US3485623A

US3485623A - Continuous tone thermoplastic photography

Info

Publication number: US3485623A
Application number: US541835A
Authority: US
Inventors: Robert A Wilferth; Peter B Keenan
Original assignee: Xerox Corp
Current assignee: Xerox Corp
Priority date: 1966-04-11
Filing date: 1966-04-11
Publication date: 1969-12-23
Anticipated expiration: 1986-12-23
Also published as: GB1181093A; DE1572360B2; DE1572360A1

Description

Dec., 23, 1969 'wlLFR'l-H ET AL 3,485,623

cc NT11\xlJ sv ToNE'HERMoPLAsTIc PHOTGRAPHY Filed April 11, 196e 2 Sheets-Sheet 1 455.2922... IAQ o2 A: .Emzwa mom- 19 vv O24 my Dec. 23, 1969 R; A wlLF-"ERT". ET AL i 3,485,623

cONTNUoUs TONE. THERMOPLASTIC PHOTOGRAPHY Filed April 11. 196e :e sheets-sheet 2 o"max 500 (2) vous v T 0R U 40 (l) oc (2) v0=vR=3oov am()l (3) O: (4) :Vo= VR= 400V (5)0( (6) =v=vR=5OOv (5) (3)v K (l) l lY l l I l l F/'6 4 RELATIVE-UNITS oF LOG EXPOSURE FROST DENSITY FROST DENSITY x )n A ll LOG E (mcs) INVENTORS.

ROBERT A. wxLFERTH APETER s. KEI-:NAN

a@ gfllw ATTORNEY United States Patent 3,485,623 CONTINUOUS TONE THERMOPLASTIC PHOTOGRAPHY Robert A. Wilferth, Pittsford, and Peter B. Keenan, Rochester, N.Y., assignors to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Apr. 11, 1966, Ser. No. 541,835 Int. Cl. G03g 13/00 U.S. Cl. 96-1.1 3 Claims ABSTRACT OF THE DISCLOSURE A process enabling control of the photo-response characteristics of a frostable member wherein the initial charging and recharging potentials are altered so as to adjust the spread between that exposure which results in threshold frosting and that exposure which results in maximum frosting until this exposure spread is in such ratio to the difference between image density present at threshold frosting and image density present at maximum frosting as to produce the desired gamma in the particular frostable member being utilized.

This invention relates generally to deformation imaging processes and, more specifically, to processes of the type identified as electrostatic frosting.

In U.S. Patent No. 3,196,011 to Kenneth W. Gunther and Robert W. Gundlach, a new form of deformation imaging is disclosed in which an electrostatic pattern is used for selectively frosting in image configuration a continuous deformable lm or layer. The frost processes therein disclosed result in images having properties remarkably at variance with those properties displayed by images produced by earlier known deformation processes. More specically, and as is fully set forth in the Gunther and Gundlach patent alluded to, these earlier techniques relied upon fringing fields present at the edge of boundaries separating charged and uncharged areas on the deformable surface to produce thereon a pattern of depressed and elevated areas corresponding to the charged and uncharged areas, respectively, whereby a mere relief or outline pattern resulted when one attempted to image an optical pattern including continuous tone areas. The processes of electrostatic frosting on the other hand are such as to deform both solid and/ or line areas of charge on the mechanically deformable material into relatively orderly patterns of microscopic discontinuities whereby gross light scattering characteristics are present at all points on the deformation image where an optical input was originally presented.

It is therefore apparent from the foregoing that the frost images to which the present invention is applicable display continuous-tone reproduction capabilities much like conventional silver halide images. Clarification of the type of problem to which the present invention addresses itself may in fact be best gained by considering the semianalogous silver halide images somewhat further and in particular noting that one of the most important characteristics of such images derives from the fact that the relationship present therein between image density and exposurewhich is to say the so-called gamma of the photographic emulsion-may be controllably varied by development processes, and so forth, so as to augment or lessen the gamma characteristics of the film. By virtue of such controllable variations, corresponding changes may be introduced in the contrast range characteristics of the developed image; which is to say that the tonal gradations in a reproduced scene or the like may be adjusted to suit the eye of the viewer, to emphasize lowlight features, or so forth. A specific and appropriate ICC example of this manipulative technique arises in the art of processing photo reconnaissance film or the like where it is well known to adjust both development and/or exposure of the positive print so as to best utilize the gamma characteristics of the printing stock to achieve the most desirable (usually meaning information-laden) final picture.

