US3512967A - Electrophotographic method and member for contact printing of relatively opaque documents - Google Patents

Description

y 9, 1970 c. E. HERRICK, JR 3,512,967

ELECTRQPHOTOGRAPHIC METHOD AND MEMBER FOR CONTACT PRINTING OF RELATIVELY OPAQUE DOCUMENTS Filed Nov. 9, 1966 3 Sheets-Sheet 1 [NT/ENTOR CLIFFORD E. HERRICK,Jr.

BYW U. m

ATTORNEY May 19, 1970 c. E. HERRICK, JR 3,512,957

ELECTRQPHOTOGRAPHIC METHOD AND MEMBER FOR CONTACT PRINTING OF RELATIVELY OPAQUE DOCUMENTS Filed Nov. 9, 1966 3 Sheets-Sheet 2 IMAGE CONTRAST GAIN l 0.00 2.0 '60 0.40 F|G.3 1.0 I 0.50

FRACTIONAL UGHT ABSORPTION IMAGE 31 A CONTRAST GAIN 20 0.00

FRACTIONAL LIGHT ABsORPTION 000 IMAGE I. CONTRAST 1' I (SAW l i t 0.

2.0- 1 "1' +-1I-- l I i I I I i F|G.5 1.0 E! I 1 1 L FRACTIONAL LIGHT ABSORPTION United States Patent 3,512,967 ELECTROPHOTOGRAPHIC METHOD AND MEM- BER FOR CONTACT PRINTING OF RELATIVELY OPAQUE DOCUMENTS Clifford E. Herrick, Jr., Los Gatos, Calif., assignor to International Business Machines Corporation, Armonk, N.Y., a corporation of New York Filed Nov. 9, 1966, Ser. No. 593,051 Int. Cl. G03g 13/22 US. Cl. 96-1 Claims ABSTRACT OF THE DISCLOSURE A contact printing electrophotographic method in which an opaque document such as bond paper is brought into face-to-face contact with a photoconductive layer on a highly light reflective backing, followed by uniformly exposing the photoconductive member through the back of the document. The exposing light is reflected back and forth between the highly light reflecting backing of the photoconductive member and the nonimage face surface of said document with only a per tion of the light being absorbed by the photoconductive layer during each passage of light, thereby enhancing the image contrast of the resultant copy of the document.

This invention relates to an electrophotographic method and member and, more specifically, relates to the exposure of the electrophotographic member.

In the method of electrophotography, commonly known as xerography, a photoconductive member is given a uniform electrostatic charge over its surface and is then exposed to an opaque document to be reproduced by a conventional projection technique, such as scanning an illuminated document with a photographic copying camera having a linear optical system. An image of the document to be reproduced is reflected from the document as a light image which is transmitted through an objective lens and onto the photoconductive surface. The areas of the photoconductive member exposed to the light image are discharged and an electrostatic latent image is created. Development of this electrostatic latent image is then achieved with an electrostatically charged material, such as electroscopic powder which is brought into surface contact with the photoconductive member and is held thereon electrostatically in a pattern corresponding to the electrostatic image. Thereafter, the developed electrostatic image is transferred to a surface, such as paper, to which it may be fixed by heating or any other suitable means.

Exposure of the photoconductive member by the projection system, as described above, is disadvantageous for a number of reasons. For example, lenses of good optical quality are expensive and their elimination can result in appreciable savings. Moreover, due to the photographic speed of present day photoconductors, projection exposures in most cases require exposures of a few seconds up to 20 or 30 seconds. Further, with a projection exposure system, the document must be positioned away from the photoconductive member at a distance determined by the focal length of the lens system, thereby increasing the space requirements and, consequently, the flexibility of the design of the electrophotographic apparatus.

An alternative exposure system in which the photoconductive member is exposed Without a lens system is contact printing. In this type of system, the photoconductive member is brought into contact with the document to be reproduced and is exposed to a light source through the document. One of the principal advantages of contact printing is that it is many times more efficient in it utilization of light than a projections system and, hence, is much better suited for high speed operation. However, in order to obtain a good quality reproduction by this system with photoconductors such as selenium, the document must be transparent or translucent to the exposing radiation and carry a dense, high contrast image. Otherwise, the contrast ratio of the image to nonimage areas will not be suflicient to provide a developable el'ectrostatic image of good quality.

Accordingly, it is an object of the present invention to provide a new and improved electrophotographic method employing contact printing and yielding high quality reproduction of opaque documents.

It is another object of the present invention to provide a new and improved electrophotographic member which enhances the image contrast when employed with opaque documents.

