the lamp vessel forms a stream of infrared radiation evenly distributed in a 360° angular distribution.

To this aim, the lamp vessel has the shape of a cylindrical tube, which contains in its center an elongated spiral filament and has at each of its two ends a connector pin.

The radiation emitted by a lamp with a vessel of this type and comprising an elongated spiral filament manifests itself in the form of cylindrical infrared radiation in a 360° angular distribution centered around the filament.

Yet, when such a lamp is used in combination with an infrared sensor which has a planar receiving surface, the infrared radiation distribution as received by said surface is inhomogeneous, i.e. the regions of the receiving surface closest to the axis of the envelope are submitted to a more intense radiation than the regions of the receiving surface which are farther removed from said axis. Such inhomogeneity has adverse effects, because a nominal radiation level of the regions of the receiving surface closest to the axis of the envelope will imply an insufficient level of the regions farthest removed from the axis. Therefore, in one embodiment of the invention, the radiation source is formed by at least one filament of flat shape, by a plurality of co-planar filaments or by at least one fan- folded filament. Such arrangements of filaments are known in the art.

According to an alternative variant of the invention, the electric lamp is designed so that infrared light, emitted from the filament and transmitted by the interference layer arranged on the lamp vessel, forms a directional stream of infrared radiation.

To this aim, the halogen filament lamp is provided with a lamp vessel in the shape of a spheroid, e.g. ellipsoidal, which contains in its center a coiled coil filament and has a single-ended pinch arrangement.

An elliptical or spheroidal geometry of the lamp vessel, bearing a reflective coating, causes the reflected radiation to be reflected back to the light source, the filament. The reflected radiation assists in maintaining the operating temperature of the light source, thereby improving the energy balance of the electric lamp.

To provide for a directional type of radiation distribution, this type of lamp is preferably combined with a reflector positioned around said lamp to guide the light emitted from said filament in substantially one direction. The reflector may bear a reflector coating, said coating being designed to reflect the IR-radiation that is transmitted by the optical interference coating on the lamp vessel. The reflector coating material may be selected from metallic coatings, such as aluminum, or it may be a thin film interference coating.

The radiation emitted by a reflector lamp of this type manifests itself in the form of directional infrared radiation, which originates from the filament. This arrangement greatly increases the contrast and detectability of the lamp's infrared emissions.

If an electric lamp is used as an infrared source, radiation coming from the lamp inevitably comprises continuous radiation in the near infrared range (NIR) together with radiation in the middle infrared range (MIR) and far infrared range (FIR) as well as visible light, which cause deterioration of the quality of an electric lamp as a narrowband IR- radiation source useful for remote control and authentification.

In order to eliminate this problem, an electric lamp according to the invention is provided with a interference filter coating designed so that visible light and most IR-radiation is reflected back by the interference filter, while only NIR radiation in the range from 800 to 1200 nm passes through it.

The interference filter coating is designed as a band-pass filter that reflects the emitted energy above and/or below the desired NIR infrared wavelength range back to the filament, while transmitting the desired wavelength range between 800 and 1200 nm.

According to an especially preferred embodiment of the invention, the interference band-pass filter is a spectrally narrow band-pass filter with a bandwidth of 200 nm and has a spectrally narrow high transmittance of on average at least 90% between 800 to 1200 nm and a spectrally broad high reflectance (low transmission) of on average at least 80% (20%) between 400 to 800 nm and above 1200 nmup to 2200 nm.

The area covered by the interference filter coating may cover the total surface of the lamp vessel or otherwise be defined by a contour line of an emitting aperture and omit selected parts of the lamp vessel. As the omitted parts of the lamp vessel will provide visible light during lamp operation, this will allow for an efficient and inexpensive way of knowing whether or not the system is operating correctly.

Advantageously the interference filter is a thin film optical interference filter coating 4.

