CA1074703A - Document copying apparatus with programmed computation means - Google Patents

Abstract

DOCUMENT COPYING APPARATUS WITH
PROGRAMMED COMPUTATION MEANS

Abstract of the Disclosure Copying apparatus includes a document reader for generating a series of electrical signals which are represen-tative of image and color information in an original docu-ment, and programmed computation means for enhancing the image and color information in the signals before the signals are applied to a printer. Non-linear amplifiers are used to convert such signals to forms which are linear with respect to human visual color error tolerances, whereby the signals can be more efficiently stored in a memory.

Description

10~'7~3 BACKGROUND OF THE INVENTION
Field of the Invention The present invention relates to document copying apparatus, and more particularly to means for generating and modifying electrical signals and for applying those signals to an exposure station.
Description of the Prior Art Document copiers which include reader devlces for generating electrical signal's representative of imagewise information in an original scene and for applying the signals to the controls of an exposure station are known in the prior art. For instance, co-assigned U.S. Patent No. 3,783,185, which issued to Richard A. Spaulding on January 1, 1974, "
discloses a laser printer in which a color transparency is scanned and three electrical signals, representative of the color information of the transparency, control the color output of the laser as it scans the print material. However, that patent discloses no means for correating the signals to correct for characteristics of the original dbcument which ; 20 would result in inferior prints. '~
Certain techniques for transformation and enhancement Or electrical color signa]s have been utilized in other environments. For instance, in the television arts it is well known to generate a video slgnal and to apply certain signal correction techniques thereto. However, heretofore the manner in which such techniques can be'advantageously utillzed in the document copier field has not been recognized. ' - '~ ' Also, such known techniques for correc~ing signals are not .:
directly applicable~for utilization in document copiers and ' ' 30 improved, more'efficient t~chniques, specif'ically adapted for ! ' the particular purpose of copying documents, are required to allow transformation and'enhancement in that environment.
- SUMMARY OF THE INVENTION ~ ~
By the pFesent invention I have~provided document ~ ~;

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~ 0747~3 .
copying apparatus ~or generating, transforming, storing and enhancing electrical signals which are representative of imagewise information in an original document or of the original scene represented in the original document.
In a preferred embodiment, I have provided apparatus for generating an electrical luminance signal and two ortho-gonal chrominance signals of limited bandwidth (i.e. two) chrominance signals which vary linearly independently from each other); for per~orming electrical unsharp masking of the lu~inance signal; for converting the signals to forms more convenient for buffering the signals and for applying cor-rective functions thereto in order to compensate for various characteristics of the original document and/or the scanning process which would result in inferior prints; and means to reconvert the resulting signal to a form more efficient for use by a given printer.
In the preferred embodiment described hereina~ter, I have shown the present invention in a color copier combina-tion. However, it is to be understood that certain of the novel features of the invention are equally adapted for use in black-and-white copying and for use in combinatlon with facsimile transmission equipment not necessarily associated with a printer output. Also, certain o~ the novel features, though previously known in non-analogous apparatus, are novel in the ~ield of document copying~
The invention, and its objects and advantages, will become more apparent in the detailed description of the preferred embodiment presented below.

BRIEF DESCRIP~ION OF THE DRAWINGS
In the detailed description o~ the pre~erred embodiment of the invention presented below, reference ls made to the accompanying drawings in which:

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~ -3-~ ~L0747~3 -FIG. 1 is a block diagram of varlou~ components of the present invention;
FIG. 2 is a schematic view of a document scanning device, `

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1(~747{?3 Fig. 3 is a schematic view of an electronic unsharp mask;
Fig. 4 is a schematic view o~ the timing of signals generated in the unsharp mask of Fig. 3; and Fig. 5 is a schematlc view of an image exposure station.
DESCRIPTION OF THE PREFERRED EMBODIMENT

The present description will be directed in par-ticular to elements forming part of, or cooperating more directly with, apparatus in accordance ~lth the present invention. It is to be understood that elements, components and/or subcomponents not speci~ically shown or described may take various ~orms well known to those skllled in the art.
In the following description and in the preceding discussion, I have referred to document copiers as a preferred environment of the invention. By the term "document copier", ~
I mean apparatus adapted to read the imagewise information '' on an original document (which may be a reflective print or '' transparency) and to form a second document containing such imagewise lnformation.
In Fig. 1, I have shown a block diagram of the varioùs components which make up a prbferred embodiment of ~ -the present invention. Each block'will he explalned in detall hereinafter, but generally, the apparatus of this embodiment involves (1) generating eleotrlcal signals repre-sentative of the color information in an original document, ~2) converting the electrical color signals into one luminance signal plus two essentially orthogonal ¢hrominance signals~

(3) differentially narrowing the bandwidth of the two chrominance ~' signals, t4) applying unsharp masking techniques to th~
' ' . ' ' ' , ' ' ' , "

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., ' ,. ~,' . ''', ' ' : ., 7~3 ` luminance signal to enhance edge sharpness and/or suppress noise resulting from grain, dirt, scratches or ma-tte finish on the original document or blemishes in scanner optics or electronicsg (5) converting the resultant signals to forms more efficient for subsequent buf~ering and applying correc-tive functions, (6) buffering the electrical signals, (7) processing the signals by a color table look-up to compensate for various characteristics of the original document, the scanning process and the image forming process by enhancing color reproduction from the colorant set in the original document and correcting for saturation losses, hue errors and contrast errors, (8) converting the processed signals to forms more suitable ~or controlling an exposure station.
Theory The rationale for many features of the preferred embodiment of my invention can be understood more readily if preceded by a discussion of the theory of color perception and color reproduction. More particularly, the illustrated embodiment shows apparatus by which electrical signals representative of color information are transformed into a more efficient form for handling such electrical signal~.
Black-and-white information and monochrome information are here considered to be simplified subsets of color information.
Reference is made to the following books and journal articles: (1) Gunter Wyszecki and W. S. Stiles, Color Sience Concepts and Methods,_Quan~itative Data and Formulas, John Wiley and Sons, Inc., 1967, hereinafter referenced as Wyszecki and Stiles; (2) Robert W. G. Hunt, The Reproduction of Color, Edition 2, ~ohn Wiley and Sons, In., 1967, hereinafter referenced as Hunt; (3) Ralph M.
Evans, Welsey T. Hanson and W. Lyle Brewer, Princi~ of Color Photography, John Wiley and Sons, Inc., 1953, herein-` ~ =
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after referenced as Evans, Hanson and Brewer, (4) David L~
MacAdam, Color Discrimination and the Influence of Color Contrast on , -5a-~`

