US 3914649 A
A low pressure metal or metal halide vapor lamp for photocopying applications. In a preferred embodiment, a low pressure sodium vapor lamp is operated in a pulsed mode, providing illumination of increased brightness and of a color (yellow) which is optimum for providing a copy which is an accurate representation of the original.
Claims available in
Description (OCR text may contain errors)
United States Patent Hug Oct. 21, 1975  PULSED METAL OR METAL HALIDE 3,596,237 7/1971 Barber et a1. 313/201 X LAMPS O PHOTOCOPYING 3,619,716 11/1971 Spira et al 315/244 APPLICATIONS William F. Hug, Pasadena, Calif.
Xerox Corporation, Stamford, Conn.
Filed: May 6, 1974 Appl. No.: 467,343
US. Cl 315/241 R; 313/187; 313/228; 315/115; 315/243; 315/289; 355/69 Int. Cl. G03B 27/54; HOSB 41/34 Field of Search 313/187, 228, 229, 197, 313/198, 201; 355/67, 69, 71-; 315/234, 241 S, 241 R, 243, 244, 289, 335, 358, 115
References Cited UNITED STATES PATENTS Gunto et al 355/67 x Primary ExaminerJames W. Lawrence Assistant ExaminerE. R. La Roche Attorney, Agent, or Firm-James .l. Ralabate; Terry J. Anderson; Irving Keschner A low pressure metal or metal halide vapor lamp for photocopying applications. In a preferred embodiment, a low pressure sodium vapor lamp is operated in a pulsed mode, providing illumination of increased brightness and of a color (yellow) which is optimum for providing a copy which is an accurate representation of the original.
ABSTRACT 7 Claims, 3 Drawing Figures US. Patent Oct. 21, 1975 Sheet 1 of2 3,914,649
PuLsD' MODE SPECTRAL RADI ANCE cONfiNous MODE l l I 4000 5000 6000 7000 WAVE LENGTH (ANGSTROMS) FIG. 3
US. Patent Oct. 21, 1975 Sheet 2 of2 3,914,649
PULSED METAL OR METAL HALIDE LAMPS FOR PHOTOCOPYING APPLICATIONS BACKGROUND OF THE INVENTION In the xerographic process as described in U.S. Pat. No. 2,297,691, a base plate of relatively low electrical resistance such as metal, etc., having a photoconductive insulating surface coated thereon is electrostatically charged in the dark. The charged coating is then exposed to a light image. The charges leak off rapidly in the base plate in proportion to the intensity of light to which any given area is exposed, the charge being substantially retained in non-exposed areas. After exposure, the coating is contacted with electrostatic materials which adhere to the remaining charges to form a powder image corresponding to the latent electrostatic image remaining after exposure. The powder image then can be transferred to a sheet of transfer material resulting in a positive or negative print, as the case may be. Since dissipation of the surface electrostatic charge is proportional to the intensity of the impinging radiation, light sources of uniform and sufficient intensity must be provided so that the photoconductive insulator can be properly exposed.
Low pressure metal halide lamps are a near optimum illumination source for photocopiers producing black and white output copies from black and white and multi-colored originals.
With respect to line copy, the optimum goal of any black and white photocopying apparatus is to make the image areas on the copy as black as possible. In other words, one would like a minimum of energy reflected from the image areas of the original while reflecting a maximum from the background region. Obviously, it is impossible to copy all colored backgrounds as white while concurrently copying all colored images as black.
From prior experience, it appears that most colors that are utilized as images on an original tend to be located at the extremes of the visible spectrum, i.e., blues and reds, whereas yellow, for example, is seldom utilized for images. Colored backgrounds are pastel (desaturated) and can usually be considered as tinted white paper which may be explained in part on well known principles of physiological optics (photoptic vision).
It then follows that the optimum light source for photocopying apparatus producing black and white output copies from black and white and multi-colored originals produces yellow light whereby black and reds will copy as black, while concurrently most common colored papers have considerable reflectance in yellow (it should be noted that the use of the yellow exposure lamps obviously necessitates a yellow sensitive photoreceptor). However, the typical prior art photocopying apparatus utilizes aperture fluorescent lamps which generate colored light.
Low pressure sodium lamps represent a commercially available yellow light source. Present commercial sodium lamps, such as those manufactured by N. V. Phillips, have several disadvantages for photocopying applications associated therewith.