As shown schematically in Fig. 2, a thin film optical interference filter coating 4 for selectively reflecting and transmitting different portions of the electromagnetic spectrum comprises a plurality of alternating layers of a low refractive index material (represented by L) and a high refractive index material (represented by H). Materials that can be advantageously used for the highly refractive sublayers H include titanium oxide (TiO₂), zirconium oxide (ZrO₂), hafnium oxide (HfO₂), niobium oxide (Nb₂Os) and tantalum oxide (Ta₂Os) and physical mixtures and multilayer arrangements thereof. Silicon oxide ( SiO₂) or aluminum oxide (Al₂O₅) and physical mixtures and multilayer arrangements thereof may be preferably used for the lowly refractive sub-layers L.

In a preferred embodiment of the invention, the thin film optical interference filter coating comprises at least three multiperiod, spectrally adjacent stacks, each comprising a plurality of alternating high and low refractive index layers (H, L).

It is preferred that the first two stacks are placed in a way that their first order transmission blocking regions are placed within or next to the visible region and that the transmission blocking region of the third stack is placed in the IR region.

Such a three-multiperiod filter stack is represented in the following manner, which is known to those skilled in the art: a(H/2LH/2)^x b(H/2LH/2)^y c(L/2HL/2)^z , with 0.6<a<1.6; 0.8<b<1.8; 1.6<c<3.6; 3<x<8, 3<y<8 and 3<z<12 and a reference wavelength between 450 and 550 nm, preferably at 510 nm. Materials L and H each have an optical thickness defined as one-quarter of the reference wavelength, or a quarterwave optical thickness.

Layers forming a period are surrounded by brackets, with the superscripts x, y and z being the number of times the period is repeated in the stack. The values for the denominators a, b, c and d are chosen based upon the required optical thickness T₀ of each layer according to the formula: T₀ =λ/(4 x denominator), wherein λ is the reference wavelength. The physical thickness T_p of each layer is equal to the optical thickness T₀ divided by the index of refraction of the material. Accordingly, the notation a*L represents a fraction of a quarterwave of optical thickness of the L material at the reference wavelength, i.e., one-half of a quarterwave (1/8 wave) for a=0.5.

The factors a, b and c are chosen in such a way that the first stack reflects the light that is above the desired wavelength range and the other two stacks reflect the near- visible and far- visible light that is below the desired wavelength region.

The sequence of the stacks can be exchanged (e.g. long-wave against short-wave pass stack) and further stacks can be added to narrow the bandwidth of the pass band.

Further stacks are preferably added in the IR region. For example, a long- wave pass stack with blocking features in the NIR region next to the visible region can be added. It shifts the transmission pass region to higher wavelengths. Otherwise, a further short-wave pass filter can be added to increase the amount of FIR reflected by the filter.

According to one embodiment of the invention, the highly refractive sub- layers themselves are composed of a sub-stack of two layers of one highly refractive material and a thin intermediate layer of a second highly refractive material known from the prior art. The thickness of the intermediate layer is preferably in the range from 1 to 25 nm. This intermediate layer will avoid extended crystal growth in the highly refractive layer. How many times the various stacks Sl, S2 and S3 are repeated, in other words the choice for the exponents x, y and z, is determined on the basis of an analysis of the maximum increase of the reflectance per thickness increase of the filter design. Analysis has shown that it is very favorable to choose as values for x, y and z: 3<x<8, 3<y<8 and 3<z<8, if a high reflectance is desired. A further increase of the values for y and z generally has the advantage that the width of the transmission window decreases. Otherwise, such an increase is undesirable because it causes the tolerance of the filter design with respect to variations in layer thickness occurring during the manufacturing process of the interference film to be reduced.

The (physical) layer thicknesses of the H-L interference filter coating in accordance with the invention are the result of computer optimizations, which are known per se. Computer optimization is used to balance the need for high infrared transmission and minimum layer count. The necessary calculations are applied to the complete filter design. Table 1 shows the number of layers and the physical thickness of each layer.

FIG. 3 illustrates a representation of an interference filter according to the present invention, consisting of three spectrally adjacent multiperiod stacks Si, S₂ and S₃, each having a respective reference wavelength of λr_ef. By spectrally adjacent is meant that the longest high reflectance wavelength of one stack coincides approximately with the shortest high reflectance wavelength of the other stack. A reference wavelength is defined as the wavelength at which the strongest reflection is located.

