GB1597794A - Sodium lamp - Google Patents

Description

(54) SODIUM LAMP (71) We, XEROX CORPORATION of Xerox Square, Rochester, New York, United States of America, a corporation organised under the laws of the State of New York, United States of America, 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: This invention relates to a low pressure sodium vapor discharge lamp.

Prior art low pressure sodium vapor lamps, from which the present invention is to be distinguished, are typically operated at relatively low power and it is generally desired to retain the heat generated within the lamp to maintain efficiency. Additionally, such lamps typically emit in all directions for such purposes as street lighting or general illumination.

Sodium vapor illumination has certain advantages for use in xerography. For producing black and white copy, the monochromatic object illumination from a sodium vapor lamp produces the maximum in object contrast for exposure to a photoreceptor. This subject matter is more fully described in U.S. Patent No.

3,869,205, issued to Charles F. Gallo on March 4, 1975.

It is an object of the present invention to provide a low pressure sodium vapor lamp capable of operation at relatively high power levels for correspondingly high intensity of light output.

According to the invention there is provided a low pressure sodium vapor discharge lamp including: an elongate envelope having an aperture extending the length of the envelope through which light can pass out of the envelope; a cylindrical discharge tube disposed within said envelope on an axis generally parallel to that of said envelope; said envelope containing a gaseous heat transfer medium for the transmission of heat from said discharge tube, through said envelope.

An embodiment of the invention will now be described by way of example with reference to the accompanying drawing in which: Figure 1 is a longitudinal cross section of a lamp; and Fig. 2 is a transverse cross section of the lamp of Figure 1, taken through its central portion.

Referring now to Figure 1, a low pressure sodium vapor discharge lamp is generally indicated at 2 and includes an inner discharge tube 4 mounted within an outer envelope 6 by means of a suitable support structure shown schematically at 8. Electrodes 10 and 12 at opposite ends of the discharge tube are operatively connected respectively to electrical connection elements 14 and 16.

Referring now to Figure 2, lamp 2 including inner discharge tube 4 and outer envelope 6, is shown in a transverse cross section. As is more clearly shown in this view, outer envelope 6 is coated with a diffusely reflective coating 18 defining a generally straight light aperture 20 along the length of the envelope 6. Examples of diffuse coating materials for coating 18 are titanium dioxide (TiO2), barium sulfate (BaSO4), and magnesium oxide (MgO). Aperture 20 can be sized to suit the requirements of a particular system. In general, the aperture angle, that is the angle subtended by the aperture at the axis of the envelope, will be smaller than 1800. An aperture angle of roughly 60 is an exemplary configuration by a lamp used as a xerographic exposure light source.

The effect of the apertured reflective coating 18 on the envelope 6 is to inwardly reflect light generated by the discharge tube 4 so that it ultimately exits or escapes the lamp in the desired direction through the aperture 20.

In operation, when an electrical discharge is passed through the sodium vapor in the discharge tube 4, radiation is emitted in both the visible and infrared portion of the spectrum.

As is well known, the visible radiation is essentially monochromatic, nominally at 589.3 nm.

Typically, the operating temperature at the wall of the discharge tube is approximately 260"C. In the prior art, for the sake of effici ency, the infrared radiation from the lamp tube is conserved in the system, as by reflection inwards, to help keep the discharge tube at its optimum operating temperature. By contrast to the prior art, the purpose of the present lamp is to operate at substantially increased power levels for the purpose of maximizing output light intensity, albeit at a sacrifice of efficiency. Now in order to permit operation at such higher power levels, it is no longer desirable to conserve the heat generated by the lamp; rather it is desired to dissipate it.

In order to achieve the desired heat dissipation or heat transmission away from the discharge tube 4, the envelope 6 is air or gas filled, rather than evacuated as in the prior art, to permit heat transmission by conduction and convection through the air as well as by radiation through the space. For air filling the outer envelope 6 is simply provided with one or more small apertures 22 so that the interior space 24 is in communication with the surrounding air. The higher power level of operation permitted by the addition of air effects a substantially greater intensity of visible light output of the lamp. At the same time, the reflective coating provides directionality to the light output.

These are of course desired conditions for use of a sodium discharge lamp as an exposure lamp in a xerographic system. The increased light intensity enables the associated xerographic process to be operated at correspondingly higher speed, and the tailored illumination profile permits improved efficiency of object illumination.