0747~3 Visual Acuity", Revue d'Optique, Vol. 28, No. 3, pages 161 to 173, March 1949, hereinafter referred to as MacAdam-l; (5) David L. MacAdam, "Colorimetry", pages 6-182 to 6-197 in American Institute of Physics Handbook, edition 3, McGraw-Hill, 1972, hereinafter referred to as MacAdam-2; (6~ David L. MacAdam, "Role of Luminance Increments in Small Color Differences", section 14, pages 160-170, Color Metrics, AIC/
Holland, 1972, hereinafter referred to as MacAdam-3; and ~7) John H. Ladd and Jack E. Pinney, "Empirical Relationships with the Munsell Value Scale", Proceeding of the Institute of Radio Engineers, Vol. 43, No. 9, page 1137, September 1955, hereinafter referred to as Ladd and Pinney.
Human visual perception of color can be divided into three attributes which in the Munsell Renotation System are designated as hue H, value v, and c~roma C ~Wyszecki and Stiles, pages 478 to 500~. The Munsell value V scale is a scale which is linear with respect to human perception of neutral grays from black to peak white under specified ~iew-ing conditions, and is relatable to reflectance R or to re-flected luminance L by a nonlinear, nearly cube-root function.
Ladd and Pinney compare the precision of fit for linear, log-arithmic, square-root, cube-root, and other functions of re-flectance to the Munsell value scale.
Chrominance is the visual attribute of color which remains when the luminance attribute is excluded. Chrominance is two-dimensional and may be represented in polar coordinates, such as hue H and chroma C (or saturation) in the Munsell ~ - 6 -_ _ .

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- ~ ~7~7~3 Renotation System, or in rectangular coordinates, such as x and y in the Commission Internationale de l'Eclairage ~CIE) chromaticity diagram (Wyszecki and Stiles, pages 238 to 351). David L. MacAdam has shown (,Wyszecki and Stiles, pages 461, 518 to 521, 528 to 538, 557, and 558; also MacAdam-l, 2 and 3) that human perception of chrominance, ' ' ''. '' ~'.', - '. ' '" ' ' '~ '. ' ' ' ~ 0~7~703 .~ ~
~~ under specified viewing conditions, ls not equally critical for all azimuths ln the CIE chromaticity diagram. Rather, the locus of just noticeable differences (JND) near a given color form an ellipse. The size, eccentricity, and angular orientation of each MacAdam JND ellipse depends upon its location on the CIE chromaticity diagram.
Any nonlinear ~ransform of a given three-dimensional color space forms a new color space. A color space is defined by a set of three attributes into which color can be di~ided. For example, the Munsell Renotation System defines a color space in terms of hue, value and chroma attrihutes and is not linearl~ related to the CIE color space which is de~ined in terms of luminance and two chrominance attributes hereinbefore called X and Y (the transform from Munsell to CIE
is not merely a redescription of the same color space in rectangular and in c~lindrical coordinates). However, MacAdam has also shown that no practical, realizable color space can transform the MacAdam JND ellipses into circl~es of equal diameter for all the hues, satura~ionsg and luminances possible in an~ color space. When the third axis, such as either the luminance axis or the Munsell value axis, is included, the MacAdam JND ellipses become ellipsoids.
The effect of the size of the field of view upon the luminosity equivalent to a chromatic contrast is dis-cussed in MacAdam-l, page 172. The CIE 1964 color-difference foI~ula, chromaticity difference, color difference, and lightness are discussed in MacAdam 2, pages 6-194 to 6-195 The dependence of color-dif~erence tolerances (tolerances which define the bounds of human acceptance of color error in a given application) upon luminance is discussed in MacAdam-3, pages 165-166.
Color tolerance ellipsoids are conceptually ; ;~
analogous to JND ellipsoids~ but define the locus of equal color error tolerances around a given point in a reference s ~ ~ -7-074~3 color space, for a particular application and for a specified set of viewing conditions. We are assured by D. L. MacAdam that the shapes of color tolerance ellipsoids, in a reference color space and under given viewing cond~tions, are often assumed to be the same as the shapes of JND ellipsoids in the same reference -7a~

, ~7~7~
color space and under comparable viewing conditions. A
linear scaling factor converts from JND to tolerance.
Define herein an "efficient" color space as a color space for which the electrical representation thereof can be more ef~icient than for other, less ef~icient~ color spaces.
The idea of efficient electrical representation is applicable to analog electrical signals and to digital electrical signals. Here, e~iciency of digital representation means ability to represent the same color information with fewer data bits, or to represent more color information with the same ~umber ofdata bits; and efficiency of analog represen-tation means the ability to represent color information using a scale wherein a small increment of signal change has the same visual e~fect regardless of the absolute magnitude of the signal (i.e. where the signal is substan-tially linear with respect to human visual response). An efficient color space is important for analog signals for which the signal-to-noise ratio ls to be maximized, and is important for digital signals which are to address a memory, be stored in a digital memory, or be transmitted from one location to another by telephone line, coaxial cable, microwave relay link, or other means of data transmission.
For the purposes of digital data handling~ all alternative color spaces which have the same bit efficiency (i.e. require the same number of binary data bits to represent the coordi-nates of points within the color space to required tolerances) are about equally efficient for the applications assoclable with those tolerance specifications.
Digital representation of each coordinate in color 3 space is precise to plus or minus one-hal~ of the least significant bit. The magnitude o~ the lea~t signi~icant bit has a fixed ratio to visual tolerance along that coordinate ~ ~8~

79~7~3 if and only if the scale of that coordinate is linear with respect to visual color difference tolerance~ It is commonly assumed that a scale which is linear with respect to visual perception of differences is also linear with respect to visual tolerances. For example3 the Munsell value scale is parallel to the luminance (or luminous reflectance) axis but is not linear with respect to luminance. However, the Munsell value scale is linear with respect to human preception of luminance differences. Therefore, a digital -8a-.

7~)3 representation which is linearly proportional to Munsell value provides the most efficient digital measure of luminance.
The two digital scales which represent the chromi-nance information must necessarily be essentially orthogonal to the scale of luminance information and orthogonal to each other in order to be efficient (i.e. to represent the two components of the chrominance data with a minim~m number of bits). As used herein, the phrase orthogonal signals means signals which are measures of respective color attributes along orthogonal axes. Furthermore~ the least signi~icant bit for the digital representation of each chrominance factor should relate to the visual tolerance for color error for that factor by the same linear factor of proportionality as does the least signi~icant bit on ~he luminance function scale to the visual tolerance for luminance error. This latter requirement is equivalent to requiring that, insofar as possible, the digital representations of luminance function and of chrominance should be in a color space ~or which the ellipsoids of visual tolerance become spheres o~ essentially constant diameter throughout the most important portions of the color space. In many applications, the colors occurring most often and the most important colors are desaturated colors near the neutral axis from black to peak white~
Document Reader Document reader devices suitable for;use with the present invention for producing electrical signals represen-tative of the color information in the original document may take various forms well known to those skilled in the art.
For instance, U.S Patent No. 3,783~185 discloses a document 3 reader in which a scanning light source directed through a color transparency impinges upon a set of three photosensors which are each sensitive to a different one o~ the selected 1~ _9_ .

~ 7 ~ 3 wavelengths, e.g. red, green or blue. The photosensors produce electrical signals representative o~ the red, green and blue information in the transparency.

