GB1603846A - Point light source - Google Patents

Description

(54) POINT LIGHT SOURCE (71) We, THORN EMI LIMITED, formerly known as THORN ELECTRICAL INDUSTRIES LIMITED, a British Company of Thorn House, Upper Saint Martin's Lane, London, WC2H 9ED., do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to electric discharge lamps of the type including, supported in a sealed light transmitting envelope, a first arc electrode constituted by a body of refractory metal and a second electrode spaced from the first by an arc gap so that in operation the refractory metal body is heated to incandescence by an arc between the electrodes, the incandescent body and not the arc being the principal source of light emitted.Such a lamp, which will be called herein a lamp of the type specified, is frequently called a 'point' light source in view of the relatively small light emitting area.

Point source lamps are of particular value in optical and photographic applications.

Earlier lamps of this kind included the 'Pointolite' lamp which was essentially a small arc lamp enclosed in a sealed envelope or bulb with an inert gas fill. The source brightness of Pointolite (Registered Trade Mark) lamps varied between 700 and 1500 candles/cm2 (stilb) with lamp lives of 300-500 hours.

Among their disadvantages was the fact that bulb blackening necessitated the use of relatively large envelopes, which restricted their usefulness in optical systems.

Although these lamps have been superseded by mercury or xenon arc discharge lamps having a higher source brightness, there is still a need for a point source lamp having the qualities of uniform brightness, stability and continuous spectral quality of light output.

In accordance with this invention there is now provided an electric discharge lamp of the type specified comprising a sealed lighttransmitting envelope, a first arc electrode constituted by a body of refractory metal supported within the envelope, a second electrode spaced from the first by an arc gap, and a gas fill, in the envelope, comprising an inert gas and at least one halogen which in operation maintains a regenerative transport cycle with the said refractory metal.

Halogen regenerative cycles are, of course, already well known in tungsten filament incandescent lamps. In this invention, however, the halogen cycle operates in very different circumstances, namely in the environment of an arc discharge and in conjunction with an essentially two- or three-dimensional refractory metal body, as contrasted with the resistance-heated and essentially linear or one-dimensional filament in an incandescent lamp. A filament of an incandescent lamp may be considered to be essentially one-dimensional since, considering it as a length of wire, the measurement in one dimension is several orders of magnitude greater than the measurements in the orthogonal dimensions (as represented by the cross-section). The said refractory metal body, however, has at least two dimensions of similar orders of magnitude, the third being of a similar order or smaller.The novel results achieved by the invention are notably different from those of the incandescent lamp.

In the lamps of this invention the first electrode, which is raised to incandescence by the arc to provide the light source, may take the form of a sphere, disc or other shaped body of a metal such as tungsten. The second electrode may be a simple wire adapted to provide electron emission, for example by the inclusion or application of an emissive material.

The inert gas filling may be substantially all neon, argon, krypton or xenon, or a mixture thereof, together with one or more of the halogens, iodine, bromine and chlorine. These may be added as the element or as any halogencontaining compound, including, for example, hydrogen halides, hydrocarbon halides, halophosphonitriles and like compounds, and metal halides, all of which yield halogen(s) at the operating temperature of the device. It should be noted that the incandescent mass is principally heated by ion bombardment, which distinguishes it from a resistively heated incandescent and tungsten-halogen lamp and that the light is principally emitted by the incandescent mass (not from the discharge), which distinguishes the lamp from a metalhalide discharge lamp.The halogen is added to keep the bulb wall clean and return evaporated metal to the electrodes, and plays a negligible part as a light emitter.

It has been found possible, in a lamp con taininghalogen in accordance with the invention, to operate with a spherical tungsten body at a temperature only slightly below the melting point of the metal. In this way it is possible to achieve source brightness up to 5000 stilb or above, which compares very favourably with the brightness of tungsten halogen incandescent lamps, which is in the region of 3800 stilb with a life of only some 50 hours.

The luminous efficacy of a spherical tungsten bead will be influenced by the heat transfer from the bead to the wire supporting it, but measurements show that an efficacy of at least 25 ImrW at relatively low wattages can readily be achieved, together with a life of many hundreds of hours. The more important parameter, however, if the lamp is viewed as a source for projection apparatus, is the increase in brightness over the 'Pointolite' (RTM) lamp of at least 3-5 times and in comparison with a tungsten halogen source of at least 30%. The optical utilisation may, however, be still greater if the optical system is designed to utilise the new smaller and brighter source, while still affording a longer life expectancy.

