Colour selectable pulsed discharge lamp

Abstract: The present invention is directed to a novel system for producing selectable color bands from a single emission source. More particularly, this invention is directed to a small emission source consisting of a gas discharge means and a technique for activating this discharge means. The emission source in the present invention is preferably a single-ended fused silica capsule which encloses one or more predetermined metal halide salts and an inert gas, preferably argon gas.

Description: The present invention relates to a colour selectable pulsed discharge lamp. The invention is particularly concerned with a system for producing selectable colour bands from a single, preferably small, emission source in a relatively low pressure pulsed arc discharge lamp. Conventional (i.e., non-pulsed) metal halide arc lamps typically comprise a fused silica tube with two electrodes, a rare gas for starting, a charge of mercury, and one or more metal halides, generally iodides. In operation, a starting voltage of about 300V is applied across the electrode gap causing the contents of the arc tube to vaporize, resulting in a high temperature, high pressure, wall stabilized arc in a gas, consisting principally of mercury vapour, ionized metal atoms and iodine molecules. The output spectrum (i.e., the colour of the discharge) consists predominantly of the spectrum of the added metals. Colour output for such lamps is tailored by varying the metal halides added to the arc tube. See for example, Waymouth, "Electric Discharge Lamps", Chapter 8, MIT Press, (1971). In the case of pulsed arc discharge lamps, the experience with conventional metal halide arc discharge lamps is inapplicable as regards what colour can or will be obtained based upon a given selected metal halide. The pulsed arc discharge lamp can operate at a much lower temperature than the conventional metal halide discharge lamp and thus conventional theories do not apply. A low pressure metal or metal halide pulsed discharge lamp is disclosed in US-A-3 914 649. The lamp particularly described is a low pressure sodium vapour lamp comprising an arc tube in which are disposed a pair of electrodes spaced apart by a gap. A coil of resistance wire is used to heat the arc tube to maintain it at a temperature of approximately 260°C, causing the vapour pressure of the sodium to be approximately 1.31 x 10⁻⁵ bar (0.01 Torr). This same resistance wire is used to provide a trigger to induce breakdown in the sodium vapour. The pulse length is in the range of 30 to 1000 microseconds. The lamp is used in photocopying applications, and produces a yellow light. US-A-4 137 484 and US-A-3 624 447 disclose pulsed high pressure sodium lamps. According to one aspect of the present invention, there is provided a pulsed metal halide arc lamp system according to Claim 1. Viewed from another aspect of the present invention, there is provided a method of producing a pulsed metal halide arc lamp according to Claim 15. It has thus been discovered that for pulsed metal halide discharge lamps of the type described herein, the colour of the emission is directly related to the duration of the pulse. Short pulses (e.g., about 5 microseconds and below, preferably about 1 microsecond) and longer pulses (e.g., greater than about 5 microseconds, preferably about 10 microseconds) have different effects upon the emission colour. The emission source for use in the present invention is preferably a single-ended fused silica capsule which encloses one or more predetermined metal halide salts and an inert gas, preferably argon. Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which:- Figure 1 represents in schematic form, testing apparatus for investigating the principles underlying of the present invention; Figure 2 illustrates electrical/optical plots of colour output for pulsed arc discharge lamps prepared according to the present invention; and Figure 3 illustrates a self contained lamp in accordance with invention. The present embodiments concern a novel system for producing selectable colour bands from a single emission source in a pulsed arc discharge lamp. One preferred capsule and source excitation testing scheme is illustrated in Figure 1. An oven 10 is used to heat the arc tube 12 and its contents, thereby adjusting the vapour pressure of the metal halide disposed therein to an optimum level for the production of a fairly diffuse, well-behaved discharge during excitation. The vapour pressure in the arc tube is generally maintained within the range from about 1 to 10 Torr. Once the requisite vapour pressure has been achieved by heating the arc tube, the power supply 14 generates a high voltage (0.5 - 2.0 kV) pulse which causes a high current discharge to form across the electrode gap of the arc tube. Current and voltage are monitored during the testing