Still another object of the present invention is to provide a new and improved electrophotographic apparatus which is inexpensive, compact in size, and capable of operating at a high rate of speed. a

In general, the foregoing and other objects and other advantages of the present invention are achieved by an electrophotographic method in which an opaque document is brought into face-to-face contact with a photoconductive member comprising a photoconductive film and with a highly reflecting backing, and exposing through the back of the document to light which is only weakly absorbed by the photoconductive film and to which the photoconductive film is substantially transparent.

Other and further objects and advantages of the invention will be apparent in the following more particular description of the preferred embodiments of the invention, as illustrated in the accompanying drawing in which:

FIG. 1 represents an electrophotographic apparatus embodying the exposure system of the present invention.

FIG. 2 is an enlarged fragmentary cross-section of the photoconductive member of the exposure system of FIG. 1.

FIGS. 3, 4, and 5 are graphs to be utilized in explaining the mechanism of the exposure technique of the present invention.

FIGS. 6, 7 and 8 are graphs to be utilized to show the improvements in image contrast with the photoconductive member of the present invention.

The electrophotographic apparatus shown in FIG. 1 embodying the exposure system of the present invention comprises a rotatable drum 1 carrying around its periphery an electrophotographic photoconductive member 2 having situated adjacent its periphery a corona unit 3 for electrostatically charging the surface of the drum. Herein, the drum 1 rotates in a clockwise direction and after its surface is electrostatically charged by the corona unit 3, a document 5 carrying an image 6 to be reproduced is fed into virtual contact with the electrostatically charged surface past the exposure station 7 wherein the photoconductive member 2 is illuminated through theback 8 of the document 5 by a light source 9 having an exposure slit 10. The electrostatic charges on the area of the photoconductive member 2 corresponding to the nonimage areas of the document 5 are dissipated to a greater degree than the electrostatic charges on the areas of the photoconductive drum corresponding to the image areas, thereby forming a developable electrostatic image. After exposure, the document 5 is separated from the drum 1 and the drum passes a development station 11 at which a toner-carrier mixture 12 is gravity fed across the electrostatic image on the surface of the photoconductive member 2. The toner having a charge opposite from the polarity of the electrostatic image is attracted to the image rendering it visible. Herein, an endless conveyor belt 13 carries the toner-carrier mixture to a position for gravity feeding it across the surface of the photoconductive member. Continuing the clockwise rotation of the drum 1, a copy paper 14 is fed into contact with the developed electrostatic image. Preferably, a corona unit 15 is disposed beneath the paper at the area of contact and with a polarity opposite that of the toner, thereby attracting the toner to the copy paper. After this so-called corona transfer, the paper 14 is separated from the drum and fed past a fusing element 20 to permanently fix the toner to the paper. The drum 1 continues to rotate past a cleaning brush 16 which wipes the surface of the photoconductive member so as to remove any excess toner. This completes a cycle of the drum.

In accordance with the present invention, the photoconductive member comprises a film of weakly light absorbing photoconductive material being substantially transparent to light and with a highly reflecting backing so that, in being exposed through an opaque document, the light penetrating the document passes through the photoconductive film and is reflected between the highly reflecting backing and the reflective nonimage areas of the document, two portions of the light being absorbed by the photoconductive material during each reflection cycle. In the image areas, however, the light reflected from the backing of the photoconductive material is absorbed rather than being reflected by the image. Therefore, there is a substantial increase in the ratio of light absorbed in the nonimage areas to the light radiation absorbed in the image areas as compared to the ratio achieved with a strongly absorbing photoconductive material.

As best shown in FIG. 2 for purposes of illustration, the photoconductive member 2 comprises a photoconductive film 17 and a backing 18, both of which are carried on the drum 1. The exposing light, as shown by the arows 19, strike the back of the opaque document and a portion of the light rays are reflected off of the document depending upon the reflectivity, R,, of the back surface 8 of the document. The remainder of the light rays pass through the document with a portion being absorbed depending upon the transmission density, D,,, of the document. Another portion of the light rays in the image area 6 are absorbed by the image depending upon its transmission density, D At this point, if the photoconductor would totally absorb the remaining light striking its surface, the exposure contrast, C (i.e.the ratio of light absorbed in the photoconductive material in the nonimage areas to the light absorbed in the image areas) is essentially dependent only upon the transmission density of the image area, D.