Now in accordance with the present invention we have discovered that the photographic response properties of frostable photoreceptor members too may be controlled by a process to be hereinafter set forth, whereby it becomes possible to adjust not only the gamma characteristics, but to some extent the speed as well of these materials, so that the resulting frost image will display the characteristics and treatment of information -content desired by a user.

It is thus an object of the present invention to provide a process whereby the response characteristics of a frostable ,member may be varied to suit the requirements imposed by the optical input thereto and/ or the subsequent use thereof intended by a user.

It4 is a further object of the present invention to provide a process whereby the gamma characteristics of a frostable member may be adjusted to suit the requirements of a user.

It is an additional object of the present invention to provide a process whereby a frost image corresponding to an optically imaged scene may be established with a contrast gradation range of determinative width.

It is yet an additional object of the present invention to provide a process whereby the speed of a frostable photoreceptor may be determinatively adjusted.

In the present invention these explicit objects, and other implicit objects as will become apparent in the ensuing specification, are brought about by a process wherein the initial charging potential and the recharge potential utilized in the frosting process are so adjusted that the curves representing the physically measurable quantity frost density as a function of light exposure are made to assume a desired shape; more specifically these curves--representative of the physical quantity frost density-are made to acquire a desired span width between the points at which such curves represent a frost threshold value and the points of maximum frosting. In purely physical terms this means that the initial charging and recharging potentials are determinatively altered so as to adjust the spread between that exposure which results in threshold frosting and that exposure which results in maximum frosting until this exposure spread correlates with a desired exposure range or gam-ma in the particular frostable member utilized.

' A full understanding of the manner in which the present inventive process is practiced may now be gained from a reading of the following detailed specification and from a simultaneous examination of the drawings appended hereto in which:

FIGURE l is a graphic representation of a three layer frostable photoreceptor member of the type utilized in the present invention.

FIGURE 2 is a representative curve illustrating the relationship between frost density and surface charge density achieved on a member of the type depicted in FIGURE l.

FIGURES 3 and 4 are analytical curves depicting how in the practice of the present invention, surface potential and surface charge densities on members as in FIGURE l may be expected to vary with the log of exposure.

FIGURES 5 and 6 are empirical `curves depicting variations of the type shown in FIGURES 3 and 4 for the exemplary member set forth in Example I.

In FIGURE 1 a typical frostable photoreceptor device is depicted, generally resembeling the frostable members described in connection with the Gunther and Gundlach patent previously alluded to. This frostable member 1 is seen to include a conductive substrate 2 conveniently comprising a thin sheet of aluminum, a photoconductive layer at 4, and an overcoated frostable layer at 6. For present purposes, the photoconductive layer may be considered to comprise a layer of vitreous selenium having a thickness designated as ds and a permittivity designated as es; however, in some instances this layer will actually be compound in nature and consist of a lower portion of selenium alone intended to exercise a charge storage function, and an upper portion comprising a relatively panchromatic selenium-tellurium alloy and intended to function as the photoconductor proper.

The upper most layer 6, the frostable component, is shown in FIGURE 1 as having a thickness dp and a permittivity designated as ep. A list of materials suitable for this frostable component is indicated in the Gunther and Gundlach patent previously mentioned. In a very typical example such layer will comprise a few microns thick coating of Staybelite Ester-10 (glyceryl tri-ester of 50% hydrogenated wood resin) available under the trade name indicated from the Hercules Powder Company, Wilmington, Del. It might be noted that in some instances an interlayer comprising several hundredths of a micron of an organic material may be applied between the photoconductive layer 4 and the lfrostable layer 6, as such interlayer appears t operate in some obscure manner to increase resolution in the final frosted image. However, `for purposes of the present invention, this interlayer need not be specically considered or included and in any event may for purposes of analysis be thought of as included in the frostable layer 6.

