Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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- BACKGROUND OF T~E INVE~TION
In the xeroyraphic process as described in
U.S. Patent 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 ~orm
a powder image corresponding to the latent electro-
static image remaining after exposure. The powder
image then can be transferred to a sheet of tr~ sfer ~;
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 90 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
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maximum from the ~ackground 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
utilize~ aperture fluorescent lamps which generate
colored light.
. . .
Low pressure sodium lamps represent a commercially
available yellow light source. Present commer~ial sodium
lamps, such as those manufactured by ~. 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
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optimum efficiency, i.e., at an operating temperature
; of 260C. For example, whereas unassisted fluorescent
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lamps require only a matter of seconds t~ reach peak
radiance, unassisted sodium lamps require several
minutes. Additional problems arise if the sodium lamp
is operated in a continuous mode tmost photocopying
apparatus operate with the illumination lamp continuously
energized). For example, the p~otoreceptor 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 customer's 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 tilluminance) of a continuously operated
low pressure sodium lamp is less than desirable.
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SUMMARY OF THE PRESENT INVENTION
In accordance with one aspect of this invention
there is provided apparatus for forming a latent electrostati~
charge pattern on a photoconductive insulating medium, said
latent electrostatic charge pattern corresponding to the
radiation pattern projected from an infonnation 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 in- ~;
sulating 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 in-
~ormation 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
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' electrostatic charge pattern.
BRIEF DESCRIPTION OF THE DRAWINGS
. For a better understanding of the present invention
~ reference is made to the following description which is to be
read in conjunction with the accompanying drawings wherein:
Figure 1 is a schematic diagram showing a low
pressurG metal halide vapor lamp connected in operating
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relation to an electric circuit with a direct current
power source in combination with a trigger circuit and
a standby heating source;
Figure 2 illustrates an electrostatic photocopying
~; S apparatus in which the present invention may be utilized;
and ( ~; r 5~ 5~ e ~ s)
: Figure 31is a graph depicting the brightness
levels obtained when the lamp is operated in the :~
continuous and pulsed modes.
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DESCRIPTION OF THE PREFERRED EMBODIMENT
Referring now to Figure 1, an envelope 10 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 l:L 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 17 is
cloqed. The other, or cathode, electrode 13 is connected
~ 15 to the negative terminal of capacitor 16.
:~; When switch 17 is closed, the voltage to which
the capacitor 16 is charged by battery 15 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 `~
~ 25 conductivity of the gaseous medium within envelope 10 after
:~ the lamp has been operated for a time. A triggering circuit
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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
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conditions of flashlamp 1. (Note that the operation of a
lamp in a mode which is non-continuous, i.e., alternately
on and off, is generally referred to in the art as a
flashlamp.) I'he 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 trigyering, 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 1, 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 5900 A range and corresponds to yellow light.
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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 (appro~imately
0.01 Torr) to be correspondingly oplimized for optimum
sodium line output, i.e., spectral output is primarily
centered about 5900 A. In the apparatus shown in Figure 1,
the resistance wire 1 is used both to heat the envelope 1
and maintain the sodium vapor pressure at its optimum
~; 10 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 embddiment, the pulse Erom source
20 is o~ a width ~rom about 30 microseconds to about 1000
microseconds and the radiation produced by said pulse has an
energy density range from about 10-4 to about 10-2 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.
Patent 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 La~ps, Illumination Engineerin~, Volume 64, Page 94,
February, 1969. Particular temperatures, operating
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pressures, glass envelope composition, etc. described
in the article will similarly characterize the low
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pressure sodium vapor lamp in accordance with the
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teachings of the present invention. Further, the sodium
vapor lamp disclosad in U.S. Patent No. 3,400,2~8 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.
Figure 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 Figure 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
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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 oE 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 note~ that the apparatus of
Figure 2 assumes that the original 18 is a transparency
since the illumination will pass therethrough t~ expose
photoconductive layer 32. Obviously, illumination lamp 1
can be positioned 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 electroscopi~ toner particles
; 6~ and a roll 66 having a brush 68 on its surface. As
roll 66 rotates, the brush 68 passes through the toner
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: 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 uncharged ar~as, resulting in
development to visual form of the latent electrostatic
image corresponding to document 16. ALternate development
techniques may be utilized, such as powder cloud development
as described, for example, in U.S. Patent 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 30 for a short distance
; by guide rolls 76 and 7~. Transfer of the toner particles
, constituting the developed image may be aided by an .
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appropriate electrical ~ield or by charging of the web 72
by corona charging electrode 70', as i~ well understood in
- the art. After the image is transferred to the web 72, .
the web may be 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 illumin-
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25 ation source, to erase any residual electrostatic image.
Before returning to the optical exposure station, the
: surface of the photoconductor is exposed to a general
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.~ corona discharge at station 50, to provide a uniform
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. electrostatic charge over the photoconductive layer 32
and thereby enable electrostatic optical recording of an
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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.
Figure 3 is a graph illustrating the difference
in brightness levels (spectral radicmce) 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.
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.
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