by monitor 16 and light output is correspondingly analyzed and recorded for all different voltage and/or current levels by a light monitor 18 and a digital transient recorder 20. A printer 22 provides hard copy results of all test data generated. The current was limited to approximately 1.8 amperes for the duration of each pulse, and the duration of the pulse was found to establish the dominant colour of the emission. For the preferred metal halide salts employed herein, the duration ranged from about 1 to 10 microseconds. For other metal halide salts and mixtures of such salts, longer or shorter duration pulses may be employed to vary the colour output according to the teachings of this specification. During short pulses, e.g., from about 1 to 5 microseconds, the emission appears to be produced by a fairly diffuse glow. For long pulses, e.g., from about 5 to 10 microseconds, a glow-to-arc transition occurs. Table 1 illustrates the dominant colours, approximate capsule temperatures, metal halide vapour pressures, and number densities for the two most preferred metal halides used in conjunction with the present invention, namely cadmium and zinc iodide. Figures 2 (A), (B) and (C) illustrate in the case of cadmium some of the electrical and optical differences between short and long pulse lengths, i.e. 1 and 10 microseconds. Figure 2A illustrates current v. time; Figure 2B illustrates voltage v. time; and Figure 2C illustrates light (colour) output v. time. The time scale of Figures 2 is in microseconds, and thus the long pulse extends beyond the end of the plot. As illustrated, with a 1 microsecond pulse, a reddish light is given off. Green light is the dominant colour for pulse widths beyond 1 microsecond. It will be noted that the green light intensity is roughly twice that of the red, although both light curves have been scaled to be the same size for shape comparison. Notice that the red light fades exponentially from its peak, while the green light levels out to a non-zero value for the duration of the longer pulse. The transition between red and green appears to occur at the point in time where the initially high voltage collapses and the current increases, i.e., the glow-to-arc transition. The dominant red colour appears to be associated with the higher electron energy of the glow state, while green is associated with the lower average electron energy of the arc state. While not wishing to be bound by theory, this interpretation of the colour transition is consistent with the Grotrian diagram of available energy states of cadmium. The initial excitation channel involves singlet excitation with the strongest transition being the 3D-2P (6438Å). For longer pulse widths the increase in collisional interaction allows the triplet states to be populated. Associated observed transitions are 2S-2P, (5086Å), 2S-2P₂ (4800Å) and 2S-2P₃ (4578Å). Presumably a large fraction of the triplet states are populated by recombination of highly excited or ionized cadmium. Zinc exhibits characteristics similar to cadmium. The colour transition for zinc is from a peach colour to blue. The dominant observed wavelengths are 6362 Angstroms (singlet state) and 4811, 4722 and 4680 Angstroms (triplet states). Mercury, another member of Group IIB of the Standard Periodic Table of the Elements, is expected to exhibit similar emissions. Similarly, other metal halides can readily be analyzed using the testing apparatus and methodology discussed in connection with Figure 1. A practical lamp design incorporating both the new gas discharge means and the activation method is illustrated in Figure 3. The lamp comprises a clear glass vacuum outer jacket 10, and a light transparent fused silica arc tube 12, which contains the selected metal halide salt and argon gas. Two electrodes 14 and 16 extend into the arc tube 12 from opposing ends thereof. At least partially surrounding the arc tube 12 is a heating coil 18 which is used to achieve the necessary vapour pressure of the contents of the arc tube prior to the addition of the voltage pulse. As set forth above, depending upon the selected metal halide fill and the pulse duration, one may select a desired colour output for lamps of this type. Lamps such as those illustrated in Figure 3 provide a compact source which can be used, for example, as a signal flasher. Alternatively, lamps having double-ended geometry could be designed which would lend itself to axial focusing optics that can be used in projection systems. Potential applications for a multi-color pulsed arc discharge system in accordance with the invention are very widespread, ranging from signal and warning lighting to color projection of graphic information. The ability to select any of several narrow colour bands from a single source greatly enhances the multi-colour options that can be employed in signal and projection applications.