Although the heat dissipation or cooling effect can be achieved with different gases at different pressures, it is especially convenient to leave the outer envelope open to atmospheric air since this minimizes constructional detail and expense.

In order to control and continue the location of condensed sodium within the discharge tube, protruding sodium reservoirs or "dimples" 26 are constructed on the discharge tube 4. These dimple protrusions tend to be cooler and are filled with an excess of metallic sodium. In operation, the discharge tube is hot and the small amount of sodium is vaporized and serves as the radiating species of the gas discharge medium. To ensure that the radiation is reasonably uniform along the length of the discharge tube, the dimples are unifoImly distributed along the tube length.

Similar dimples as such are known in prior art. In the present embodiment, however, in order to ensure that the dimples remain comparatively cool so as to confine and localize the condensation of the opaque metallic sodium, the dimples are located in the "down" position as defined by gravity, preferably at or near the vertical. There is a combined beneficial effect of the sodium dimples thus located and the air presence in the tube-envelope cavity 24.

That is, to the extent that there is heat convection within the cavity 24, the lower part of the tube 4 is cooler than the top part where the light aperture 20 is located. Thus, the portion of the discharge tube facing the aperture when the lamp is appropriately positioned in a copying machine is kept clear of condensation of opaque sodium.

From a cold start, it takes an appreciable length of time for a sodium vapor lamp to warm up to its optimum operating temperature.

For xerographic application, it is desirable that the full radiant output of the lamp be available on demand. To achieve this, the inner discharge tube is heated by auxiliary means. In a preferred embodiment, the discharge tube is wrapped with heater wire 28 which is independently powered. Thus, during warm-up and standby, the heater wires are energized while the lamp itself is off. When the lamp is energized for use, supply to the heater wires may be shut off. For some applications, where it is desirable to operate the lamp at currents which do not produce the optimum discharge temperature, simultaneous application of both lamp and heater power can be used to produce the desired optimum tube temperature.The diameter and resistivity of the heater wire are selected so that the required power is conveniently obtained while minimizing the wire temperature and thermal gradients. In one exemplary and successful combination, Alumel wire of 0.01" diameter, is wound around the discharge tube 4 in a helix of approximately 1/2" pitch.

With respect to the thermal coupling between the heater wire and the discharge tube, there is a beneficial effect from the combination of the heater wire and the air in the cavity 24. In the conventional evacuated outer cavity, the thermal coupling between the heater wire and the discharge tube is quite poor resulting in thermal gradients along the discharge tube and sometimes glass cracking, whereas in the present case the air puts the heater wires in better thermal contact with the discharge tube to reduce the thermal gradients therealong. The presence of air permits relatively straightforward wrapping of the heater wire around the discharge tube. Otherwise, more elaborate and expensive constructional techniques would be necessary.

In typical sodium vapor lamps of the prior art, the lamp filament electrodes are not externally heated. This could result in the lamp ends being somewhat cooler than the lamp central portion with the possibility of a resulting correspondingly lower sodium vapor pressure and a resultant drop in the radiant profile at the ends. In embodiments of the present invention, the filament electrodes 10 and 12 can be externally powered, independently of the lamp discharge, to provide ohmic heating to keep them appropriately warm at all times.

This technique improves the longitudinal radiant profile of the lamp. In addition, for xerographic application in which on-off lamp cycling is frequent, the life of the filament electrodes is extended by keeping them hot but independent of lamp operation.

In other embodiments, the envelope 6 itself is formed of reflective material, or is transparent and has a reflective layer on its outer surface instead of its inner surface. The envelope may be formed such that the aperture 20 is defined by a part of the envelope cutaway or otherwise absent from the construction of the envelope 6. That is, the aperture 20, may be formed by a slot removed from the surface of the envelope 6 or otherwise.

In the embodiment described the envelope is of at least generally circular cross-section. In other embodiments the envelope may be noncircular. Satisfactory results can be provided with an ellipsoidal shaped envelope, for example.

WHAT WE CLAIM IS:- 1. A low pressure sodium vapor discharge lamp including: an elongate envelope having an aperture extending the length of the envelope through which light can pass out of the envelope; a cylindrical discharge tube disposed within said envelope on an axis generally parallel to that of said envelope; said envelope containing a gaseous heat transfer medium for the transmission of heat from said discharge tube, through said envelope.