-9a-~ -" 107~703 ~ ig. 2 shows another form of document scanning apparatus 42 (see also Fig. 1). A multicolor beam 46 from laser source 44 is incident on cylindrical optics 48 which spreads the beam ~o form a long, narrow sheet beam and "line" image.on the document being scanned so that, after being reflected by a pair of mirrors 50 and 52, the line image scans along an original document 54 from end to end.
B A relay lens ~ transfers the shee~ beam to an intermediate .

pupil plane on the surface of page scan mirror 52. The 10 sheet beam passes through a final imaging lens 56 to provide .
a good planar image field at docùment 54. While in the illustrated embodiment illumination is derived from.a poly-chromatic laser source 44, tungsten or fluorescent lamps or xenon flash lamps or other sources may be used as well.
. . .
The original document is shown as-a reflective print~ and the reflected light beam is once more reflected by page scan mirror.52, through an imaging lens 58 to a neutral beam-splitting mirror 60. Page scan mirror 52 is driven by a page scan galvanometer motor 62 so that successive line scans by each of three linear photo~ensor arrays 64, 66 and 68 progress down original document 54 in synchronism with page scan of the illuminating sheet beam. Linear photo-sensors 64, 66 and 68 may be replaced with-self-scanned area photosensor arrays, and sheet beam illumination be replaced by area illumination.
Beam-splitting mirror 60 or ~ ~econd mirror 70 or both, may be dichroic mirrors, and hence have the character-1stics of primarily passing light of a ~elected band of wavelengths and primarlly reflecting other light. However, ~0 .the pre~erred embodiment of the present ~nvention utilizes neutral beam-splitting mirrors to minimize possible changes : in transmittance for a given wavelength of light as a function `` 10- ,~

-` 11)7476~3 of angle of incidence to the mirror surface, especially for the elliptically-polarized, coherent radiatlon from one or from a multiplicity of lasers.
There are twelve ways to divide a beam of light into three components by means of two beam-splitting mirrors:
the first beam-splitting mirror can isolate component 1, 2, or 3 (three alternatives); isolation at the first beam-splltting mirror can be either by transmission or by reflec-tion (two alternatives, independent from the three alternatives listed above); the second beam-splitting mirror can isolate either of the two remaining components by transmission (two independent alternatives) and the remaining component by reflection. The total possible number of combinations is therefore 3 x 2 x 2 - 12. In the preferred embodiment of the present invention, the most important color component is isolated by the first beam-splitting mirror to avoid possible :
optical degradat~on associable with the second beam-splitting mirror.
With a neutral beam-splitting mirror, it is often convenient to let transmittance be greater than reflectance.
In the preferred embodiment of the present invention, the most important color component is isolated by transmission through the first beam-splitting mirror 60 in order to maximize the signal-to noise ratio of that signal. The least important color component is isolated by reflection at the-second beam-splitting mirror 70 3 because the signal associated with the least important color compohent has the least bandwidth and hence can tolerate an optical path which would yield the worst signal-to-noise ratio if the bandwidths of all the sigffals were equal. The above selection .
of beam-splitter combination (of the twelve possible com-binations) is preferred, provided that areas of the original document which are intended to represent peak white reflect -~1 . .
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~747~3 (or transmit) approximately equal radiant power In red, green, and blue wavelength bands and ~he three photosensors 64, 66 and 68 are equally sensitive to red, green and blue light.
One possible embodiment of the present invention may include three separate, monochromatic lasers as the light source~ such as, for example, a helium-neon laser emitting at 632.8 nanometers, an argon laser emitting at 528.7 nanometers, and a helium-cadmium laser emitting at 441.6 nanometers. Such a light source, at the present state of the art, is likely to emit much less blue light than red or green light. Furthermore, silicon photosensors are often more sensitive to red than to blue light. Hence, a beam-splitting mirror combination other than the one described above may be preferred~ in order to provide adequate signal-to-noise ratio for each of the three color signals.
The preferred embodiment of the present invention-employs a luminance L, red R and blue B color space at the .
output from document scanner. The L~ R, and B signals are ;20 generated as follows: Lens 58 in Fig. 2 focuses light transmitted by beam-splitting mirror 60 upon linear charge coupled device (CCD) photosensor array 64 to form an image of thè pictorial content of that part of original document 54 which is temporarily illuminated by the sheet beam of light. If original document 54 is itself a black-and-white or color reflection print which is also ~ntended to be viewed *irectly by humans, photosensor array 64 is spectrally filtered to generate a luminance L signal for CIE Source C
; ~ (Wyszecki and Stiles, pages 274, 275, and 471) and photopic vision (Evans, Hanson and Brewer~ pages 48-50). That is, the red, green and blue spectral transmittances of the filter in front of photosensor array 64 are so selected in con~unction with the re].ative red, green, and blue radiant ' ' `:

~ ~ 107~7~?3 wer densities actually incident on original document 54, and in conjunction with the spectral transmittance function - of the optics between the original document and photosensor array 64, that, ~or a neutral gray area represented on the original document, the relat-ive red~ green, and blue con-tributions to the luminance signal L are in proportion to what the red, green and blue contributions would be if the original document were illuminated by CIE Source C and if the electrical luminance signal L were electrically generated from red, green and blue electrical signals as in conventional color television practlce.
If the original document to be copied is a color . ....
photographic negative, foP example,.the pictorial content ~hereon is not intended to be viewed and interpreted directly by humans. Furthermore, such a color photographic negative may inclùde colored couplers (reference Evans, Hanson and Brewer, pages 265 .to 266, 307, 544 and 568 to 571; and Hunt, pages 179, 183, 216, 2.33 to 263 and 301 to 303), which further confuse attempts at direct human viewing and lnterpretation.
For this type of original document, it would be desirable but is theoretically impossible, ln general, to determine rigorously from the original document the color temperature of the original scene and the spectral reflectances of ob~ects in the original scene depicted in the original document. ..
However, it is p.ossible bo assume a scene illuminant and to assume that all the colors i~ the original scene are derived . from a-particular (somewhat arbitrary) set of three dyes;
cyan, magenta, and yellow. It is a preferred embodiment of ~ .
the present invention that, for original documents of this ~`30 type, the red, green, and blue relati~e contributions to the luminance signal ~ be referred back to a mathematically ... ~:
m realizable approximation of peak white, grays and black in the -:, , .

original scene, rather than peak.white, grays and black in .:

the original document per se.
. .
.
.... . .. _ Only the "L" signal is necessary for copying black-and-white documents and ~or the neutral gray scale of colored documents. A "color killer" switch (not shown) on the copier hardware can eliminate other, chrominance signals when black-and-whi~e documen~s are to be copied and when black-and-white copies are desired from colored original documents.
Lens 58 in Fig. 2 focuses light reflected by beam-splitting mirror 60 and transmitted by beam-splitting mirror 70 upon photosensor 66 which is spectrally filtered to be responsibe only to the red spectral component of the light beam reflected from (or transmitted by) original document 54.
Similarly, lens 58 in Fig. 2 focuses light reflected by beam-splitting mirrors 60 and 70 upon photosensor 68 which is spectrally flltered to be responsible only to the blue spectral component of the light beam reflected from (or transmitted by) the original document.
Three electrical pre-amplifiers 72, 74 and 76 in Fig. 2 amplify the luminance signal L, red signal R, and i blue signal B, respectively. The output signal from the voltage gain stage of each such pre-amplifier is unipolar, ;
has low a-c impedance, h`as an output of zero volts for each black area in the original document scanned, and is normalized to plus one volt output for each peak white area in the original document scanned. The output stage of each such pre-amplifier is a phase splitter to provide two output voltages of equal magnitude and phase, opposite polarity, and equal bandwidths. However, ~he bandwidth for signals +L and -L is greater than the equal bandwidths for signals +R, -R, -~B
3 and -B. The signal bandwidths of the lat,ter four signals are - equal-to,-or slightly greater than, the signal bandwidth required for chrominance signa~ M, which in turn is greater than the bandwidth required for chrominance signal N. The '.

