Rotating loop half-tone reproduction
US RE28255 E
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Description (OCR text may contain errors)
Nov. 26, 1974 J. E. AUGHTON RG- 25, 255
:mums Loop HALF-Tons nnrnonucnon Orghal Filed Dec. 27, 1971 4 Sheets-Sheet 1 Nov. 26, 1974 J. E. AUGHTON R- 23. 255
ROTTIHG LOOP HALF-TONE REPRODUCTION Original Filed Des. 27, 1971 4 Sheets-Sheet 2 Nov. 26, 1974 J. E. AuGHToN R 23, 255
nourzns Loo? HALF-'roms nnrnonucnon original Filed nw. 27. 1971 4 sheets-sneet 3 Nov. '26, 1974 .1. E. Ausl-nou R- 25, 255
ROTTIHG LOOP HALFTOHE REPRQDUCTION Y @Moda/0000 J/gno/ United States Patent O 28,255 ROTATING LOOP HALF-TONE REPRODUCTION John E. Aughton, London, England, assigner t Crosfeld Electronics Limited, London, England Original No. 3,737,225, dated June 5, 1973, Ser. No. 212,434, Dec. 27, 1971. Application for reissue Sept. 4, 1973, Ser. No. 394,274 Claims priority, application (lieat Britain, Jan. 6, 1971,
Int. Cl. G0311 27/48 U.S. Cl. 355-48 5 Claims Matter enclosed in heavy brackets appears in the original patent but forms no part of this reissue specification; matter printed in italics indicates the additions made by reissue.
ABSTRACT OF THE DISCLOSURE An image reproduction apparatus employing a scanning light spot to expose a light-sensitive sheet, a halftone image is produced by moving a light-transmitting half-tone screen element through the light path between the light source and the light-sensitive sheet at a point in the light path conjugate with the light spot on the sheet. In this way an image of the moving screen element, which may be a rotating loop of film, is formed at the light-sensitive surface.
The usual method for producing half-tone images consists in exposing a suitable light-sensitive film (for example a lithographie tilm) through a halt-tone screen, for example by superposing a negative image, the half-tone screen and the light-sensitive sheet for contact printing. The density characteristics of the screen and the sensitivity of the tilm are such that each clement of the image is reproduced as a dot on the light-sensitive sheet, the density of the dots being constant but the area or percentage of each dot varying as a function of the density of the corresponding element of the image negative. Halftone screens are expensive and for color printing there is required either a set of half-tone screens with lines running at carefully selected angles or a single circular screen capable of being indexed to the selected screen angles, as otherwise the dots of different screens combine to produce a moire pattern which may be very noticeable. If a black printer is used in addition to the three color printers, the screen angles are generally 0, 15, 45 and 75.
In some forms of image reproducer, the image to be reproduced is scanned by a photo-electric device which generates electric signals varying in amplitude with the density of the scanned elements. In color reproducing apparatus of this kind, the scanner also analyzes the light into its color components and the resulting electric signals from the photo-electric devices represent by their variations the densities of the corresponding color components of the scanned elements. With such apparatus color correction and tone correction can be effected by electrical mixing of the signals and the corrected signals can then be used to modulate a light source which exposes a lightsensitive sheet which is to be used in the preparation of a printer for the color component in question. Half-tone reproductions were produced by placing a half-tone screen in contact with the light-sensitive sheet.
According the present invention in apparatus having means for obtaining relative motion between a light-sensitive sheet and a light spot derived from a light source, to expose the sheet element by element to the light spot, and means for modulating the light spot in accordance with tonal values of successively scanned elements of an image to be reproduced on the light-sensitive sheet, we provide means for moving a light-transmitting half-tone ICC screen element through the light path between the light source and the light-sensitive sheet at a point in the light path conjugate with the light spot on the light-sensitive sheet, so that an image of the moving screen element is formed by the iight spot at the light-sensitive surface, the movement of the screen element through the light path being synchronized with the movement of the light spot over the light-sensitive sheet. Thus, it can be arranged that the half-tone screen element moves across the aerial image of an aperture in the optical system or that an aerial image of the half-tone screen pattern moves across the aperture itself. The light which reacties the light-serlsitive sheet is modulated in two different senses, being time-modulated in accordance with the tonal values of the image to be reproduced and spatially modulated by the screen element.