In the practice of electrostatic frosting as taught in the Gunther and Gundlach patent, the frostable member 1 is initially provided with charge on the surface of layer y6 by moving the member relative to a corona charging device or the like. The member is then exposed to a pattern of light and shadow as, for example, by means of a photographic enlarger or the like, which permits migration of charges in portions of the photoconductive layer 4 adjacent to light exposed areas on the member. These migrating charges become bound at the interface between

layers

4 and 6 with a resulting lowering of potential at corresponding points on the surface of the deformable layer 6. Subsequently, the layer 6 is recharged by the same or a similar corona device so as to bring the surface `6 once again to an equipotential. In areas of previous exposure--and thus of internal charge migration-the surface of 6 accepts additional charge, as a result of which the electric field and the potential difference across such regions is also greatly increased. Upon softening of the deformable layer 6 those areas subjected to this increased electric iield frost to a degree that bears a relationship to the eld or to the charge density present thereon.

In FIGURE 2. a representative curve is shown illustrating the typical experimental frost density achieved upon such a frostable member as a function of the charge density present on the member after recharging. As used herein the term density refers to the light attenuation, 1.e.,

log I produced in an optical path including the frosted member; which is to say that density may -be regarded as a direct indication of light scattered out of the optical path by the frost deformations. The curve shown is generally of the type shown in FIGURES 6A and 6B of the Gunther and Gundlach patent and is intended to illustrate firstly that in any frost process a certain minimum value of charge density -must be achieved before .4 any frosting takes place; and secondly that a certain maximum value of charge density exists after which frost density shows no appreciable increase. These two critical values are depicted in FIGURE 2 by the designations 0T and Umax, respectively, the subscripts being chosen to suggest the threshold and maximum useful values of this parameter.

The essence of the present invention resides principally in the discovery of a technique whereby the range of frost densities extending between aT and Umax may be made to coincide with exposure ranges of varying widths. In the conventional parlance of photographic engineering this implies a method for varying the so-called gamma of the photosensitive member, and this is in fact the result that is achieved by the present discovery. The discovery will further be seen in what ensues to indicate a method for adjusting the sensitivity of the photoresponsive member-at least to a reasonable degree.

An understanding of the precise method by which the present invention is practiced may now best be gained by a step-by-step analysis of the manner in which the density of charge present on a frostable member after recharge URE may be expected to vary in a generalized frost process including the possibility of differing values in the initial and recharge potentials utilized.

es fp es In terms of surface potential the effect of dark decay cannot be distinguished from effects of exposure; therefore, the change in surface potential AV includes exposure and dark decay. The charge at the interface UE includes the charge due to exposure and dark decay. Hence,

VE=U0 da The photoreceptor member 1 is then recharged to restore the condition of an equipotential surface. Charging to a potential VR(VR V0) will result in a surface charge density URE in the exposed area and RD in an unexposed area. Hence, in the dark R RD el en and in the exposed area magg-ag) l D B By combining the values of AV and VR the surface charge density in the exposed area is found to be This analysis shows that the final surface charge density is a function of--among other thingsthe two independent processing parameters representd by the initial potential V0 and the recharge potential VR, as well as of the exposure.

FIGURE 3 illustrates a representative set of curves that result on utilizing Equation 1 to determine surface charge density URE as a function of log exposure for a representative device constructed in accord with FIGURE 1. Variation in surface potential, VE as a result of exposure and before recharge is also shown on the same set of axes. Exposure units in FIGURE 3 may be assumed to be relative, absolute values not being of any particular relevance to the present discussion.

Curves

1 and 2 in FIGURE 3 show the values for initial and recharge potentials equal to 400 volts each.

