Method of electrophotography using low intensity exposive

My invention comprises an improved method of electrophotography which enables me to increase the effective speed of a photoconductor. The speed at which copies may be made is a function of the quantum of light falling on the photoconductive surface and the conductivity of the photoconductor under illumination. Since the rate at which a given photoconductor discharges the surface potential on the photoconductor through the action of light is limited, speed can be increased for a given photoconductor only by increasing the illumination. This requires energy and produces heat. My process deliberately underexposes a charged photoconductor to a light and shade image of the original to produce a weak latent electrostatic image of low contrast which is insufficient to make a satisfactory copy. I then mask the latent image with a liquid-carried toner while preventing deposition of the toner on the background areas. I then discharge the background areas with a blanket illumination of low intensity. The optical mask prevents the image areas from discharging while enhancing the constrast of the weak latent image. The enhanced image is then easily developed by any known developing method for making latent electrostatic images visible at a development station.

BACKGROUND OF THE INVENTION 
One of the main problems with electrophotographic copiers arises from the 
desire for speed in copy production. After a photoconductor has been 
charged, the energy required to produce a latent image in light and shade 
of the original of sufficient contrast to produce an acceptable copy is a 
function of the quantum of light falling upon the photoconductor and the 
light sensitivity of the photoconductor. Ideally, the illumination of the 
photoconductor should be such that the brightest part of the image will be 
fully discharged while the darkest part of the image will leave the 
photoconductor fully charged. In practice, this is never achieved, owing 
to the limits of the light response of known photoconductors. In the 
current state of the art of photocopying machines, when the speed of 
producing copies exceeds about 30 copies per minute, the energy required 
to operate the copier approaches 1500 watts. Since the ordinary potential 
in office and house wiring is 110 volts, the power from a given outlet is 
limited to 1500 watts. Accordingly, to produce satisfactory copies at a 
higher rate, a special electrical installation will be required. This 
means that the copying machine cannot be decentralized, but must be 
located in the region of the higher voltage outlet. Furthermore, the high 
energy will produce thermal problems, both in respect of the 
photoconductor and in the environment, aside from the expense of energy 
consumption. Because of these problems, many efforts are being made to 
increase the light sensitivity of photoconductors. 
1. Field of the Invention 
My invention relates to a novel method of increasing the effective 
sensitivity of photoconductors, thus enabling me to increase the speed of 
electrophotographic reproduction of documents. 
2. Description of the Prior Art 
The following art is of interest in respect of or is referred to in this 
specification: 
Steinhilper--U.S. Pat. No. 2,756,676 
Schaefer et al--U.S. Pat. No. 3,892,481 
Hayashi et al--U.S. Pat. No. 3,907,423 
Brooke--U.S. Pat. No. 3,912,387 
Brooke--U.S. Pat. No. 3,994,723 
Steinhilper, which will be discussed more fully hereinafter, proposes to 
make multiple copies of an image produced from a single light exposure of 
an original. He recharges the photoconductor after each transfer of a 
developed image and enhances the recharged image by subjecting it to 
illumination. There is no teaching of increasing the speed of the 
xerographic reproduction process. The apparatus shown by Steinhilper has 
only one development station. There is no optical masking station. There 
is no showing of a biased toner applicator at a toning station where 
optical shielding is achieved. 
Schaefer et al show an automatic control system for biasing a development 
electrode. This system can be used both for the mask-forming step, which 
is a salient feature of my invention, and for the development step as 
taught by Schaefer et al. 
Hayashi et al show a reverse roller designed to remove excess liquid from 
the photoconductor after the latent image has been developed. I employ a 
roller of this type, insulated from ground and biased to a voltage of the 
same polarity as the charge on the photoconductor, but at a potential 
higher than the background potential, in order to ensure that no toner is 
deposited on the background areas of the image when the mask-forming step 
is performed. 
Brooke U.S. Pat. No. 3,912,387 and its divisional U.S. Pat. No. 3,994,723 
show detecting background areas which are underexposed and discharging 
them by light before development of the latent electrostatic image. 
SUMMARY OF THE INVENTION 
One object of my invention is to provide an improved method of 
electrophotography which will increase the speed of the copying operation. 
Another object of my invention is to provide an improved method of 
electrophotography which will increase the speed of copying and reduce the 
quantum of energy required in the operation. 