' de~inition and generation of M and N by a color differnence matrix 78 are explained below. tM can denote mlddle band-width and N can denote narrow bandwidth components of the chrominance signal.) Color Difference Matrix De~ine electrical chrominance electrical signals M
and N by the equations M = All ~ (R - L) ~ A12 ~ (B ~ L) (equation 1) and N = A21 * (R - L) + A22 * (B - L) (equation 2) where the constants A11, A12, A21 and A22 form a 2 x 2 matrix ~A~. The color dif~erence signals (R - L) and (B - L) may be formed by`conventional analog summing or'differencing circuits, since both the ~L and the -L elec~rical signals are available from ampli~ier 72,' both the +R and the -R electrical signals are available ~rom amplifier 74, and both the +B and -B
electrical signals are available from amplifier 76. Reference is made to E. M. Grabbe, S. Ramo, and D. C. Woolridge, Handbook of Automation, Computation, and Control, John Wiley and Sons, 1961; three volumes hereinafter ldentified as'Grabbe, Ramo and Woolridge 1, 2 and 3. Analog summing circuits are described in Grabbe, Ramo and Woolridge 2, pages 22-04 to 22-09.
The two coefficients All and A12 o~ matrix ~A3 are --selected such that middle-bandwidth chrominance signal M is' ~ ~
a vector for which the amplitude is measured in a direction ;~ '' nominally parallel to the minor axis of MacAdam JND ellipse's for near-neutral colors. The two coefficients A21 and A22 of matrix ~A] are selected such that narrow-bandwidth chromlnance ~' signal N is a vector normal to M. Therefore, narrow-bandwidkh chrominance signal N is a 'vector for which the amplitude is measured in a dlrection nominally parallel 'co the maJor axis of MacAdam JND ellipses for near-neutral colors. It is understood that to the extent that input signals L, R~ and B
have differing bandwidths, they also have di~ferent inherent __ .__ -phase delays. Hence it may be required to add externalphase delay to wide-bandwidth luminance signal L so that its total phase delay equals that inherent in signals R and B.
Equalization of phase delay assures that the simultaneous input amplitudes of L, R, and B to the color di~erence matrix correspond to the same location on the original document being scanned. Similarly, external phase delays may be required for output signals L and M so that their total phase delays equal that inherent in narrow-bandwidth ehrominance signal N.
Again, for the purposes of this disclosure, it will be assumed.that the electrical output signals R, L, and B from document.scanner 42 are linear measures o~ relative red, luminance, and blue reflectances of each successive location in a raster on a black-and-white or color original ~ .
document. It is to be understood that the invention is equally applicable to the geheration and processing of signals derived from document transmittances. Furthermore, the electrical signals may represent red, green, and blue ~20 instead of red, luminance, and blue information. The direct .
optical generation of the luminance signal at photosensor 64 .:
has the advantage, over prior art, that signal-to-noise.
ratios for`the required.signals in a document copier can -thereby be maximized... Also, for a black-and-white or monochrome ~.:
do.cument copierj only the luminance signal is required. For such a copier the spectral sensitivlty of the luminance photosensor is broad enough to assure optimal gray scale rendition in black-and-white copies of colored original : .
documents.
Unsharp Mask .
Luminance signal L has the greatest bandwidth and . hence may contain the most "noise" resulti.ng from grain, dirt, or scratches on the original docurnentj rrom matte , , - . ' ' -:, ' .. , ' ~ ' ' 7D~7~3 ~inish on the original document, and/or from blemishes in the documen~ scanner optlcs and electronics. I have provided apparatus for the reduction of such noise and for the enhance-ment of print sharpness by electronic unsharp masking techniques, one form of the apparatus being shown ln Fig. 3. It will be recognized that ~ig. 3 shows an unsharp mask 81 for the luminance signal of Fig. 1.
As a scan line, designated (J - 1), Or the original document is scanned simultaneously and in synchronism by photosensors 64, 66, and 68 of Fig. 2, the electrical signals representing the luminance for each successive point along the scan line are stored in an analog shift register 82 of ~ig. ~-3. For example, the Fairchild CCD311 130/260 analog shift register is a monolithic charge coupled device intended to be used in analog signal processing that includes analog storage and delay applications. As the next scan line, designated J, is scanned, the electrical signals from that line replace those of the preceding scan line in register 82, and the replaced signals are shifted into a second shift register 83.
Now, as the next line, designated (J + 1), is -scanned, its electrical signals replace those from line J in register 82 and are simultaneously applied along conductor 84 to a scan line cross correlator 85. ~he signal outputs ~rom register 82 replace those from the (J - 1) line in ;`
register 83 and are simultaneously applied along conductor 86 to scan line cross correlator 85. Simultaneously, the signals from scan line (J - 1) shift`from register 8~
along a conductor 87 to scan Iine cross correlator 85.
It is now seen that scan line cros`s correlator 85 of Fig. 3 serially receives the electrical signals from scan lines (J + 1), J, and (J - 1), with all three inputs syn-chronized such that corresponding samples from each scan line arrive simultaneously.
~ . . . . .
.

- ~74~3 Referring to Fig. 4, s~an line cross correlator 85 of Fig. 3 per~orms edge enhancement and noise suppression , by comparing the amplitude of the signal associated with one sample in a two-dimensional array of sample`d informatlon with the means among eight adjacent samples in the array.
Fig. 4 depicts samples (J + 1, K + 1), (J + 1, K), and (J +
1, K - 1) from scan line (J + l); samples (J, K + 1), (J, K), and (J, K - 1) from scan line J; and samples (J - 1, K +
1), (J - 1, K), and (J - 1, K - 1) ~rom scan line (J - 1).
Considering these nine amplitudes as a 3 x 3 matrix, the location of each amplitude in the matrix may be indicated as ~ollows:
Bll B12 B13 ~here the matrix ~B] is a moving matrix, centered at (J, K).
If column sum differences are less than a reference threshold, it is inferred that there is no vertic-al edge through (J, K). If row sum differences are less than the reference threshold, it is inferred that there is no horizontal edge through (J, K). Similarly~ the possible presence of a dia~onal edge through Bll, B22, and B33 may be tested by comparison of the absolute values of the differences among (Bll + B22 + B33), (B12 ~ B13 + B23), and (B21 + B32 ~ B31); and the possible presence of a diagonal edge through B13? B22~ B31 may be tested by comparison of thè absolute values of the dif~erences .:
among (Bll + B12 + B21)9 (B13 + B22 ~ B31), and (B23 ~ B32 + B33). I~ no edge is detected, B22 is reassigned an amplitude equal to the mean value of the nine elements of the matrix.