Curves

3 and 4 show the values for an initial potential of 200 volts and recharge potential of 550 volts. It may be noted in these curves that, as would be expected, the charge density curves at 2 and 4 are mirror images of the potential curves at 1 and 3. It will also be noted as a general feature that curves 1 and 2 join at the vertical axis whereas 3 and 4 are separated by a degree that results from the voltage differential between the initial potential and the recharge potential utilized.

Also shown on the same set of axis are the charge density threshold mr and the charge density rmx to yield maximum density. We note here that the intersection of

curves

2 and 4 with aT line gives the exposure to produce a threshold value of frost density. Furthermore, the intersection of

curves

2 and 4 with the cmax line gives the exposure to produce a maximum frost density. We therefore see that in the case of the processing parameters utilized in connection with curve 4, the useful exposure range is represented by the spread at 10 whereas in the case of processing parameters in accord with curve 2 the useful exposure range has narrowedand has also been displaced-to the spread at 12.

The average slope of

curves

2 and 4 between 1T and max multiplied by the slope of the curve relating r and frost density D, is approximately the gamma. As is clearly apparent from the ligure the gamma value obtainable under conditions producing curve 2 differs markedly from that obtainable under conditions producing curve 4.

In addition it may be noted that the displacement of the exposure range to the left as in curve 4 has resulted in increasing the effective sensitivity of the frostable member in that threshold frosting occurs in this case at a much lower exposure than would be the situation where curve 2 were applicable, This factor is more strikingly illustrated in FIGURE 4 where curves are depicted showing surface charge density and surface potential as a function of log exposure for instance whereas the recharge and initial charging potentials are in each curve set equal, but are made successively higher in adjacent sets of curves. Thus, in that ligure curves 1 and 2 illustrate the effect where V: VR=300 volts, curves 3 and 4 where and in curves 5 and 6 where V0: VR=50O volts. In each instance the slopes of adjacent curves are roughly equal; however, the speed or sensitivity of the particular frostable member to which these processing parameters are applied increases from curve 2 to curve 4 to curve 6. In fact, curve 6 actually shows VR exceeding the threshold value for frosting even in the absence of exposure implying a background level of fog and the highest speed of the lot.

The method taught by the present inventive process should by this point be quite clear. Essentially one utilizes in a frost process charging and recharging parameters in accord with the gamma response desired. So, for example, if one desires relatively flat response characteristics, processing parameters resembling those associated with curve 3 in FIGURE 3 can be utilized by charging prior to exposure to a relatively low initial potential, and subsequently frosting over the resulting relatively fiat-response regions utilizing a higher recharge voltage-sufficient to elevate the flat portion of the charge density curve into the desired exposure range as at 10. On the other hand, one may achieve the short spread high gamma exposure as in, for example, curves 1 and 2 by utilizing relatively high initial potentials and suitable recharge potentials to properly position the exposure range as at 12.

It will, of course, be appreciated by those skilled in the art that the approach to describing the present invention has thus for been completely general and is accordingly in no way limited to particular frostable members possessing particular layer thicknesses, permittivities, or so forth. Thus, in particular, curves such as appear in FIG-

URES

3 and 4 may be readily constructed for any given frostable member. If sufficient data is available this can be done analytically; however, as a practical matter, it is more likely that with a given structure these curves will initially be determined empirically--that is by direct experimentation. However, once such curves are determined, whatever the process may be, they thereafter may be utilized in precisely the manner that has been indicated since the determinative physical processes that underlie establishment of these curves are repeatable and not subject to unknown variations.