Another object of my invention is to increase the effective speed of 
photoconductors. 
Another object of my invention is to provide an improved method of 
electrophotography which will enable me to copy originals having poor 
contrast. 
Another object of my invention is to copy originals of a color to which the 
photoconductor has a great sensitivity. 
Another object of my invention is to provide a novel apparatus for carrying 
out my improved method of electrophotography. 
Other and further objects of my invention will be apparent from the 
following description.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In general, my invention contemplates charging a photoconductor in the 
dark. The charged photoconductor is then exposed to a light and shade 
image of the original. This will form a latent electrostatic image on the 
photoconductor. The original exposure is only a fraction, such as 5% or 
10%, of the quantum of light normally required to produce a satisfactory 
image--that is, one having sufficient contrast so the image areas are dark 
and the background areas are white. The latent image thus formed has 
sufficient contrast, however, so that it can be toned--that is, 
developed--with any appropriate toner such as resinous powder or, more 
preferably, by a toner dispersed in an insulating liquid, as is well known 
to the art. This masking step produces a shield substantially opaque to 
light over the image areas of the original being copied. A critical 
feature of the masking step is that a shield will be produced only over 
the image areas, and not over any of the background areas. This is 
accomplished by ensuring that the development electrode, or means for 
applying the toner, during the masking step is biased to a potential above 
that existing on the background areas and below that present on the image 
areas of the latent electrostatic image. 
I then expose the photoconductor to a blanket of light. The electrostatic 
charge of the masked image on the photoconductor will decay marginally or 
not at all, while the electrostatic charge of the background areas will be 
discharged to a very low voltage, such as 50 volts or the like. This has 
the effect of enhancing the charge of the latent electrostatic image by a 
very large percentage with the requirement of about one-tenth of the 
energy which would normally be necessary to produce a latent image having 
the strong contrast now achieved by my method. The enhanced electrostatic 
image can then be developed in any appropriate manner known to the art 
and, if desired, readily transferred to a carrier sheet. 
More particularly, referring now to FIG. 2 in which apparatus for carrying 
out my invention is shown, a metal drum 10 carries a photoconductive layer 
12 which may be selenium. The metal drum 10 is supported by apertured 
disks 14 which are mounted on a shaft 16 and keyed thereto for rotation 
therewith. The shaft 16, which may be grounded, is driven by any 
appropriate means known to the art to rotate the drum 10 in the direction 
of the arrow. A charging corona 18 is adapted to charge the surface of the 
selenium photoconductor 12 to a voltage of between 800 and 1000 volts. To 
accomplish this, the charging corona is energized to a positive potential 
of 5000 or 6000 volts. The elements of the corona discharge unit cause 
ionization of the circum-ambient atmosphere and place a uniform positive 
charge over the surface of the selenium. If my process were being 
practiced with a zinc oxide-coated paper, the corona would be powered to 
produce a negative charge, as will be readily understood by those skilled 
in the art. The photoconductor 12 is then carried past the exposure 
station indicated generally by the reference numeral 20. Projection 
optics, indicated diagrammatically by the lens 22, project an image of the 
original to be copied upon the photoconductor 12. In my method, the 
exposure time is extremely short. I have made copies with an exposure of 
as little as 5% of normal and have been able to achieve completely 
satisfactory copies of the originals. 
A selenium photoconductor will generally discharge to about one-fifth of 
its original charge in about three foot-candle seconds. This can be 
readily seen by reference to FIG. 3, which shows a surface potential on a 
selenium photoconductor of 800 volts being discharged to 160 volts in 
between two to three foot-candle seconds. Normally, sufficient energy is 
employed in the light source of photocopying machines so that the 
background areas of the photoconductor will be discharged to about 50 or 
60 volts. This will require about five foot-candle seconds. Five percent 
of this quantum of light is about 0.25 foot-candle seconds. By referring 
to FIGS. 3 and 4, it will be noted that, after this short exposure, the 
background areas (B) will have dropped in voltage about 100 volts from the 
image areas (I). If this image were toned, a very low-contrast image would 
be achieved. If this image, after being toned, were to be transferred onto 
paper from the drum, the density of the toned image would be so small that 
a poor transfer or a failure to transfer would result and only a faint 
image would appear. The low-contrast image, however, when developed, has 
sufficient optical density so that it provides a mask or shield for the 
latent electrostatic image which is sufficiently dense for the practice of 
my process. 