The principal problem is that a long warm-up period is required before the lamp may be operated at its optimum efficiency, i.e., at an operating temperature of 260C. For example, whereas unassisted fluorescent lamps require only a matter of seconds to reach peak radiance, unassisted sodium lamps require several minutes. Additional problems arise if the sodium lamp is operated in a continuous mode (most photocopying apparatus operate with the illumination lamp continuously energized). For example, the photoreceptor may fatigue when it is continuously flooded with light produced by said sodium lamp, the heat generated by the sodium lamp may harm the photoreceptor, and continuous operation of the lamp may add to the cost of a customers electrical bill. Although the equipment may be devised to protect the photoreceptor from the heat and light (when copying is not in progress) generated by the lamp, additional apparatus to effectuate this protection would add to the cost of the photocopier and the complexity thereof. Further, the brightness (illuminance) ofa continuously operated low pressure sodium lamp is less than desirable.
SUMMARY OF THE PRESENT INVENTION The present invention provides apparatus for operating a low pressure metal halide lamp in a pulsed mode for use in photocopying apparatus. In particular, a low pressure sodium vapor lamp is operated in a, noncontinuous, or pulsed mode whereby an original is illuminated only when a copy is desired. The pulsed mode of operation eliminates the need for mechanical apertures which would be required to prevent illumination on the photoreceptor and/or operator between copies, increases lamp brightness by increasing the current density over nominal value for d.c. operation, and increases potential copy speeds because of increased lamp brightness and total output. The use of sodium vapor lamps, in particular, enables copies to be made of most originals, the copies being an accurate representation of the original.
It is an object of the present invention to provide apparatus for utilization in a photocopying apparatus wherein a low pressure metal or metal halide lamp is utilized, in a pulsed mode of operation, as the illumination source.
It is a further object of the present invention to provide apparatus for utilization in photocopying apparatus wherein a low pressure sodium lamp is utilized, in a pulsed mode of operation, as the illumination source, maximum lamp brightness and efficiency being obtained.
It is still a further object of the present invention to provide apparatus for providing a low pressure sodium vapor lampoperable in a pulsed mode of operation which provides a spectral output which is particularly useful in photocopying applications.
BRIEF DESCRIPTION OF THE DRAWINGS For a better understanding of the present invention, as well as otherobjects and further features thereof, reference is made to the following description which is to be read in conjunction with the accompanying drawings wherein:
FIG. 1 is a schematic diagram showing a low pressure metal halide vapor lamp connected in operating relation to an electric circuit with a direct current power source in combination with a trigger circuit and a standby heating source;
FIG. 2 illustrates an electrostatic photocopying apparatus in which the present invention may be utilized; and
FIG. 3 is a graph depicting the brightness levels obtained when the lamp is operated in the continuous and pulsed modes.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to FIG. 1, an envelope of material capable of being formed into a sealed container to withstand evacuation to partial vacuums and capable of transmitting desired wavelengths of radiation generated by the metal or metal halide gaseous medium within the envelope is shown. Lead-in wires 11 are embedded in the envelope 10, each lead-in wire bearing an electrode 12 and 13 in spaced-apart relationship.
The anode 12 is connected by electrical conductor 14 through an inductor 2 to the positive terminal of capacitor 16 that may be charged through a charging resistance 19 from an energy source, such as battery 15, when switch 17 is closed. Theother, or cathode, electrode 13 is connected to the negative terminal of capacitor 16. I
When switch 17 is closed, the voltage to which the capacitor 16 is charged by battery may, depending upon various characteristics of lamp 1, in and of itself be sufficient to effect a discharge through the gaseous medium within the envelope 10 between the anode electrode 12 and the cathode electrode 13. This initial voltage required to operate the lamp is referred to as the breakdown voltage and is usually greater than later lamp operating voltages because of increased ionization and electrical conductivity of the gaseous medium within envelope 10 after the lamp has been operated for a time. A triggering circuit including source 20 and external winding 21 provides an alternative technique for igniting lamp 1. For example, capacitor 16 may be charged as described hereinabove to a voltage below the breakdown voltage for the particular conditions of flashlamp 1. (Note that the operation of a lamp in a 7 mode which is non-continuous, i.e., alternately on and off, is generally referred to in the art as a flashlamp.) The flashlamp 1 may then be triggered by means of trigger circuit 20 and 21 by transmitting a'pulse from source 20 to external winding 21 to cause partial ionization of the gaseous medium within envelope 10 making the medium conductive enough to permit the voltage stored in capacitor 16 to become discharged through the gaseous medium from the anode 12 to the cathode 13, thereby producing high-intensity radiation of a wavelength dependent upon the gaseous medium utilized. Using this alternate technique of triggering, once the triggering pulse is completed on the winding 21, the conduction path between the electrodes 12 and 13 continues until the energy stored in capacitor 16 is dissipated. Capacitor 16 is then charged via battery 15 as described hereinabove. Therefore, if source 20 comprises a repetitive source of voltage which has the proper amplitude to trigger the ionization of lamp 1, a technique is provided whereby the firing of the flashlamp may be controlled in accordance with predetermined conditions.