Another significant feature of the lamp of this invention is the very small size of envelope that can be used, so that a physically small optical reflector system can be used, which was not possible with the 'Pointolite' (RTM) lamp.

However, it may prove that as the source size becomes smaller, the optical quality and shape of a pressed glass reflector with an aluminised or dichroic coating, as used at present with tungsten halogen filament lamps, may not be of a high enough optical quality for efficient utilisation of the smaller brighter source. Should a more accurate optical surface be required, any mechanical polishing process may become a practical and economic possibility, since the reflector can be of small physical size.

The invention will be described in greater detail, by way of example, with reference to the drawings accompanying the Provisional Specification, in which: Figure 1 shows one form of 'point' light source embodying the invention; and Figure 2 shows the source of Figure 1 in position in a reflector lamp fitting.

Figure 1 shows a basic form of a singleended lamp 1 with a pinch seal 2 and envelope 3 enclosing a two-electrode assembly. One electrode 4 has a spherical ball 5 of tungsten of about 1.5 mm in diameter and forms the light-emitting source which is heated by positive ions from the current passed in an arc discharge, the cathode 6 supporting the arc being a simple wire of thoriated tungsten to provide the electron emission.

The gas filling, its pressure and halogen content can be selected to provide a lamp free from tungsten deposition on the envelope wall, giving a high light output and source brightness through a long life of many hundreds of hours.

One suitable filling is krypton at a cold filling pressure of3 atmospheres with a 0.4% (by mass) content of hydrogen bromide.

The lamp is essentially a D.C. source and a voltage pulse is required to break down the short arc gap to establish an arc discharge.

Typically, such a lamp can be operated with an arc voltage of 12-13 V with a current of about 3 amps to give a tungsten sphere brightness of about 5000 stilb, with a life of many hundreds of hours.

Other alternative methods of starting the discharge, as by providing a directly heated cathode to avoid the use of a voltage pulse to establish an arc discharge, can be used.

In the novel lamp described light is generated from an incandescent sphere of tungsten by an arc discharge to provide a small intense source of high brightness greater than can be obtained from a tungsten filament lamp.

By way of further example, it has been found possible, because of the compact and substantially reduced physical dimensions of the light source of Figure 1, to employ such a source 1 in a small lamp as shown in Figure 2, when the sphere 5 is disposed at the focus of a reflector optic 7. We can show that the utilisation of the light flux through the gate 8 of a film projector is increased from about 11% with a helical tungsten filament of dimensions 2 mm x 2 mm to at least 24% with the incandescent spherical light source. This improved utilisation is partly accounted for by the slightly higher brightness of the tungsten sphere, with reduced light scatter at the gate due to the smaller source dimensions, and also by the curvature of the spherical source providing a more uniform flashing of the optical contour of the reflector.

WHAT WE CLAIM IS: 1. An electric discharge lamp of the type specified comprising a sealed light-transmitting envelope, a first arc electrode constituted by a body of refractory metal supported within the envelope, a second electrode spaced from the first by an arc gap, and a gas fill, in the envelope, comprising an inert gas and at least one halogen which in operation maintains a regenerative transport cycle with the said refractory metal.

2. A lamp according to claim 1 wherein the first electrode, which is raised to incandescence by the arc to provide the light source, has the form of a sphere, disc or other shaped two or three-dimensional body.

3. A lamp according to claim 2 wherein the second electrode is a wire adapted to provide electron emission.

4. An electric discharge lamp of the type

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (4)