Referring again to FIG. 2, I show apparatus for providing a mask or shield 
for the latent electrostatic image. It comprises a tank 24, from which a 
developing liquid 26 containing dispersed toner particles, which may be 
charged, is drawn through pipe 28 and pumped by pump 30 through pipe 32 to 
nozzle 34, adapted to discharge the developer between the photoconductor 
and a reverse roller 36. If the toner particles are conductive, they may 
acquire a charge by induction, owing to their passage adjacent the latent 
image under the action of its electric field. In this case, the 
electrostatic charge pattern serves first to charge the particles and then 
to trap them. In the case of a liquid-carried toner particle, the 
continuous phase is an insulating liquid--such as a hydrocarbon liquid, a 
fluorinated hydrocarbon liquid, or the like--having low vapor pressures at 
room temperature, and the disperse phase is composed of the minute 
particles of toner adapted to make the latent electrostatic image visible. 
As is known in the art, the polarity of the charged particles may be 
controlled by materials added to the developing liquid. These act by 
adsorption onto the surface of the particles and alter the magnitude and 
polarity of the charge acquired by the particles, depending on the 
environment of the particles at the time of their formation and the method 
of their preparation. The toner particles must be applied by a development 
electrode biased to a potential of the same polarity as that of the latent 
electrostatic image and to a potential greater than that of the background 
areas and below that of the image areas. Preferably, I employ a reverse 
roller as the development electrode when a liquid developer is used. This 
reverse roller is made of metal and is mounted on shaft 38 for rotation in 
a direction opposite to the rotation of the photoconductor. The reverse 
roller is insulated from ground and is positioned closely adjacent the 
surface of the photoconductor to provide a gap ranging from 0.05 to 0.1 
millimeter. The reverse roller is driven by a prime mover and is 
controlled in speed so as to remove excess developing fluid from the 
photoconductor. The biasing of the development electrode is critical to my 
process, since there can be no masking of the background areas in my 
process, as will be pointed out more fully hereinafter. It will be 
appreciated that the close proximity of the insulated metal reverse roller 
to the surface of the charged photoconductor is such that it will float to 
assume the average potential of the photoconductor and thus be 
auto-biased. Since the average potential on the reverse applicator roller 
36 will be above the background potential on the photoconductor, toner 
particles will migrate to the applicator roller instead of to the 
background areas on the photoconductor. If desired, instead of permitting 
the reverse applicator or metering roller 36 to float electrically, it may 
be biased to a potential from any appropriate D.C. voltage source to above 
the potential of the background areas but below the potential of the image 
areas. The bias on the toner applicator roller will substantially 
eliminate the deposition of masking toner on the background areas of the 
photoconductor. In the usual developing liquid, however, the carrier 
liquid has a low boiling point, so that it is easily vaporized to ensure 
that the developed image, when transferred to a carrier sheet such as 
paper, will produce a copy dry to the touch. In the masking step, however, 
a hydrocarbon carrier liquid having a higher boiling point which will not 
vaporize may be employed. This, of course, will reduce the danger of 
atmospheric pollution during the masking step. If desired, instead of a 
liquid-carried toner, dry toner may be used for the masking step. Such dry 
toners are well known to the art. One example of a reverse roller which 
can be used in my invention is shown in Hayashi et al U.S. Pat. No. 
3,907,423. The excess toner from the masking step will be caught in the 
tank 24 for recycling. A wiper blade 40 keeps the metal reverse roller 
clean. 
After the optical shield is produced in any appropriate manner such as 
described, the photoconductor is subjected to a blanket of light. This may 
be accomplished by an elongated incandescent lamp 42 placed adjacent the 
photoconductor bearing the masked image and extending thereacross. In an 
office copier, a quartz-halogen lamp having an output of about 500 watts 
is usually employed. In my process, a much lower-energy exposure lamp to 
project the image may be employed. Furthermore, the illumination of the 
background areas to discharge them to a residual voltage of about 50 volts 
requires comparatively small energy. As will be readily appreciated by 
those skilled in the art, the exposure step subjects the photoconductor to 
an image of the original by reflected light. The illuminated original is 
focused by the optical system upon the charged photoconductor. Since the 
light gathered by the optical system is a small fraction of the light 
which illuminates the original, a bright illumination of the original is 
required. In the background discharging step, after the optical shield is 
in place, the illumination of the photoconductor is by direct light, which 
accounts for the small energy required to discharge the background areas. 