An illustration of the manner in which such empirical curves may be established and utilized will now be set forth:

EXAMPLE A frostable photoreceptor member essentially in struc- 'tural accord with that depicted in FIGURE 1 was prepared wherein the substrate 2 comprised an anodized aluminum sheet 0.05 inch thick. The layer in the instant example was compound and included a selenium charge storage layer 25 microns thick adjacent the conductive substrate. Upon the selenium layer was coated a .3 micron thick photosensitive layer comprising an alloy having a nominal composition of 75% selenium and 25% tellurium, the latter layer by virtue of its composition exhibiting generally panchromatic light response. In the present example an interlayer of an organic material about 0.05 micron thick was applied over the photosensitive selenium tellurium layer, its function being to obtain the maximum possible resolution with the device. The mechanism of operation of this interlayer is not clear.

top layer was coated over the interlayer as the thermoplastic frostable layer corresponding to 6 in FIGURE 1. In the present example, this layer comprised about 2 microns of Staybelite Ester-10 (glyceryl tri-ester of 50% hydrogenated wood resin). This particular material is available under the trade name indicated from the Hercules Powder Company, Wilmington, Del. Storage layers and photosensitive layers were applied by vacuum deposition. The interlayer and the frostable layer were applied by dip coating from a solution.

Experimental data was obtained of the variation of frost density D with log E for this frostable member. Two such experimental curves are shown in FIGURES 5 and 6, the rst depicting the D-log E curve for the processing condition V0=400 volts, VR=400 volts, the second indicating the curve for the processing condition Vozvolts, VR=600 volts. Table I below shows a compilation of data on this member for various other combinations of initial and `iinal potentials. It will be noted that not only has variation in gamma been achieved in accordance with the analysis previously made but the speed or sensitivity of the member has also been varied in accord with such analysis.

TABLE I Final Speed Initial potential potential [L8/Ems Gamma Dm" In the foregoing data speed is intended to imply an ASA type of definition and is specifically defined as 0.8 divided by the exposure meter candleseconds to produce a ydensity 0.1 unit above base plus log. To obtain the gamma a line is drawn from a point on the D-log E curve 0.1 density unit above base plus log to a similar point 7V 0.1 density unit below Dmax. The slope of this line is taken as gamma.

Having once determined the gamma and/or speed of a frostable member under given processing conditions, we may thereafter set out processing conditions so as to utilize this frostable member in accordance with our nee'is. This is to say that we may thereafter decide what gamma response or sensitivity is desired in a given exposure situation, and by referring to the previously determined gamma and sensitivity vs. voltage curves readily determine the proper processing parameters--viz., the proper charging and recharging voltages-appropriate to achieve the desired results.

Having thus described the present inventive process it will be apparent that those skilled in the art may now readily devise numerous variations thereupon and deviations therefrom that will yet fall Within the province of the present invention. Accordingly, the invention is to be broadly construed and limited only by the spirit and scope of the claims appended hereto.

What is claimed is:

1. A method for adjusting the gamma response in a frostable photoreceptor member including a conductive substrate overcoated with a photoconductive layer of thickness ds and permittivity es in turn overcoated with a deformable frostable thermoplastic layer of thickness dp and permittivity ep comprising:

(a) charging the surface of said member to a rst potential V;

(.b) exposing said -member to a light pattern;

(c) recharging said member to a recharge potential VR where the absolute value of VR is greater than the absolute lvalue of V0;

(d) and softening said frostable thermoplastic layer so as to enable formation of the desired frost image.

2. A method for varying the gamma characteristics of a frostable photoreceptor member comprising initially charging said member to a potential V0 and, subsequent to exposure to an image, recharging said member to a potential VR where the ratio of the absolute values V0 to VR is less than unity.

3. A method for changing the response characteristics of a frostable photoreceptor member from an initial characteristic of threshold and maximum frost densities which correspond to first and second levels of exposure illumination, respectively wherein the member is charged to an initial potential, exposed suitably at one of said exposure illumination levels, recharged to an initial recharge potential, and softened, comprising the steps of:

(a) charging said member to said initial potential;

(b) exposing said member to a light pattern;

(c) recharging said member to a recharge potential, the absolute value of which is greater than the absolute value of said in initial potential, and

(d) softening said member to deform an exposed surface thereof in accordance with said light pattern.

References Cited UNITED STATES PATENTS 7/1965 Mihajlou et al 96-1.1 7/1965 Gunther et a1. 96--1.1

U.S. Cl. X.R.