However, in accordance with the present invention, a weakly absorbing photoconductive material or one which only absorbs a fraction of the light striking its surface is employed. Hence, in addition to D the exposure contrast, C becomes dependent on the fractional light absorption, A, of the photoconductive material; the reflectivity, R of the backing of the photoconductive material; and the reflectivity, R,,', of the face surface in the nonimage areas of the document. That is, in the process of the present invention, the initial light striking the photoconductor which is not absorbed by it passes through the photoconductor and is reflected back from the backing of the photoconductor. Again, a portion of the light is absorbed and the remaining portion passes out of the photoconductor. This portion, depending on whether an image or nonimage areas is adjacent the photoconductor, either is reflected back from the surface of the nonimage area of the document or is substantially absorbed by the image. The document reflected light passes through the photoconductor, with a portion being absorbed, and is reflected back oif of the backing of the photoconductor. Also, a portion of this light is absorbed by the photoconductor as it travels back toward the document.

Thus, it can be seen from the foregoing that the light initially reaching the photoconductor is reflected back and forth between the backing of the photoconductor and the nonimage surface of the document, as shown in FIG. 2. During each complete reflection cycle, two portions of the light are absorbed by the photoconductor. In addition, in the image areas of the document adjacent the photoconductor, each time a portion of the light is reflected out of the photoconductor, a major portion of it is absorbed by the image with the amount of the absorption dependent on the image density, D Even with documents having relatively poor image density, the amount of light which is not absorbed and, hence, is reflected back towards the photoconductor, is significantly less than the amount of light reflected from the nonimage surface.

To further illustrates the contrast enhancement or gain of the present invention, reference is now made to the graphs shown in FIGS. 3, 4, and 5. It will be recalled that, with a photoconductor which totally absorbs the light striking it, the image contrast, C is only a function of the image density, D More accurately, C, is equal to 10 1. Conversely, the image contrast, 0,, in the process of the present invention is also a function of R R and A, as defined above. Therefore, setting C =1O i as the reference, the enhancement of image contrast over this reference or gain, 6,, can be expressed as follows:

wherein the terms are defined as stated above. That is, the term, G,, is the ratio of exposure contrast of the contact exposure method of the present invention to the exposure contrast of a contact printing method with a strongly absorbing photoconductor.

Turning now to FIG. 3, there is shown a graph, based on the above expression, plotting contrast gain, G versus the fractional light absorption of the photoconductor for a document having an image transmission density, D of 1.0. The curves are plotted for different products of reflectivity, R R ranging from 0.30 to 0.85. The lower of these two reflectivity products is representative of a low reflective document, such as vellum, and the higher numbered reflectivity product represents an opaque highly reflective document. It will be seen from this graph that the contrast gain increases as the fractional absorption of light of the photoconductor decreases and the product of reflectivity increases. This is also shown to be the case when the image transmission density is low, such as 0.2, as shown in the graph of FIG. 4.

In FIG. 5, a graph is shown in which the contrast gain, 6,, is plotted versus the image transmission density for a photoconductor having a 5% light absorption. This graph illustrates that the contrast gain, G,, is constant over a wide range of image densities. FIG. 5 also amplifies the influence of the reflectivity product on the gain, but it will be noted that, even with low reflective documents (the 0.30 curve), such as vellum, there is an image contrast gain over the use of a strongly light absorbing photoconductor.

Because of the importance of the reflectivity product, R R in obtaining high quality reproduction of highly opaque documents, the backing of the photoconductive film must provide a smooth surface at the interface of the photoconductive film and the backing so as to be highly reflective. In other words, the reflectivity, R of the backing must be sufliciently high so that the reflectivity product, R R approaches the reflectivity, R of the document. Preferably, the material forming the backing of photoconductive layer should have a surface capable of reflecting greater than of the light in the wavelength range absorbed by the photoconductor. Ninety percent or greater is more preferred.

Suitable materials for the backing of the photoconductive member of the present invention are aluminum, gold, silver, copper, magnesium, calcium, and rhodium, which may be coated, preferably by evaporation, on a suitable substrate of plastic, metal, or paper. Other reflective materials can be found on pages 6-104 through 6-110 of the American Institute of Physics Handbook (1957). In addition, it is preferred that the reflective backing material be specular reflecting as well as highly conductive. For that reason, the backing material herein is aluminum deposited on a substrate of polyethylene terephthalate. If desired, however, a low conductive or nonconductive backing material can be employed in conjunction with the dual corona device of U.S. Pat. 2,922,883.

The reflectivity, R of the nonimage areas of the document is also important, but can vary over a wide range from transparent up to extremely highly opaque documents. Preferably, the reflectivity, R ,.of the nonimage areas of the document is such that the reflectivity product, R R is greater than 0.25. More preferably, the documents should reflect greater than 70% of the light in the nonimage areas. If, however, very low reflective documents are to be copied, a highly opaque sheet may be placed in back of the document to form a composite which has a greater reflectivity thanthe document itself and provides a reflectivity product, R R greater than 0.25.