2. The sodium discharge lamp of Claim 1 wherein the envelope has at least one opening through which air can pass.

3. The sodium discharge lamp of Claim 1 or 2, in which the material forming the envelope is reflective material.

4. The sodium discharge lamp of Claim 1 or 2, in which the internal or external surface of the envelope is provided with a layer or coating of reflective material.

5. The sodium discharge lamp of Claim 3 or 4, in which the reflective material is a diffusely reflective material.

6. The discharge lamp of any of the preceding claims, in which the aperture subtends an angle of 45" to 900 at the longitudinal axis of the envelope.

7. The discharge lamp of any of the preceding claims, including a resistive wire around the discharge tube in contact therewith and spaced therealong, the resistive wire being arranged to be connected to a source of power to energise the wire to maintain the temperature of the discharge tube at a predetermined level when the lamp is inoperative.

8. The discharge lamp of any of the preceding claims, wherein the discharge tube includes on a surface thereof a plurality of protrusions defining a plurality of concave reservoirs within the discharge tube for the deposit of metallic sodium when the lamp is inoperative and condensed excess sodium when the lamp is operative, the protrusions being disposed on the surface of the discharge tube generally opposite the aperture.

9. The discharge lamp of any of the preceding claims, wherein said discharge tube includes an electrode at each end portion thereof to initiate and maintain an electrical discharge within said tube; said electrodes being arranged to be operatively connected to an external source of electric current independent of the current for the lamp discharge for ohmic heating of said electrodes to maintain an elevated temperature to correspondingly maintain the vapor pressure of sodium in the end portions of said discharge tube to enhance the uniformity of light output intensity at the ends of said lamp.

10. Sodium discharge lamps substantially as herein described with reference to the accompanying drawing.

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

Claims (10)

**WARNING** start of CLMS field may overlap end of DESC **. xerographic application in which on-off lamp cycling is frequent, the life of the filament electrodes is extended by keeping them hot but independent of lamp operation. In other embodiments, the envelope 6 itself is formed of reflective material, or is transparent and has a reflective layer on its outer surface instead of its inner surface. The envelope may be formed such that the aperture 20 is defined by a part of the envelope cutaway or otherwise absent from the construction of the envelope 6. That is, the aperture 20, may be formed by a slot removed from the surface of the envelope 6 or otherwise. In the embodiment described the envelope is of at least generally circular cross-section. In other embodiments the envelope may be noncircular. Satisfactory results can be provided with an ellipsoidal shaped envelope, for example. WHAT WE CLAIM IS:-

1. A low pressure sodium vapor discharge lamp including: an elongate envelope having an aperture extending the length of the envelope through which light can pass out of the envelope; a cylindrical discharge tube disposed within said envelope on an axis generally parallel to that of said envelope; said envelope containing a gaseous heat transfer medium for the transmission of heat from said discharge tube, through said envelope.

2. The sodium discharge lamp of Claim 1 wherein the envelope has at least one opening through which air can pass.

3. The sodium discharge lamp of Claim 1 or 2, in which the material forming the envelope is reflective material.

4. The sodium discharge lamp of Claim 1 or 2, in which the internal or external surface of the envelope is provided with a layer or coating of reflective material.

5. The sodium discharge lamp of Claim 3 or 4, in which the reflective material is a diffusely reflective material.

6. The discharge lamp of any of the preceding claims, in which the aperture subtends an angle of 45" to 900 at the longitudinal axis of the envelope.

7. The discharge lamp of any of the preceding claims, including a resistive wire around the discharge tube in contact therewith and spaced therealong, the resistive wire being arranged to be connected to a source of power to energise the wire to maintain the temperature of the discharge tube at a predetermined level when the lamp is inoperative.

8. The discharge lamp of any of the preceding claims, wherein the discharge tube includes on a surface thereof a plurality of protrusions defining a plurality of concave reservoirs within the discharge tube for the deposit of metallic sodium when the lamp is inoperative and condensed excess sodium when the lamp is operative, the protrusions being disposed on the surface of the discharge tube generally opposite the aperture.

9. The discharge lamp of any of the preceding claims, wherein said discharge tube includes an electrode at each end portion thereof to initiate and maintain an electrical discharge within said tube; said electrodes being arranged to be operatively connected to an external source of electric current independent of the current for the lamp discharge for ohmic heating of said electrodes to maintain an elevated temperature to correspondingly maintain the vapor pressure of sodium in the end portions of said discharge tube to enhance the uniformity of light output intensity at the ends of said lamp.

10. Sodium discharge lamps substantially as herein described with reference to the accompanying drawing.