The first stack Si is the broadest wavelength stack and is a conventional long-wave pass stack filter having a stack design generally expressed as *[L/2 H L/2]^x. Stack Si is considered a longwave pass filter since it has very high reflectance at wavelengths shorter than the reference wavelength and then a region of substantial transmission at wavelengths longer than the design wavelength.

The first stack is placed in such a way that its first order transmission- blocking region is placed within or next to the visible region. The number of periods y in the first stack Si is generally greater than or equal to 5.

The second or middle stack S2 is a second conventional long-wave pass stack filter having a stack design generally expressed as b*[L/2 H L/2]^y. The second stack is also placed in such a way that its first order transmission-blocking region is placed within or next to the visible region. The reference wavelength of the second short-wave stack is typically longer than the reference wavelength of the first short-wave pass stack Si. The number of periods y in the second stack S2 is generally greater than or equal to 3.

The third stack in the IR is a short-wave pass filter having the structure

c * (— H — Y @ reference wavelength. The short-wave pass filter stack

in the IR is placed in such a way that at least one higher order transmission blocking region of the stack is placed in the visible region between 380 nm to 780 nm

The long and/or short-pass filters in the visible region are placed in such a way that their transmission blocking regions are adjacent to each other and to the blocking region of the short-wave pass stack in the IR. In this way the transmission of the light in the visible region can most effectively be blocked directly without further optimization of the filter.

If another reference wavelength is chosen, the denominators a, b and c must be adapted accordingly to reach the same result for every one of the stacks given.

In the case that the order of the stacks is to be interchanged, it is preferred that the outermost layer next to ambient is chosen to be the low index material layer L and in the case that the outermost layer is not a low index material layer L, such a layer is added to the filter design.

Table I shows the filter designs, i.e. layer structure, layer materials and layer thickness of the optical interference band-pass filter coatings presented in Figures 4 to 17 having a transmission bandwidth in the range from 800 nm to 1200 nm (designs A to A4), 800 to 1000 nm (designs C to C6) and 1000 nm to 1200 nm (designs D and Dl) and reflecting visible light and far infrared radiation in accordance with the invention. Designs D and Dl in addition also reflect near infrared radiation.

Table 1

Figures 4 to 17 show the transmission spectra of the fourteen embodiments A to Dl of Table 1 to illustrate the scope of this invention.

Figure 4 shows the transmission spectrum as a function of the wavelength λ (in nm) of an infrared-reflecting interference film composed of 37 alternate layers of SiC>2 and Nb₂Os of filter design A having the structure

Substrate 0.9 * (— L— )⁶ 1.3 * (— L— )⁶ 3.0 * (-H-) air

2 2 2 2 ' ^{' " v} ° ° ^' where L and Η denote quarter waves (λ/4) at a reference wavelength of 5 lOnm (abbreviated @5 IOnm in the following), for SiC>2 and Nb₂Os respectively. The transmission range is in the infrared wavelength range from 800 to 1200nm.

In the visible wavelength range from 300 to 760 nm and in the FIR range from 1200 to 1800 nm, the transmittance of the interference film in accordance with the invention is below, on average, at least 10%. The transmittance is high, approximately 90%, in the wavelength range from 800 to 1200 nm.

Figure 5 shows the filter design Al with the same structure as design A in an optimized design.

Figure 6 shows the filter design A2 having the structure,

Substrate 0.9 * (— L— )⁴ 1.3 * (— L— )⁵ 3.0 * (-H-) air @510nm.

2 2 2 2 ' ^{' " v} ° ° -

The transmission range is between 800 and 1 lOOnm. This design has a reduced number of layers as compared to design A, but a higher transmittance level in the range from 1300 nm to 1800 nm.

Figure 7 shows the filter design A3 with the structure

Substrate 0.9 * (-H-)⁷ 1.3 * (— L—f 3.0 * (-H-)³ air @480nm.