If the light-sensitive sheet has a suitable characteristic, the required half-tone image is produced when the sheet is developed. The light-transmitting halttone screen element may be one of a series of elements on a film loop which is fed continuously across the light path. The enlargement of the screen elements on this tilm loop depends upon the characteristics of the optical system.
The invention utilizes the observation that if a set of equally spaced parallel lines is drawn at any angle to the ruling on a screen pattern, by varying the pitch. ie., the spacing between these equally spaced parallel lines it can be arranged that the screen pattern is split up into identical strips, assuming that the screen stretches to intinity in all directions` This will be explained more fully later in this specification.
In one form of scanner, the image to be analyzed and the light-sensitive sheet to be exposed are placed around the periphery of a rotating drum, axially spaced from one another. The drum is rotated and the analyzing and exposing heads of the scanning apparatus are moved slowlyr parallel to the drum axis so that each scanning head traces out a helical path on the drum. With such an arrangement, for a given angle the `whole screen pattern can be built up from a repeated unit half-tone screen cell and can thus be reproduced from a single continuous loop of pattern of the same width and length equal to an integral multiple of repeating lengths.
With such an arrangement the user will not require expensive sets of complete screens and there is no need for a screen to be physically located over the light-sensitive layer each time a separation is made. ln addition, there is no need for a vacuum contact system between the screen and the light-sensitive layer.
It will be appreciated that it is necessary to use a suitable relative traverse rate between the film loop and the light sensitive sheet to produce a required screen pattern and that adjacent scanning lines must be correctly phased.
In order that the invention will be better understood, two examples will now be described with reference to the accompanying drawings, in which:
FIG. l illustrates an image reproduction system embodying the invention;
FIGS. 2 and 3 illustrate screen elements of the kind used for the screen loop shown in FIG. l;
FIG. 4 illustrates the possible strip widths for a given screen angle: and
FIG. 5 illustrates the optical system of a second irnage-reproducing apparatus embodying the invention.
In FIG. 1, a transparency 1 to be reproduced is wrapped around the transparent surface of a drum 2 rotated by a motor 3. A light source 4 (the analyzing light source) illuminates the transparency 1 by way of a mirror 5 placed at 45 to the light beam, within the drum 2. Light passing through a spot on the drum and through the transparency element above this spot is received by a lens 6 in an analyzing head 7 and is focused on to a photomultiplier B which provides a corresponding electric signal. As the drum rotates the spot 9 traces a circumferential path around the drum and therefore around the curved transparency 1. The analyzing head 7 is mounted on a lead screw l rotated by a motor 11 so that in the course 3f a drum revolution the analyzing head moves parallel to the drum axis. As a consequence the spot 9 traces a helical path around the drum surface and the transparency s scanned in a number of side-by-side lines, the pitch seing governed by the movement of the analyzing head ilong the lead screw in l revolution. The mirror is attached to the analyzing head and moves with it. The motor 11 is phase-locked to the motor' 3. This is effected Jy means of a disc 12 mounted on the driven shaft 13 and having a ring of peripheral apertures, a light source 14 on one side of the disc and a photoelectric device 15 Jn the other side of the disc which receives light through he apertures and generates a train of pulses having a frequency governed by the rotational speed of the drum. l'hese pulses control the rate of rotation of the motor l5.
The signal from the photomultiplier 8 thus represents :he tonal values of' successively scanned elements of the ransparency 1. This signal is applied to a correction :omputer 20, which can be of known design, and the :orrected signal is applied as a modulating signal to a ight modulator 21. This may be a Pockell cell crystal employing the longitudinal electro-optic effect.
The light modulator 21 receives light from a reproiucing light source 22 through a collecting lens 23. A ens 24 concentrates the modulated light on to an adustable aperture 25 and thence on to a lens 26. A mirror I7, arranged at 45 to the axis of the incident beam, 'etlects the light on to a screen loop 28 constituted by t strip, of the form shown in FIG. 2 or 3, formed into t circle. In a typical example there are 1,000 screen elenents in a screen loop, the enlargement of the screen rlements being less than that depicted in FIGS. 2 and 3. The scren loop 28 is driven in rotation by a motor 29 vhich is also phase-locked to the motor 3 by means )f the components 12 to 15.