The photoconductive material of the photoconductive member, must only weakly absorb the light to which it is exposed. Herein, the photoconductive material should only weakly absorb light Within the Wavelength range of 4000-6500 A., which is the wavelength range of the preferred light source. By weakly absorbing, it meant that the photoconductive material preferably should absorb less than 30% of the light each time the light passes through the photoconductive film. In addition, the photoconductive material should be nonlight scattering so that totally internally reflected light rays will not be generated within the photoconductive film. Accordingly, organic photoconductors are the preferred materials for use in the present invention and include what are termed small molecule photoconductors dispersed or dissolved in an essentially transparent binder and polymeric photoconductors which can be self-supporting. Examples of such organic photoconductors are listed in copending application, Ser. No. 474,583, now abandoned and refiled as Ser. No. 847,493 on July 14, 1969, and copending application, Ser. No. 474,977, filed July 26, 1965. As a general rule, the sensitivity of organic photoconductors normally is in the ultraviolet region of the electromagnetic spectrum, but can be extended into the visible region by the addition of a dyestufl sensitizer. Also, activators can be added for increasing the photoconductivity of the photoconductor and, in some cases, to shift the sensitivity of the photoconductor into the visible region. Examples of both dyestuffs and activators can be found in abovereferenced copending applications as Well as in British Pat. 942,810 and U.S. Pat. 3,169,060.

'If the mode of operation in which the photoconductive member is employed is one in which the conductive image in the photoconductive material should not persist after exposure, suchv as is the case with conventional xerography, then the activators should be selected from the quinones, ketones, and aldehydes listed in the above-referenced patents. If, however, the photoconductive member of the present invention is to be used in a persistent electrophotographic mode in which the conductive image should persist after exposure because, for example, it is exposed prior to charging, then the photoconductive materials of the referenced copending applications are preferred.

The general nature of the invention having been set forth, the following specific examples are now presented as illustrations, but not limitations, of the present invention. The following examples will include a comparison 7 of the image quality of copies prepared using a strongly light absorbing photoconductor and the photoconductive member of the present invention in which the photoconductor is only weakly light absorbing. The examples will also show the voltage change (Av.) in the exposed areas of a strongly light absorbing photoconductor and the photoconductive member of the present invention.

EXAMPLE I Three photoconductive compositions were prepared and coated on the aluminum side of three separate films of aluminized polyethylene terephthalate. One composition contained 1:1 molar ratio of poly-N-vinylcarbazole and 2,4,7-trinitro-9-fluorenone and is described in copending application, Ser. No. 556,983, filed June 13, 1966. The other compositions contained 40:1 molar ratio and :1 molar ratio of poly-N-vinylcarbazole and 2,4,7-trinitro-9- fluorenone. The reflectivity of the aluminum surface for all three photoconductive members was about 92% or greater of the incident light in the visible range of the electromagnetic spectrum. Using a laboratory contact printing device, the prepared photoconductive members were individually tested. The device comprised an electrostatic charging station having a corona unit and a contact exposure station having a l5-watt white fluorescent light source positioned to be 1 /2 inches from the document as it passed by the exposure station. Also, the device included a toning station for developing the electrostatic image by conventional cascade development.

During the testing of the three samples, the exposure setting was optimized for each sample because they varied in light sensitivity. With the 1:1 molar sample, it was necessary to employ a 1:1 neutral density filter over the exposure aperture to limit the light intensity in order to achieve the optimum exposure. With the 40:1 molar and the 100:1 molar samples, an orange filter was placed over the aperture to block wavelengths below about 5000 A. because these wavelengths are more strongly absorbed. When measured on a Macbeth Ansco densitometer the 1:1 molar, 40:1 molar and 100:1 molar samples absorbed approximately 92%, 29%, and 20%, respectively, of the light. (Wavelength range of 4000-6500 A. for the 1:1 molar sample and 5000-6500 A. for the other two samples.)

All of the films were electrostatically charged to substantially the same voltage of 600 volts with any variation being due to the thickness of the film. The document to which the samples were exposed was a composite of three different documents, one being highly opaque and the other two of low opacity. The print density on the three separate documents forming the composite also varied, with the greatest print density being on the highly opaque document.