.. ..
If an edge detection difference exceeds threshold, B22 is assigned an amplitude whlch accentuates the edge detected.
Analog summing and differencing are discussed in Grabbe, .
Ramo and Woolridge 2, pages 22~04 to 22~09.
.
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7~7~3 Nonlinear Amplifiers Referring to Figs. 1 and 2, the document reader output electrical signals R, L, and B are inputs to color - dif~erence matrix 78. The output electrical signals from color di~ference matrix 78 are the middle bandwidth chrominance signal M and the narrow bandwidth chrominance signal N.
The three color signals L, M and N together provide the coordinates for a color space which is linear with respect to reflectance (or to transmittance) if the original document is intended also for direct viewing. However, if the original document is, for example, a color photo-graphic negative with photographic gamma o~, say, 0.70, then the three color signal outputs L, R and B from the document reader are nearly linear with respect to the square-root of transmittance. (Photographic gamma is discussed in Hunt, pages 209 and 216, and in Evans, Hanson and Brewer, p~ge ;92 ) .

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~74~03 I have provided three nonlinear amplifiers 92, 94 and 96 (Fig. 1) to perform the nonlinear transformations required on electrical signals L, M, and N to generate three new electrical signals L', M', and N', respectively. The new signals are intended to represent color information in a color space which is, insofar as possible, lin~ar with respect to human visual color tolerance errors. Furthermore, the new electrical color signals L' ? M', and N', are scaled so that equal signal amplitudes represent equal multiples of color difference tolerances. That is, signals L', M', and N' are intended to represent colors ~ a color space for which the color error tolerance ellipsoids approximate spheres of equal diameter over most of the important portions of the total color space. The approximations are, of course, subject to ' the theoretical limitations desc'ribed earlier in the preceding section titled "Theory".
Electrical color signals L','M', and N' proYide a .
most efficient way to represent points in color space, provided ''' the reIative bandwidths among these signals are consistent with the relative spatial resolutions for these color coordinates referred back to the original'document to be copied. 'For the finest detail, the human eye is essentially color-blind, and only 'the eiectrical luminance signal L' is requlred in ~' -order to reproduce that detail in each copy. For somewhat grosser details in color images, the middle-bandwidth chrominance signal M' begins to contribute to visual perception of images'.
For image in~ormation of still lower spatial frequencies, the electrical narrow-bandwidth chrominance signal N' begins to contribute to visual perception of images. ~ ' . .
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..
, - ~7~3 Analog to Digital Conversion Blocks 98, 100 and 102 (Fig. 1) are conventional electrical'sample and hold amplifiers for elec'trical signals L'j M~, and Nl, respectively. Blocks 10ll, 106 and 108 of ~ig. 1 are conventional electrical analog to digital con-verters which operate in synchronism with sample and hold amplifiers 98, 100 and 102, respectively, to digitize electrical color signals L~, M~, and N1, respectively.
In a preferred embodiment of the present invention, each o~ the three electrical color signals is sampled, held, and digitized at a rate consistent with the electrical bandwidth of that signal. The sampling of analog signals is a familiar topic, discussed in many text books. See, for example, R. B. Blac~man and J. W. Tukey, The Measurement of Power Spectra From the Point of View of Communications ':
Engineering, New York Dover Publications, Inc., 1958, and Alfred X. Susskind (ed'itor), Notes on Analog-Di~tal Conversion Techniques, Cambridge, The Technology Press, 1957.
Basically, sampling theory tells us that lf the sampllng rate is too low for a given bandwid-th of the input analoe signal, the power spectrùm above the Nyquist frequency can introduce`serious degradation of the sampled data due to aliasing of frequencies above the Nyquist frequency. On the other hand, if the sampllng rate is much higher than required for a given bandwidth of the input analog signal, the number of samples to be digitized and s~bsequently processed bec'omes unnecessarily great. Many economic disadvantages can accrue from an unnecessarily-high sampling rate; the analog to digital .
converter must function faster, the buffer memory must have more capacity and subsequent digital signal processing .
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:~L07~7~3 hardware must function faster. Some or all of these results may contrlbute unnecessarily to manufacturing cost of the hardwàre.
Also ~or the sake of economy, the number of binary bits to which each color signal is digitized is kept to a minimum consistent with the ratio of tolerable quantization ~ e~
b error to total range of the variable dititiJ~c~. For exampleS
if only thirty-two luminances need to be discri.mlnated for .~
a particular application, a five-bit analog-to-digital :
converter is required at block 104 in Fig. l; if sixteen middle-bandwidth chrominances need to be discriminated for `
that application, a four-bit analog-to-digital converter is required at block 106; and if eight narrow-bandwidth chrominances need to be.discriminated for that application, a three-bit analog-to-digital converter is required at block :i 108.
It should be noted that color signals L, R, and B ~.
~rom the document reader are unipolar; they have magnitude . ~.
but not sign. Similarly, ~he digital signal L' output from 20 luminance analog~to-digital converter lO4 is unipolar. How- .
ever, thè digital output signals M' andil N' ~rom middle band-width chrominance ~analog-to-digital converter 106 and from ~ :
narrow bandwidth chrominance analog-to-digital converter 108, respectively, are both bipolar; those two dlgital signals have both magnitude and sign. The sign bit is included in the ~ .
examples proposed above. Whether thé bipolar digital signals are handled by magnitude and sign, or by complementary offset ~ binary ~ode, is immaterial to this patent.

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~ 79L'703 Data Format Lo~ic and Data Compressor A data format logic and data compressor 110 of Fig. 1 organizes the digital data for e~ficient storage or data transmission. Design details of the data format logic depend upon the relative sampling rates of sample and hold amplifiers 98, 100 and 102. In a preferred embodiment of the present invention, those relative sampling rates are related by simple integers in order to simplify the data format logic. In the hypothetical example proposed above, the precisions of analog-to-digital converters 104, 106 and 108 were assumed to be five bits, four bits, and three bits, respectively, for a given hypothetical application.
Assume further, in that example, that the sampling rates for sample and hold amplifiers 98, 100 and 102, and that the rates of analog-to-digital converters 104 3 106 and 108, respectively, are in the ratios of ~our to two to one, respectively. Then it is convenient to work with four 5-bit words plus two 4-bit words plus one 3-bit word at a single address. Under such circumstances, word length includes ~0 (4 x 5) + (2 x 4) + (1 x 3) = 31 data bits. The data format logic can accept as input these seven analog-to-digital converter words and can combine the analog-to-digital converter outputs i .
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~0747[)3 into a single word of 31 data bits for bit-parallel input to the next stage.
In some applications, it may be desirable to add one or more check bits to each large word sent out from the data format logic. (Reference, R. W. Hamming, "~rror Detecting and Error Correcting Codes," Bell System Technical Journal, volume 26, number 2~ pages 147 to 160; April 1950.) In the Iva R~
hypothetical example quoted, the we~ output rate from data - ` wo~
format logi~c equals one-fourth the data~output rate of luminance ;
analog-to-digital converter 104 in Fig. 1.
Buffer Memory And Data Transmission Link I have shown a digital buffer memory and data trans-mission lin~ 111 at the output of data format logic and data compressor 110. Buffer memories and data transmission links in color copiers are well known, and the reader's attention is directed to U.S. Patent No. 3,679,818, which issued on July 25, 1972 to J. S. Courtney-Pratt as an example. Such devices permit (1) the original document to be handled and scanned only once regardless of the number of copies to be generated, (2) a multiplicity of original documents to be scanned at one rate and the copies thereof to be generated at a different rate not necessarily synchronous with document reading, (3) the physical separation of the document reader and the exposure station, and (4) the incorporation of a multlplicity of document readers interconnected among a multiplicity of copy exposure stat~ons.
While buffer memories and data transmission links are well known, I have located suc~ a device 111 in the data . .
- handling chain of Fig. 1 at a posltion in which the required memory size and for which the required bits per transmitted message are minimized by the no~el efficient color space and , . I .