Lamp l, in accordance with the teachings of the present invention, is a metal 'or metal halide vapor lamp, and in the preferred embodiment, comprises a low pressure sodium vapor lamp. When the gas is ionized, as described hereinabove, an extremely intense flash of actinic light of relatively short duration, such as for a period of less than 100 microseconds, is generated.
Since low pressure sodium is utilized, the spectral output is predominantly in the 5,900 A range and corresponds to yellow light.
A heating voltage 22 is provided, via diode 23, to heat the winding 21 to maintain the envelope 1 at an elevated temperature (i.e., approximately 260C) which causes the vapor pressure of the sodium (approximately 0.01 Torr) to be correspondingly optimized foroptimum sodium line output, i.e., spectral output is primarily centered about 5,900 A. In the apparatus shown in FIG. 1, the resistance wire 21 is used both to heat the envelope 1 and maintain the sodium vapor pressure at its optimum condition and to provide the trigger electrode to induce a volumetric breakdown in'the sodium vapor and to allow capacitor 16 to dump its energy through lamp 1, thereby producing the illumination pulse as described hereinabove.
In a typical embodiment, the pulse from source 20 is of a width from about 30 microseconds to about 1,000 microseconds and the radiation produced by said pulse has an energy density range from about 10" to about 10 joules per square centimeter.
Other techniques may be utilized to raise the lamp temperature such as applying a transparent conductive coating to the lamp envelope or by utilizing a heating resistance within the lamp envelope, as shown in U.S. Pat. No. 2,755,400.
Low pressure sodium vapor lamps and their operating characteristics are described in the article by W. Elenbaas et al, Improvements in Low-Pressure Sodium Vapor Lamps, Illumination Engineering, Volume 64, Page 94, February, 1969. Particular temperatures, operating pressures, glass envelope composition, etc. described in the article will similarly characterize the low pressure sodium vapor lamp in accordance with the teachings of the present invention. Further, the sodium vapor lamp disclosed in U.S. Pat. No. 3,400,288 may easily be adapted to the pulsed mode of operation in accordance with the teachings of the present invention.
Low pressure sodium vapor lamps are commercially available, for example, from North American Philips Corporation, New York, New York (manufactured by N. V. Philips, Eindhoven, the Netherlands).
In addition to the sodium vapor lamp described hereinabove, other materials, such as thalium, thalium iodide and potassium, may be utilized as the gaseous medium albeit sodium is preferred for the reasons set forth hereinabove.
FIG. 2 schematically illustrates apparatus for electrostatically photocopying documents, or originals, using the xerographic process and the subject matter of the present invention. The sodium vapor lamp apparatus described in FIG. 1 (like elements labeled with identical reference numerals) is positioned above original 18 from which copies are to be made. In the embodiment illustrated, endless belt 20 having a conductive base 31 and a coating 32 of photoconductive material, which is selected to be rendered conductive by the illumination generated by lamp 1 is arranged to run on spaced drums 40, 42, and 44 underneath document 18. Although not shown in the figure, drums 40, 42, and 44 are driven in tandem by an appropriate drive motor.