^V2 2 2 2 2 2

The transmission range is between 800 and 1200nm. It has been found advantageous to interchange the materials in one of the two long-wave pass stacks in the visible range to reduce transmission in the visible range.

Figure 8 shows the filter design A4 with the structure

Substrate 0.9 * (— L— f 1.3 * (— L— f 2.4 * (-— -H-- - O ) ' ⁵ air @480nm.

Z Z Z Z δ δ δ δ z

The transmission range is between 800 and 1200nm. For this design, thin layers are included in the short-wave pass filter in the IR range. Due to this, the transmission in the visible range is reduced, without an increase of the total thickness in comparison to design A. Figure 9 shows the filter design C

Substrate 0.98 * {—L—f 1.3 * (—I—)⁶ 2.4 * (-H-)⁵ air @520nm

having a narrowband transmission range between 800 and lOOOnm. Figure 10 shows the filter design Cl with the same structure as design C in an optimized design. Figure 11 shows the filter design C2 that has the structure

Substrate 0.9 * (— L— )⁶ 1.3 * (— L— )⁶ 2.4 * (-H-)⁵ 3 * (-H-)³ air @510nm

^V 2 2 2 2 2 2 2 2 which is the same structure as design C retrofitted with another shortwave pass filter stack to reduce transmission in the FIR range. Figure 12 shows the filter design C3 with the same structure as design C in an optimized design.

Figure 13 shows the filter design C4 with the same structure as design C in an optimized design. In this design the low transmission area in the FIR is sub-divided into two areas with a transmission well below 10% (between 1050 nm and 1500 nm) followed by a range with a transmission of 20% (between 1500 nm to 1800 nm). This filter coating yields the lowest average transmission in the visible range (380 nm to 780 nm)

Figure 14 shows the filter design C 5 with the structure

Substrate 0.9 * (—L—)⁶ 1.3 * (— L—f 2.4 * (-H-)⁵ 3 * (-H-)⁵ air @510nm

^V 2 2 2 2 2 2 2 2 and a transmission range between 800 and lOOOnm. In comparison to design C the number of layers is further increased to reduce the transmission for even higher wavelength ranges at 1800 nm to 2050nm.

Figure 15 shows the filter design C6 with the same structure as design C5, in an optimized design. The range of low transmission in the FIR is increased to 2200nm.

Figure 16 shows the filter design D with the structure

Substrate 0.76 * (— L — f 1.28 * (— L—f \.6 * (—L—Ϋ 2.84 * (-H-)⁶ air

^V 2 2 2 2 2 2 2 2

@530nm and a narrowband transmission range between 1000 nm and 1200 nm. In the near infrared region between 800 nm and 1000 nm the transmission is reduced to 10% by introduction of a third long-wave pass filter stack.

Figure 17 shows the filter design Dl with the same structure as design D in an optimized design.

Fig. 1 of the accompanying drawings illustrates a special embodiment of a lamp according to the invention. The electric lamp 1 is an incandescent lamp comprising a cylindrical lamp vessel 2 that is permeable to light and infrared radiation, e.g. a lamp vessel made out of quartz glass. The inner volume of the envelope 2 is filled in a known way with an inert gas mixture comprising a halogen additive. One end of the lamp vessel bears a dome with an exhaust tip in the center. The other end of the vessel is hermetically sealed with pinch 6. The substantially parallel outer surfaces of the single pinch 6 are arranged in the center and symmetrically relative to the lamp axis.

Inside the vessel, means are arranged for structurally and electrically mounting a coiled coil filament 12. These means comprise two lead-wires 8, 10 that extend through the pinch 6 to metal contact pins 14, 16 for connecting the lamp to mains voltage, i.e. 220-240 V in Europe and 110-130 V in the US. The filament 12 comprises a coiled coil middle section. Its two ends, which are connected to lead- wires 8, 10, each are single-coiled. Of course, it is equally possible to use the coiled coil filament according to the invention with a double-ended lamp, wherein the lead wires are arranged at opposite ends of the envelope. The outer surface of the lamp vessel 2 is bearing a niobia- silica optical thin film interference coating 4, which transmits radiation in the range from 800 to 1200 nm and reflects radiation outside of this wavelength range.