The image areas (I) will not be discharged owing to the mask or shield 
which I have provided by my process as just described. The effect of 
discharging the potential of the background areas (B) while leaving the 
image areas (I) substantially undischarged is shown in FIG. 5. 
It will be seen that my process has achieved a contrast of substantially 
750 volts between the background areas and the image areas and 
accomplished the creation of this strong field at approximately a tenfold 
reduction of the energy required to expose the original. This means, as 
will be readily apparent, that a photocopying machine which presently uses 
a 500-watt lamp of the quartz-halogen type with a tungsten filament could 
use a 50-watt lamp or, alternatively, employ a fluorescent light. It will 
also be appreciated that, where a photocopying machine presently is able 
to make only about 25 copies per minute, theoretically, I can easily make 
a photocopying machine capable of producing 150 copies per minute. As a 
practical matter, however, owing to the inertia of the parts of the 
photocopying machine and in order to avoid marginal operation to provide a 
factor of safety, a photocopying machine which will produce 75 to 100 
copies per minute can be made embodying my invention. Furthermore, this 
can be done without having to increase the energy expended significantly, 
since the only additional energy required will be that employed in the 
first developing or masking step and that in the light discharging of the 
photoconductor after the image areas have been masked. It will be further 
understood that, instead of an incandescent lamp, I may use any 
appropriate light source adapted to flood the photoconductor. 
The use of a floodlight to enhance a faint latent image on a photoconductor 
is not new in and of itself. Steinhilper U.S. Pat. No. 2,756,676 describes 
a method of making a plurality of xerographic reproductions from a single 
exposure of an original. In Steinhilper, however, the effective speed of 
the photoconductor is not increased, owing to the fact that Steinhilper 
must go through a first development step which produces a fully-toned 
image. There is no masking step as described in my process. After the 
first image is developed, it is transferred to a carrier sheet such as 
paper. The faint image which is left on the photoconductor is of a 
potential too low to be enhanced by light or to be redeveloped. 
Steinhilper does not erase this image on the photoconductor, but recharges 
the photoconductor. He then discharges the background areas by light. 
Since the faint image does produce a shield, an enhanced latent image will 
be produced. The salient feature of my process, however, is absent from 
Steinhilper. He does not form a low-contrast latent electrostatic image in 
such a manner as to leave the background areas free of developer, owing to 
the fact that his development electrode is never biased but always at 
ground. Steinhilper must carry out his process to form the residual image 
from the first transfer of the developed image at the normal slow rate. 
Thus the unobvious result which I achieve--namely, the increasing of the 
effective sensitivity of the photoconductor--is not taught, nor can it be 
achieved, by Steinhilper. 
Owing to the tremendous range through which I am enabled to obtain 
sufficient optical density to produce a mask, I can employ a single 
exposure and a biased setting in the developer system and obtain a sharp, 
clear copy from any original, whether the background is ultra white or 
dingy gray. 
It will be readily apparent to those skilled in the art that, with the 
contrast potential shown in FIG. 5, there is no problem in obtaining a 
sharp, clear image. After the background potential has been discharged by 
the lamp 42, the optical shield may be wiped from the enhanced latent 
image thus formed by a cleaning roller 44 made of sponge rubber or the 
like, if desired. This wiping action can take place with either a 
liquid-toned mask or a dry developer-toned mask. The enhanced latent 
electrostatic image may then be toned by any usual method known to the 
art. 
In FIG. 2, I have shown the toning system described in Schaefer et al U.S. 
Pat. No. 3,892,481, employing a tank 46 from which a liquid toner 48 
circulates from pipe 50 to a toner supply tank (not shown) and back 
through pipe 52 to the tank 46. A development electrode 54 is controlled 
to bias any residual voltage left on the background of the photoconductor. 
It will be readily appreciated, however, that since I have discharged the 
background potential by my method, I can use a fixed bias slightly above 
the average residual bias of the background. This will produce a clear 
white background and enable me to eliminate, if desired, the sensing and 
biasing method shown in the Shaefer et al patent. 