2~

` ~074703 the novel e~icient data format which result from the "upstream"
apparatus previously described.
Color Table Read-Only Memory The concepts o~ color masking and o~ colored couplers in photographic products are well known to persons ~amiliar with the art. Reference is made to Hunt, pages 55, 233 to 263, 445, and 464 to 470, see also Evans, Hanson and Brewer, pages 542 to 544, 567 to 578, and 592 to 610. Color masking is accomplishable optically or by chemicals in a color photo-graphic product and process. Ideally, color masking providesa basis for obtaining the best possible reproduction from a given set of color image forming materials in the original to be copied and for a given set of color image forming materials at the copy exposure sta-tions. To quote Hunt, page 55, "It will be realized that these three expedients cannot correct for the fundamental limitations of the process, which spring from the nature of the color mechanism of the eye and the shape of the spectral absorption curves of the best available cyan, magenta and yellow dyes. What is claimed for modern subtractive processes is that they produce pleasing color pictures, and that the inevitable inaccuracies are balanced in such a way as to be least noticeable." Masking was one of the expedients referred to. That is, the color table read-only memory means of the present invention provides signal processing to compensate ~or various charac-teristics of the original document, the scanning process and the image-forming process by (l) enhancing color repro-duction from the colorant set in the original document, (2) correcting for saturatlon losses, (3) correctlng for hue 3 errors and (4) correcting for contrast errors.

~ -25-~07~3 I have provided a color table read-only memory 112 (Fig. 1) to accomplish electronically the objectives sought by color masking in color photographic systems.. The following simple, linear color matrlx can solve the following set of simultaneous equations for each picture element in the original document to be copied:

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~0~'703 .

R' = Cll ~ L~ -~ C12 ~ M' ~ C13 ~ N' (~quation 3) G' = G21 ~ L~ + C22 ~ M' ~ C23 * N' (Equation 4) B' = C31 * L' + C32 ~ M' + C33 ~ N' (Equation 5) where the three color signal inputs are L', M', and N'; the ; three color signal outputs are R', G~, and B'; and the nine constants (~ to C33) form a 3 x 3 matrix rc~ . However, a 3 x 3 matrix provides only a linear, ~irst approximation to solution of the general color masking problem. A second approximation, with a 3 x 10 matrix, can lnclude some non-linear effects such as the squares of the three input color signals, the linear cross products of the three color signals, ~and a constant term for each equation.
In the present invention 3 color matrix read-only memory 112 pr-ovides a digital, table look-up method to solve a color matrix ~or each picture element of the original document to be copied in a fast, precise manner. Furthermore c`olor table read-only memory 112 has the advantage thàt any set of three color matrix equations, either linear or non-linear, can be solved thereby with no increase in hardwarecomplexity, no loss in speed, and no loss ln hardware stability when nonlinearities are included in the assumed functional a~
relationships bc~e~ the three digital input signals an~ the three digital output signals.
The concept of storing digital data in a read-only memory 1s well known to those skilled ln the art. For example, the Harris Semiconductor HPROM-0512-5B pro~rammable read-only memory is a field-programmable, read-only memory (ROM) which stores 512 digital binary data bits, organlzed with elght 3 bits per word at sixty-four addres~es. Read-only memorles of larger data storage capacity, æuch as 4096, 8192, or ~ ~

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~747~3 16,384 binary bits, are also known. Furthermore, individual read-only memory devices can be combined in an array to provide longer words and more addresses.
A pre~erred embodiment of the present invention employs one or more arrays of read-only memories to construct color table read-only memor~ 112. Digital data signals L', M', and N', either from data format logic and data compressor 110, or directly from analog-to-digital converters 104, 106 ~' and 108, provide the address input information to the ROM.
Corresponding values of digital data color signals R', G', and B' are stored at each address in the ROM. Such construction permits a "table look-up" operation'to occur. The ROM provide~ ' '' one address for each possible combination of inputs L', M', and N'. A total of 4096 addresses are required if L~ has 5 data bits, M' has 4 data bitsg and N~ has 3 data bitsg for example, as in the hypothetical'example previously mentioned.
If all the data bits of L~, M', and N~ are significant in the determination of R', G', and B',' then'a preferred embodiment of the present inventlon stores one long word~at each address, that word including the values of R',-G', and B' associable with that address. I have made excellent photo-graphic color prints with 8-bit precision for R', G', and B', ~ although the experimental hardware had a capability of 10-bit ; precision for R', G', and B'. ' Some applications may require that the precisions of L', M~, and N' be greater than 5 bits, 4 bits, and 3 bits respectively, as in the hDpothetical example previously ' mèntioned. If'the precisions required for L', M', and ~' are 6 bits, 5 bits, and 4 bits, respectively, the total number of . :

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addresses required in a single color matrix ROM become 2(6 5 4) = 215 = 32,768 addresses. However, in a given applica-tion, it may occur that one or more of the least significant - bits for one or two of the digital input signals do not contribute significantly to the digital output value of one or two of the ou~put signals. In such a circumstance, the total size-(i.e., the total number of bits stored) of the ROM can be reduced by separating the ROM into three separate ROMs, one for each o~ the original output signals.
For example, assume in a hypothetical application that the required precisions for L', M', and N' are 6 bits, 5 bits, and 4 bits, respectively. Furthermore, assume that the functional relationships between input signals L~, M', and N' and output s~gnals R', G~', and B' are as follows:
R' = f[L'(5 bits), M'(5 bits), N'(3 bits)] (Equation 6) .
G' = f[L'(6 bits), M' (Ll bits~, N'(3 bits)] (Equation 7) B' = f[L'(5 bits), M'(4 bits), N'(4 bits)] (Equation 8) where each f(Lt, M', N') denotes some general function of those three input signIs, and the number of bits indicated in parenthesis after each input signal indicates the required precision for-that input signal to that functional relation-ship. Furthermore, for purposes of illustration assume that the required precislons of the digital output signals Ri, G'~
and B' are 6 bits, 7 bits, and 5 bits, respectively. If color table read-only memory 112 of Fig. 1 is implemented by a single ROM, the required number of addresses is 32,768 and the required number of bits per address is (6 + 7 ~ 5) - 18;
.
I therefore the required ROM size is 32,768 * 18 = 589j824 .
data blts.
Howeverj it is permissible to divlde the ROM into three-separate ROMs: one in which to look up the appropriate . ' ' . :' . ' ' ~ .
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3 ~747~