Claims: A pulsed metal halide arc lamp system, comprising an arc tube (12) in which are disposed a pair of electrodes (14,16) spaced apart by a gap, an emissive material comprising a metal halide salt within the arc tube, means (18) for heating the arc tube to provide a predetermined vapour pressure of the contents of the arc tube, and means (14) for providing a voltage pulse so as to cause a discharge across the gap, characterised in that the predetermined vapour pressure of the contents of the arc tube is in a range from 3.95 x 10⁻⁴ bar to 1.32 x 10⁻² bar (0.3 to 10 Torr), the voltage of the pulse is selected within the range of 0.5 to 2.0 kV, and the duration of the pulse is selected to provide emission of light in a predetermined colour band. A system as claimed in claim 1, characterised in that the metal halide salt is selected from the group consisting of cadmium iodide, zinc iodide and mercury iodide. A system as claimed in claim 1 or 2, characterised in that the heating means (18) is such as to provide a predetermined temperature of 360°C to 380°C. A system as claimed in claim 1, 2 or 3, characterised in that the heating means (18) is such as to provide a predetermined vapour pressure of 3.95 x 10⁻⁴ to 5.26 x 10⁻⁴ bar (0.3 to 0.4 Torr). A system as claimed in claim 1, characterised in that the duration of the pulse is in the range of 1 to 10 microseconds. A system as claimed in claim 5, characterised in that the duration of the pulse is in the range of 1 to 5 microseconds. A system as claimed in claim 5, characterised in that the duration of the pulse is in the range of 5 to 10 microseconds. A system as claimed in claim 6, characterised in that the metal halide salt is cadmium iodide, and the light emitted is red. A system as claimed in claim 7, characterised in that the metal halide salt is cadmium iodide, and the light emitted is bluish green. A system as claimed in claim 8 or 9, characterised in that the temperature of the arc tube prior to the addition of the voltage pulse is 380°C and the vapour pressure of the cadmium iodide is 3.95 x 10⁻⁴ bar (0.3 Torr). A system as claimed in claim 6, characterised in that the metal halide salt is zinc iodide, and the light emitted is peach. A system as claimed in claim 7, characterised in that the metal halide salt is zinc iodide, and the light emitted is blue. A system as claimed in claim 11 or 12, characterised in that the temperature of the arc tube prior to the addition of the voltage pulse is 360°C and the vapour pressure of the cadmium iodide is 5.26 x 10⁻⁴ bar (0.4 Torr). A system as claimed in any preceding claim, characterised in that means are provided for varying the duration of the pulse so as to vary the colour of the light emitted. A method of producing a pulsed metal halide arc lamp system, comprising the steps of :- (a) providing an arc tube (12) in which are disposed a pair of electrodes (14,16) spaced apart by a gap, an emissive material comprising a metal halide salt within the arc tube, and means (18) for heating the arc tube to provide a predetermined vapour pressure of the contents of the arc tube in a range from 3,95 x 10⁻⁴ bar to 1.32 x 10⁻² bar (0.3 to 10 Torr); (b) heating the arc tube so as to provide said predetermined vapour pressure; (c) providing a voltage pulse so as to cause a discharge across the gap, the voltage of the pulse being selected within the range of 0.5 to 2.0 kV; (d) varying the duration of the pulse to provide emission of light in a plurality of colour bands; (e) selecting a particular colour band and noting the pulse duration required; and (f) constructing the metal halide arc lamp system, comprising an arc tube (12) in which are disposed a pair of electrodes (14,16) spaced apart by a gap, an emissive material comprising a metal halide salt within the arc tube, means (18) for heating the arc tube to provide the predetermined vapour pressure of the contents of the arc tube in a range from 3,95·10⁻⁴ bar to 1.32 x 10⁻² bar (0.3 to 10 Torr), and means (14) adapted to provide the voltage pulse within the range of 0.5 to 2.0 kV for said noted duration so as to provide emission of light in said particular colour band.

Inventors: GRANT JEFFREY O, ROTHWELL HAROLD L JR, GRANT, JEFFREY O., ROTHWELL, HAROLD L. JR.
Applicants: GTE PROD CORP, GTE PRODUCTS CORPORATION
Application year: 1988
Main classification: H05B41
IPC codes: H01J61