After development with a liquid-carried toner, a reverse roller 56, such as 
shown in Hayashi et al U.S. Pat. No. 3,907,423, is positioned to remove 
excess developer from the developed image. The reverse roller 56 is 
provided with a wiper 58. The reverse roller 56 is positioned and rotates 
at speeds as described in the Hayashi et al patent. 
The image is now ready to be transferred to a carrier sheet such as plain 
paper. A plain paper sheet 60 is fed by rollers 62 to a roller 64, past a 
transfer-charging corona 66. It will be recalled that the toned image 
still comprises a visible image over a high positive charge on the surface 
of the selenium drum in the image areas which have not been discharged by 
light or by the bias applied to remove the residual background potential. 
To transfer the developed image from the drum to the paper carrier sheet, 
a high positive charge is applied to the back of the copy paper. As a 
result of the application of the high positive charge to the sheet, the 
toner particles are pulled from the drum surface onto the paper. A 
pick-off 68 ensures that the paper leaves the drum, and the end of the 
paper 70, now carrying the image, may be dried and passed to a receiving 
tray (not shown). A cleaning roller 72 wipes the drum clean of any 
particles of toner which have not been removed from the drum, and a wiper 
blade 74 completes the drum-cleaning operation. 
It will be understood, of course, that if my process is applied to a zinc 
oxide-coated paper, the image will remain on the coated paper. It will 
also be understood that a dry toner, made of fusible resinous powder and 
fixed by heat, may be employed, as is well known to the art. 
It will be understood by those skilled in the art that, since the 
photoconductor now bears an enhanced latent electrostatic image having a 
strong field, it may be developed in any appropriate manner known to the 
art. 
A selenium photoconductor is very sensitive to blue light and, accordingly, 
photocopying machines using a selenium photoconductor do not produce 
copies from bluecolored originals with high contrast. That is to say, a 
selenium photoconductor "sees" blue light as almost white. My method will 
reproduce blue originals as if they were black. A yellow original is very 
light in color and reflects considerable light, so that it appears faint 
in the copies made by photocopying machines. My method of image-enhancing 
reproduces yellow effectively. Furthermore, gradations in density--that 
is, a gray scale--are also achieved with my method. 
Owing to the wide latitude of effective photoconductive sensitivity which 
my process provides, when using the automatic bias of Schaefer et al U.S. 
Pat. No. 3,892,481, I can adjust the illumination in the step which forms 
the lowcontrast image prior to the mask-forming step to produce a 
satisfactory reproduction with the brightest background--that is, a pure 
white background--in the original document. This will produce an image of 
contrast too low to be effectively developed to a satisfactory copy, but 
such that the mask-forming step can be easily performed. The bias 
potential applied to the applicator roller will be well above that 
required to eliminate background potential of the latent electrostatic 
image completely, but will be below the potential of the image areas. This 
permits the image areas to be optically masked by toner during the optical 
shield-forming step. The illumination of the image-forming step may be 
readily controlled by varying the intensity of the light source or by a 
shutter in the optical path of the image-projection system. 
It will be further understood by those skilled in the art that, while I 
have shown and described the toning of an image and then its transfer to a 
carrier sheet, my image-enhancing process can be used in any method of 
electrophotography. For example, the enhanced latent electrostatic image 
can be transferred to a dielectric sheet and then toned or developed into 
a visible image on the dielectric sheet, as is well known in the art. 
It will be seen that I have accomplished the objects of my invention. I 
have increased the effective sensitivity of photoconductors. I have 
provided an improved method of electrophotography which will greatly 
increase the speed of copying operations. My method achieves this increase 
in speed with a reduction of the quantum of energy required. I am enabled 
to copy originals having poor contrast which will produce copies having 
surprisingly increased contrast as compared with the originals. I am 
enabled to copy originals formed in colors to which the photoconductor has 
great sensitivity and which, accordingly, do not ordinarily produce copies 
having the desired contrast. I have provided a novel apparatus for 
carrying out my improved method. 
It will be understood that certain features and subcombinations are of 
utility and may be employed without reference to other features and 
subcombinations. This is contemplated by and is within the scope of my 
claims. It is further obvious that various changes may be made in details 
within the scope of my claims without departing from the spirit of my 
invention. It is, therefore, to be understood that my invention is not to 
be limited to the specific details shown and described.