value of R'g one in which to look up the value of G', and one in which to look up the value of B''. The required number of addresses in the R' ROM is 2(5 + 5 + 3) = 213 = 8192 addresses, with 6 bits of digital data stored at each address. Hence the required size of the R' ROM is 8192 ~ 6 = L~9,152 data bits under the hypothetical requirements as-sumed herein.
Similarly, the required number of addresses in the G' ROM
is 2(6 3) = 213 = 8192 addresses, with 7 bits of digital data stored at each ad'dress. Hence the required size 10of the G' ROM is 8192 ~ 7 = 57~341~ data bits. Similarly, the required number of addresses in the B' ROM is 2(5 4 I) = 213 = 8192 addresses, with 5 data bits stored at each address. Hence the required size of the B' ROM is 8192 ~ 5 = 40,960 data bits. The total size of the three separate ROMs is 147,456 data bits, ~ust one-fourth as large as wouId be required if color'table read-only memory 112 were organized as one large ROM instead of three separate ROMs.
'Therefore, it will be preferred that color masking 20 be accomplished in color table read only memory 112 by digital ' table look-up either in one ROM or in three separate ROMs, one for each digital output signal, whatever organization yields ' the'smallest required ROM size.
The information to be stored in color table read~
only memory 112 is determinable by calibration techniques - analogous to those for photographic color masking, described '' by Hunt~ by Evans, Hanson and Brewer and by others in the published technical literature.
~ Exposure Time Delays 30 'Exposure time delay networks (not shown) may be - provided to permit non-simultaneous exposur:e of' the three - :
, ; ~29-~4~)3 color image formin~ materials at three physisally separated image exposure stations. It is immaterial which two digital signals are delayed, among R', G', and B'. Furthermore, it is immaterial which of the two delayed digital signals is delayed longer than the other. These alternatives are determined by practical considerations at the image exposure station. Indeed, it may be practical to accomplish two or all three exposures simultaneously at the same image exposure station and eliminate the need for one or ~oth of the digital time delays.
Each exposure time delay can be built from conven-tional digital, serial-in, serial out shift registers; with one shift register for each data ~it in the digital data signal to be delayed and with one shift location ~or each picture element associable with the time delay required.
Red, Green, and Blue Read-OnLy Memories ~
I have provided a red read-only memory 114, green read-only memory 116, and blue read-only memory 118 (Fig. 1) to convert the output signals R', G', and B' from the color space in which color matrix read-only memory 112 generates its outputs into the color space which is linear with respect to the control signals R", G", and B", respectively, required to control exposure at an exposure station 120.
These three color read-only memories function in the same manner as the color matrix read-only memory described above. Indeed, the functional objectiv~s of the three color ~ - 30 -.

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7~3 read~-only memories can he incorporaked into the information stored in color table read-only memory 112. The choice of separate color read-only memories ~ersus incorporation of the functional objective into the color table read-only ' .
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memory is primarily a matter of economics for the hardware for a given set of applicat~ons. The choice depends, in part, upon the required number of input bits and required number of output bits ~or each color read-only memory. I have generated excellent color photographic prints with 8-bit precision for R", G", and B"; although the experimental hardware was wired to provide a capability for 12-bit precision.
~ The digital output words from the red, green and blue read-only memories 114, 116 and 118 are converted from digital-to-analog control currents by respective digital-to-analog converters 122, 124 and 126. The analog control currents control the red, green and blue components of t~e radiant power in color exposure apparatus 120.
Alternative Embodiments In some applications, lt may be desirable and satisfactory to relocate color difference matrix 78 of Fig. 1 to a position following nonlinear amplifiers 92, 94 and 96.
Further, it may prove economical in certain cases to eliminate -color differnce matrix 7~ altogether. In such a case, the functional relationships expressed by matrices rAJ and [C]
hereinabove are then combined into matrix W in color table read-only memory 112.
A simplified alternative to the above is to let the color output signals from document reader 42 be representative of green, red and blue information read from the original document. The document reader output signals can then be designated as G, R, and B and the digi.tal signals from analog-to-digital converters 10~l, 106 and 108 can provide ` dlrect inputs to color table read-onIy memory 112.

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Image Exposure Station Image exposure station 120 may take many forms.
For instance, the laser color printing apparatus disclosed in U.S. Patent No. 3,783,185 may be used, receiving its control information directly from digital-to-analog con-verters 122, 124 and 126 as controlled ~y buffer memory 111.
In Fig. 5, I have shown a copier apparatus which, in operation, ~orms a direct positive image on the surface of an imaging roller 140 by the process of electrophoresis.
Imaging roller 140, commonly referred to in the art as the injecting electrode, is made of Nesa glass. (Nesa glass is a trademark of PPÇ Industries used to designate an optically transparent glass overcoated with a thin, optically trans-parent, electrically conductive lay~r of tin oxide)O In ; close proximity to the injecting electrode 140 and parallel thereto is a second roller 144, commonly referred to in the art as the blocking electrode~ ~locking electrode 144 has a conductive center core 146 covered with a layer of insulating material 48 such as Baryta paper. The conductive core 146 is connected to a power source 150, the opposite side of which 2Q is connected to the injecting electrode 140 through a switch 152.
Located above the injecting electrode 140 is a reservoir 154 containing a mixture O~ ~inelY divided, magenta, cyan and yellow pigment particles dispersed in an insulating carrier liquid. The pigment particles are also photosensitive ~ ` - 32 -`` ' ~ ' .
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10'~7~)3 and are selected so that the different color particles respond to different wavelengths of light corresponding to their principal absorption (i.e., the cyan color particles are responsive to red light, the magenta color particles are re-sponsive to green light and the yellow color particles are ~- :
.