Molar composition of poly-N- vinylcarbazole and 2,4,7-trinitro- Q-fluorenone Quality or copy Original Image Background 1:1 molar. (a) Highly opaque document:

(1) High print density.-- Very good... Fair. (b) Low opacity document:

(1)High print density... Fair (2) Low print density..- Very poor... 40:1 molar. (a) Highly opaque ducument:

(1) High print density Very good--- Good. (b) Low opacity document:

Very poor.

(1) High print density.-- Good+. Do.

Low print density... Good- Do.

100:1 molar- (a) Highly opaque document:

(1) High print density..- Very good... Do. (1)) Low opacity document:

(1) High print density.-- do Good+. (2) Low print density-.- Good Go0d+.

conductor is distinctly superior to the strongly light absorbing photoconductor.

EXAMPLE II Using the 1:1 molar and the 40:1 molar samples of Example I, plus a sample comprising a 150:1 molar ratio of poly-N-vinylcarbazole and 2,4,7-trinitro-9-fluorenone, the three samples were individually exposed through a gray scale step Wedge prepared on black developing diazo paper. Using the same densitometer of Example I, the absorption of the 15021 molar sample was measured and found to be approximately (Wavelength range of 5000-6500 A.). Each sample was exposed at exposure settings ranging from 3.2 to mm. and again the orange filter was used for the 40:1 molar ratio and the 150:1 molar sample, but not the 1:1 molar sample, the neutral density being used for the latter. Prior to each exposure of the 1:1 molar sample, the sample was charged to 600 volts and prior to each exposure of the other two samples, those samples were charged to 700 volts. The reason for this voltage difference was that the 1:1 molar sample was a slightly thinner sample.

After each exposure, the voltage change (Av.) in the exposed step areas was measured with a feedback electrostatic voltmeter manufactured by Monroe Electronics. These measurements were then used to prepare the graphs shown in FIGS. 6, 7, and 8, in which the diffuse transmission density of the step wedge is plotted versus voltage. FIG. '6 is the graph for the measurements of the 1:1 molar sample. FIG. 7 is the graph for the measurements of the 40:1 molar sample. FIG. 8 is the graph for the measurements of the 150.1 molar sample.

By comparing the 12.7 mm. exposure setting curves of FIGS. 6 and 8, it can be quickly seen that the slope of the FIG. 8 curve is much steeper than that of the FIG. 6 curve. This is an immediate indication that the voltage change (Av.) is greater with the 150:1 molar sample or with a weakly absorbing photoconductive material. As is well known in electrophotography, the greater the voltage change -(Av.), the greater capability of developing a high image quality. To further illustrate this comparison, the following table has been prepared from these graphs for the image transmission densities of 0.2 and 0.4.

Voltage Change (Av.) in volts For 0.2 image transmission For 0.4 image transmmm.

1:1 molar: 20

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that variations in form may be made therein without departing from the spirit and scope of the invention. For example, the contact exposure method and member of the present invention may be employed in persistent electrophotographic methods, such as that disclosed in US. Pat. 2,845,- 348 or any other method where the photoconductor is exposed b'efore charging. Also, the contact exposure method and member of the present invention can be used in conjunction with the charge transfer technique disclosed in US. Pat. 2,825,814.

In addition, while the term opaque document is understandable in the context of the foregoing specification, to make certain that there is no misinterpretation of this term, opaque document is a document permitting less transmisson of light than that of a vellum document, such as bond paper. The term opaque document, however, does not mean one through which no visible light can be transmitted.

What is claimed is:

1. In an electrophotographic methodof forming an electrostatic charge pattern on a photoconductive member, the stepscomprising:

bringing an opaque document into face-to-face contact with a photoconductive member comprising a photoconductive layer and a light reflecting backing having a reflectivity greater than 0.80, the reflectivity of the surface of the-document being such that the reflectivity product of the light reflective backing and the surface of the document is greater than 0.25, and

exposing the photoconductive member through the back of the document to wavelength of light only weakly absorbed by the photoconductive layer such that less than 30% of the total exposing light is absorbed during a single passage through the photoconductive layer whereby the light is reflected back and forth between said highly light reflecting backing and the nonimage face surface of said document, with a portion of the light being absorbed during each passage through the photoconductive film.

2. The method of claim 1 wherein the document is translucent and is backed by an opaque sheet. I

3. The method of claim 1 wherein the reflectivity product is greater than 0.70.

4. The method of claim 1 wherein the photoconductive layer is a nonlight scattering material.

5. The method of claim 4 wherein the photoconductive layer is an organic photoconductor.

References Cited UNITED STATES PATENTS Snelling 96- 1 GEORGE F. LESMES, Primary Examiner J. C. COOPER HI, Assistant Examiner r U.s.*c1. X.R. 96-15; 117-175

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