When the power switch for the halogen lamp 1 is turned on to energize the latter, the filament 12 begins to glare and emits radiation, which is transmitted through the lamp vessel 2 and then mostly reflected by the interference layer 4.

More specifically, radiation emitted from the filament 12 passes through the lamp vessel 2 and then hits the interference layer stack 4 arranged on the outer surface of the lamp vessel 2 to reflect visible light and far infrared radiation and transmit infrared radiation in the range from 800 nm to 1200 nm. Since the lamp vessel 2 and the interference layer 4 arranged on the outer surface of the lamp vessel 2 are substantially cylindrical, the transmitted beam of infrared radiation is securely directed in a direction where the sensor is located. Consequently, the system will present enhanced efficiency.

The visible portion of the light that has reached the interference layer 4 is reflected by it. Therefore, the radiation source as well as the sensor or the object of illumination is totally free from visible light emitted by the lamp and hence detection of the system by unauthorized persons is obviated.

The scope of protection of the invention is not limited to the examples given herein. It will be clear that, within the scope of the invention, many variations are possible to those skilled in the art.

BRIEF DESCRIPTION OF THE DRAWINGS In the drawings:

FIG. 1 is a side view of an electric incandescent lamp. FIGS. 2 and 3 show an interference filter design in a schematic view. FIG. 4 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention.

FIG. 5 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention. FIG. 6 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention. FIG. 7 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention. FIG. 8 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention. FIG. 9 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention.

FIG. 10 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention. FIG.11 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention. FIG. 12 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention. FIG. 13 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention. FIG. 14 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention.

FIG. 15 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention. FIG. 16 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention. FIG. 17 shows the transmission spectrum of a lamp comprising an interference filter in accordance with the invention.

Claims

CLAIMS:

1. A system comprising an infrared radiation source, an infrared radiation sensor and means for generating and processing optical infrared signals, wherein the infrared radiation source is an electric lamp comprising an integrated band-pass interference filter coating that is transmissive to infrared radiation in a wavelength range from 800 nm to 1200 nm.

2. A system according to claim 1, wherein said electric lamp comprises an integrated band-pass interference filter coating of the narrowband type that is transmissive to infrared radiation in a wavelength range from 800 nm to 1200 nm with a pass-band wavelength width of 200 nm.

3. A system according to claim 2, wherein the integrated narrowband interference filter coating is transmissive to infrared radiation with a pass-band wavelength range from 800 nm to 1000 nm.

4. A system according to claim 2, wherein the integrated narrowband interference filter coating is transmissive to infrared radiation with a pass-band wavelength range from 1000 nm to 1200 nm.

5. A system according to claim 1, wherein the integrated narrowband interference filter coating is a thin film optical interference filter coating comprising a plurality of alternating high and low refractive index layers (H, L).

6. A system according to claim 5, wherein the thin film optical interference filter coating comprises at least three multiperiod, spectrally adjacent stacks comprising a plurality of alternating high and low refractive index layers (H, L).

7. A system according to claim 6, characterized in that the interference filter coating has the following structure: a(H/2LH/2)^x b(H/2LH/2)^y c(L/2HL/2)^z , with O.ό≤a≤l .6; O.δ≤b≤l .8; 1.6<c<3.6; 3<x<8, 3<y<8 and 3<z<12 and a reference wavelength between 450 and 550 nm.

8. A system according to claim 1, wherein said band-pass interference filter coating is arranged on the inner or outer surface of the lamp vessel.

9. A system according to claim 1, wherein said electric lamp is an incandescent halogen lamp containing halogen gas or a halogen compound in the lamp vessel.

10. A system according to claim 1, wherein the electric lamp comprises a reflector bearing an IR-reflective coating.

11. An electric lamp comprising an integrated band-pass interference filter coating that is transmissive to infrared radiation with a wavelength range from 800 nm to 1200 nm.