In operation, the photoconductive belt 30 which essentially comprises a layer made of selenium on an alloy thereof overlying a conductive substrate is initially charged at charging station 50. The charging station comprises a generally well known corona charging device as shown in the prior xerographic art, for example. The belt 30, shown in an endless belt configuration, is driven in the direction of arrow 51. When the lamp 1 is energized in the manner described hereinabove (i.e., when a copy is to be made), the illumination generated thereby (the sodium spectral line in the preferred embodiment).illuminates' document 18. It.
should be noted that the apparatus of FIG. 2 assumes that the original 18 is a transparency since the illumination will pass therethrough to expose photoconductive layer 32. Obviously, illumination lamp 1 can be posi tioned below original 16 if the original is opaque, the generated light being reflected therefrom. In any event, the portions of the document 18' corresponding to image areas or dark areas are absorbed'and the background or transparent areas illumination are passed through to discharge the appropriate portions of the belt 32. The belt is advanced to a development station 60 whereat a housing 62 contains a charge of electroscopic toner particles 64 and. a roll 66 having a brush 68 on its surface. As roll 66 rotates, the brush 68 passes through the toner particles and then across the surface of belt 30, distributing the toner particles over the surface of the belt. The toner particles adhere to the belt in areas containing a residual charge, but not in the unment 16. Alternate development techniques may be utilized, such as powder cloud development as described, for example, in US. Pat. No. 2,701,764.
At station 70, this image is transferred to image receiving web 72. Web 72 is drawn from supply roll 74 and is guided in contact with belt for a short distance by guide rolls 76 and 78. Transfer of the toner particles constituting the developed image may be aided by an appropriate electrical field or by charging of the web 72 by corona charging electrode 70', as is well understood in the art. After the image is transferred to the web 72, the web maybe passed through a heater 75 to fuse the toner particles to the web, and the web is guided by roll 79 to a delivery station.
Since the photoconductive layer 32 is to be reused for a subsequent imaging cycle, after the image transfer operation residual toner particles are removed from the surface of belt 30 by brush 82 at station 80. The photoconductive layer 32 may then be exposed to an illumination source, to erase any residual electrostatic image. Before returning to the optical exposure station, the surface of the photoconductor is exposed to a general corona discharge at station 50, to provide a uniform electrostatic charge over the photoconductive layer 32 and thereby enable electrostatic optical recording of an image of the document 18.
If a multiple number of copies of original 18 are to be made, the same operation is repeated. If a copy of a different original is desired, the'original 18 is replaced with the different original.
FIG. 3 is a graph illustrating the difference in brightness levels (spectral radiance) between a low pressure sodium vapor lamp operated in the continuous (a.c. or d.c.) mode as opposed to the lamp being operated in the pulsed mode, the increased brightness which is obtained from a low pressure sodium vapor lamp operated in the pulsed mode being clearly shown.
While the invention has been described with reference to its preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teaching of the invention without departing from its essential teachings.
What is claimed is:
1. Apparatus for forming a latent electrostatic charge pattern on a photoconductive insulating medium, said latent electrostatic charge pattern corresponding to the radiation pattern projected from an information bearing member comprising:
a charged photoconductive insulating member,
a' low pressure metal vapor source for generating radiation of a predetermined spectral line, said information bearing member being interposed in the optical path between said metal vapor source and said charged photoconductive insulating member, and
. means for operating said low pressure metal vapor source in a pulsed mode whereby pulses of radiation are emitted therefrom, a radiation pulse exposing said information bearing member to' selectively dissipate the charge on the surface of said photoconductive insulating member in accordance with the intensity of the radiation pulse projected from said information bearing medium to form said latent electrostatic charge pattern.
2. The apparatus as defined in claim I wherein said low pressure metal vapor source comprises a metal halide vapor source.
3. The apparatus as defined in claim 1 wherein said low pressure metal vapor source comprises a sodium vapor lamp.
4. The apparatus as defined in claim 3 wherein said low pressure sodium vapor lamp comprises:
an envelope having spaced-apart electrodes and low pressure sodium vapor therein,
means for coupling a voltage of an amplitude below the breakdown voltage of said sodium vapor across said lamp electrodes, and
means for applying a voltage to said lamp of an amplitude sufficient to cause said sodium vapor to breakdown whereby a radiation pulse is emitted therefrom.
5. The apparatus as defined in claim 4 further including means for heating the sodium vapor within said envelope.
6. The apparatus as defined in claim 5 wherein each radiation pulse is generated for a period in the range from about 30 microseconds to about 1,000 microseconds and in the energy density range from about 10 to about 10 joules per square centimeter.
7. The apparatus as defined in claim 6 wherein the pressure of the sodium vapor within said envelope is approximately 0.01 Torr.