**WARNING** start of CLMS field may overlap end of DESC **. incandescent and tungsten-halogen lamp and that the light is principally emitted by the incandescent mass (not from the discharge), which distinguishes the lamp from a metalhalide discharge lamp. The halogen is added to keep the bulb wall clean and return evaporated metal to the electrodes, and plays a negligible part as a light emitter. It has been found possible, in a lamp con taininghalogen in accordance with the invention, to operate with a spherical tungsten body at a temperature only slightly below the melting point of the metal. In this way it is possible to achieve source brightness up to 5000 stilb or above, which compares very favourably with the brightness of tungsten halogen incandescent lamps, which is in the region of 3800 stilb with a life of only some 50 hours. The luminous efficacy of a spherical tungsten bead will be influenced by the heat transfer from the bead to the wire supporting it, but measurements show that an efficacy of at least 25 ImrW at relatively low wattages can readily be achieved, together with a life of many hundreds of hours. The more important parameter, however, if the lamp is viewed as a source for projection apparatus, is the increase in brightness over the 'Pointolite' (RTM) lamp of at least 3-5 times and in comparison with a tungsten halogen source of at least 30%. The optical utilisation may, however, be still greater if the optical system is designed to utilise the new smaller and brighter source, while still affording a longer life expectancy. Another significant feature of the lamp of this invention is the very small size of envelope that can be used, so that a physically small optical reflector system can be used, which was not possible with the 'Pointolite' (RTM) lamp. However, it may prove that as the source size becomes smaller, the optical quality and shape of a pressed glass reflector with an aluminised or dichroic coating, as used at present with tungsten halogen filament lamps, may not be of a high enough optical quality for efficient utilisation of the smaller brighter source. Should a more accurate optical surface be required, any mechanical polishing process may become a practical and economic possibility, since the reflector can be of small physical size. The invention will be described in greater detail, by way of example, with reference to the drawings accompanying the Provisional Specification, in which: Figure 1 shows one form of 'point' light source embodying the invention; and Figure 2 shows the source of Figure 1 in position in a reflector lamp fitting. Figure 1 shows a basic form of a singleended lamp 1 with a pinch seal 2 and envelope 3 enclosing a two-electrode assembly. One electrode 4 has a spherical ball 5 of tungsten of about 1.5 mm in diameter and forms the light-emitting source which is heated by positive ions from the current passed in an arc discharge, the cathode 6 supporting the arc being a simple wire of thoriated tungsten to provide the electron emission. The gas filling, its pressure and halogen content can be selected to provide a lamp free from tungsten deposition on the envelope wall, giving a high light output and source brightness through a long life of many hundreds of hours. One suitable filling is krypton at a cold filling pressure of3 atmospheres with a 0.4% (by mass) content of hydrogen bromide. The lamp is essentially a D.C. source and a voltage pulse is required to break down the short arc gap to establish an arc discharge. Typically, such a lamp can be operated with an arc voltage of 12-13 V with a current of about 3 amps to give a tungsten sphere brightness of about 5000 stilb, with a life of many hundreds of hours. Other alternative methods of starting the discharge, as by providing a directly heated cathode to avoid the use of a voltage pulse to establish an arc discharge, can be used. In the novel lamp described light is generated from an incandescent sphere of tungsten by an arc discharge to provide a small intense source of high brightness greater than can be obtained from a tungsten filament lamp. By way of further example, it has been found possible, because of the compact and substantially reduced physical dimensions of the light source of Figure 1, to employ such a source 1 in a small lamp as shown in Figure 2, when the sphere 5 is disposed at the focus of a reflector optic 7. We can show that the utilisation of the light flux through the gate 8 of a film projector is increased from about 11% with a helical tungsten filament of dimensions 2 mm x 2 mm to at least 24% with the incandescent spherical light source. This improved utilisation is partly accounted for by the slightly higher brightness of the tungsten sphere, with reduced light scatter at the gate due to the smaller source dimensions, and also by the curvature of the spherical source providing a more uniform flashing of the optical contour of the reflector. WHAT WE CLAIM IS:

1. An electric discharge lamp of the type specified comprising a sealed light-transmitting envelope, a first arc electrode constituted by a body of refractory metal supported within the envelope, a second electrode spaced from the first by an arc gap, and a gas fill, in the envelope, comprising an inert gas and at least one halogen which in operation maintains a regenerative transport cycle with the said refractory metal.

2. A lamp according to claim 1 wherein the first electrode, which is raised to incandescence by the arc to provide the light source, has the form of a sphere, disc or other shaped two or three-dimensional body.

3. A lamp according to claim 2 wherein the second electrode is a wire adapted to provide electron emission.

4. An electric discharge lamp of the type

specified substantially as herein described with reference to the drawings accompanying the provisional specification.