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7C~3 responsive to blue light). ~eservoir 15~1 supplies the sus-pension 156 to a coating roller 158 which applies a uni~ormly thick layer of the suspension to the surface of the in;ecting electrode 140.
A mirror 160 located within in~ecting electrode 140 receives the ligh~ image pro~ected by a laser 161 controlled by the signal outputs from buffer memory 111. The laser beam is modulated at 163 by those.signals and is deflected at 165 in a manner as described in Spaulding U.S. Patent ~o. 3,783,185. .
10 Mirror 160 reflects the light image through an exposure slit ' 162 onto imaging suspension 156 at a line of contact`where inJecting electrode 140 contacts the blocking electrode 144.
During the imaging process, suspension 156 is ' exposed by raster scanning the laser in synchronism with the rotation of in;ecting electrode 140 and blocking electrode 144 by a motor 164 while a po,tential is applied across the ` electrodes through the closed switch 152. The pigment particles are believed to bear an initial charge when suspended ' in the liquid carrier which attracts them to injecting electrode ~ 20 140. Upon exposure, the particles ~ ge~ polarities by : exchanging charge with in~ecting electrode 140 and migrate away from this electrode.to blocking electrode 144 thereby : ` forming a pos,itive image on inJecting electrode 140 and a complimèntary negative image on blocking electrod~ 144 by partlcle subtraction.
The image formed on the sur~ace of injecting ' ~, electrodè 140 is then carried into contact with an'adhesive . copy sheet 166 f'ed'from a supply, not ~hown~.so that the.copy sheet moves at the same speed as the periphery of in~ecting '` 3 electrode 140. A pressure roller 168 ls placed behind sheet ; .
' ; , ' ' 33 - ' ' ~

~L079~7~3 166 at the point of contact with in~ecting electrode 140 to assist in the transfer of the image from the surface of the injecting electrode onto the surface o~ the copy sheet. The trans~erred image is then fixed in place by transporting the sheet past a dryer unit, not shown, whereat a binder material, dissolved in the liquid carrier comes out of solution as the relatively volatile`carrier liquid evaporates. The remnants of the suspension 156 are then removed from the surface of injecting electrode 140 by a brush 170 and from the surface of the blocking electrode 144 by a brush 172 in preparation - for the next imaging cycle of the apparatus.
S~ynchro_i~ation As will be understood by those skilled in the art, characters from deflector 165 must be applied in timed B relat~ion withJfthe movement ~i~,in~ecting electrode 140 and - blocking electrode 148 so that the relative position of each diserete image portion on receiver medium 166 corresponds to the relative position of its respec~ive counterpart area of original document 54. It should be noted that the electro-photographic apparatus as shown schematically is being drivenby motor 164. Also shown schematically is a dotted-line . conhection 173 with the main drive shaft of blocking electrode `
144 which applies a signal to a shaft encoder 174. The shaft - encoder in turn ap~lies a series of signals to logic and con-trol unit 176 (Fig. 1) along dotted-line connection 177. The logie and control apparatus is adapted to determine ~ust how fast the~members 144 and~140 are moving. After it has computed -;
- this rèlationship logic and control unit 176,.in turn, feeds ~baek signals to buffer 111 as shown in Fig. 5 along dotted-line connection 178 and also to document reader 42 shown in ~ig. 1.
- Thus, it should now be clear ~hat the operation of all the ' ~,~
devlees àre actually in timed synchronizakion with the movement ' of the members 140 and 144.

_34_ - , . , . , , , ~ .. _ _ .
. . .

An e~ample of a logic and control apparatus which can perform the aforementioned described operations is set forth in detail in co-assigned U.S. Patent No. 4,025,186 issued 24 May, 1977, entitled SYNCHRONIZING CGMTROL APPARATUS
FOR ELECTROGRAPHIC APPARATUS USING DIGITAL COMPUTERS, filed in the names of Hunt et al on February 13, 1975. Briefly reviewing that application, it does not in fact describe the shaft encoder,-but what it does show are perforations along an electrophotographic member which are read by sensing apparatus to produce two series o control signals which in turn apply to logic and control apparatus. Although a shaft encoder has been shown schematically in Fig. 5, it will be understood that the perforations or marks on a moving member technique shown in the aforementioned co-pen~ing application can be used to advantage in this application.
It should further be clear to~one skilled in the art that the reading operation of the apparatus shown in Fig. 2 will have to also be synchronized to the operation of the electrophotographic apparatus. Towards this end page scan mirror 52, which is controlled by a galvanometer drive motor 62, will be understood to be under the direct control of logic and control unit 176. Reference is also called to commonly assigned U.S. Patent No. 3,864,697, issued February 4, 1975 to Dillon et al, which also shows synchronization and control apparatus that can, as will be understood to those skilled in the`art, be modified to provide the aforementioned described operations.

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:10'7~3 The inyention has been described in detail with part-icular reference to preferred em~odiments thereof, but it will be understood that variations and modifications can be effected within the spirit and scope of the i~vention.

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Claims (14)

1. In a document copier comprising reading means for producing electrical signals representative of the image and color information present in a document to be copied, and printing means responsive to applied electrical signals to imagewise expose a photosensitive recording element to reproduce thereon the image and color information present in such document, the improvement comprising:
(a) means for converting electrical signals produced by said reading means to forms which are sub-stantially linear with respect to human visual color error tolerances, whereby the converted signals are in more efficient form for storing;
(b) memory means for storing the converted electrical signals, (c) programmed computation means for applying a corrective function to the converted electrical signal to enhance the image and/or color information represented thereby; and (d) means for applying the image and color enhanced electrical signals to said printing means.

2. The improvement as defined in claim 1 wherein said programmed computation means includes means for enhancing the color information in the signals produced by said reading means by correcting for characteristics of the colorant set in the original document.

3. The improvement as defined in claim 1 wherein said programmed computation means includes means for correcting for color saturation losses in the original document and/or the document copying apparatus.

4. The improvement as defined in claim 1 wherein said programmed computation means includes means for correcting for characteristics of hue in the original document and/or the document copying apparatus.

5. The improvement as defined in claim 1 wherein said programmed computation means includes means for correcting for characteristics of contrast in the original document and/or the document copying apparatus.

6. The improvement as defined in claim 1 wherein said programmed computation means comprises digital table look-up means for applying said compensating function to said converted signals.

7. The improvement as defined in claim 6 wherein said digital table look-up means comprises a read-only memory.

8. The improvement as defined in claim 6 wherein said converted signals are in analog form, and said improve-ment further comprises means for converting such analog signals to digital form.

9. The improvement as defined in claim 1 further comprising electronic unsharp masking means for suppressing undesirable noise in the signals produced by said reading means.

10. The improvement as defined in claim 1 further comprising electronic unsharp masking means for modifying at least a portion of the signals produced by said reading means to enhance the edge sharpness of images produced by said printing means.

11. The apparatus as defined in claim 1 wherein said reading means produces a luminance signal and first and second chrominance signals, and wherein said improvement comprises means for converting said chrominance signals to two essentially orthogonal signals.

12. The improvement as defined in claim 11 wherein said means for converting said chrominance signals comprises a color difference matrix.

13. The improvement as defined in claim 11 wherein said reading means comprises:
means for producing a light beam containing red, green and blue color components;
a photosensor in the path of said light beam; and a filter between said light beam producing means and said photosensor, said filter having red, green and blue spectral transmittance characteristics selected so that, for a neutral grey area of the docu-ment, the relative red, green and blue contributions to the luminance signal are in proportion to what the red, green and blue contributions would be if the docu-ment were illuminated by CIE Source C.

14. The improvement defined by claim 1 further comprising:
means for converting signals produced by said reading means to one luminance signal and two essentially orthogonal chrominance signals; and means for differentially narrowing the bandwidth of said two chrominance signals.