The arrangement is such that an aerial image of the aperture 25 is formed in the plane of the portion of the creen loop 28 through which the beam passes. Consejuently, light passing through the screen loop carries he modulation imposed by the output of the correction :omputer as well as the modulation imposed by the tcreen loop. This light falls on a final lens 30 in an exaosing head 31 also mounted on the lead screw 10 and herefore moving axially at the same speed as the anayzing head 7. The lens 30 focuses the light to a spot 32 Jn a light-sensitive sheet 33 wrapped around the drum l. The spot 32 scans the sheet 33 in the same way that he spot 9 scans the transparency 1 and consequently he sheet 33 is exposed element by element.
We have already drawn attention to the difference in he forms of modulation present in the beam which 'caches the light-sensitive sheet. The modulation of the ight by the output signal from the correction computer lppears as a time-modulation of the beam as a whole, Jvhile the effect of the screen loop is to introduce a spaial modulation varying over the cross-section of the eam. The intensity modulation derived from the comiuter output combines with the spatial density variations m the screen sample, ultimately to produce the required iot percentages" in the developed image.
The screen loop is driven by the phase-locked motor t9 so that its peripheral speed is equal to that of the lrum 2 carrying the sheet 33 divided by the magnificaion of the exposing lens. The width of the aperture is :et to correspond to the section of the screen pattern. [he lens 30 is a zoom lens which enables fine adjustnent of the line width. The screen sample is selected n accordance with the required screen angle, and the aperture Width, traverse rate and phasing of adjacent scanning lines are set in accordance with the screen angle.
FIGS. 2 and 3 illustrate the form of' the screen element, FIG. 2 representing a screen element at 0 and FIG. 3 a screen element at an angle of arc tan 1A. The screen element of FIG. 3 is a simple example of a screen angle other than 0. Its angle is approximately 15 but is not close enough for printing at the conventional 15 screen angle, for which it is preferable to use a screen element at an angle of arc tan 15j/16, or an even closer approximation to 15. The strip shown in FIG. 2 cannot be further longitudinally divided because it is essential to find corresponding points in the dot pattern on both edges of the strip. The strip shown in FIG. 3 can be further sub-divided; for example, it can be divided into four similar strips of equal width and in each strip corresponding points in the dot pattern will be found on both edges. The example shown in FIG. 3 is the widest strip which would normally be chosen at this screen angle to maintain picture resolution.
FIG. 4 shows a series of strips of different widths for a screen having an angle t9=arc tan 2/3. This angle is not a conventional screen angle but is useful for demonstration purposes. The diagonal lines indicate the screen rulings and the points at which the lines cross one another are corresponding points in the dot pattern, for example dot centers. Three possible strip widths are shown, with points A, B and C as sets of corresponding points in the dot pattern; the distances between successive points A, B or C on a single strip represents the repeating length and the strip area enclosed between these points constitutes a unit cell. The displacement of adjacent strips in the direction of their length indicates the required phase delay between successive scanning lines.
If 0=the angle between the screen ruling and the strip edge and tan H=P/Q where P, Q are integers with no common factors when the possible strip widths are N/Q(a cos 6) where a=dot spacing (a square grid iS assumed) and Nzl, 2, 3
If tan 6 is irrational then any strip width is possible. In practice the screen is of tinite size and it is required to simulate it on a rotating drum. If the restriction is made that tan 6 is rational then the strips are seen to consist of identical repeating lengths, and the basic If the required phase delay is small in relation to the strip length (for example less than 1,'000th of the strip length), then the speed of rotation of the sample can be changed slightly (relatively to that of the drum) t0 achieve this phase delay, the resulting distortion in dot shape would not be significant.
Another way of achieving the phase delay is to select the ratio between strip length and cylinder circumference so that adjacent scanning lines begin on the correct part of the pattern. For example, if the screen angle is 45, the required phase delay between adjacent scanning lines is one half of the repeating length. This phase delay can be achieved by making the screen loop with an odd number of repeating lengths in its circumference and arranging for the loop to rotate, for example, one and a half times for each rotation of the drum. This assumes a magnification of one between the loop and the drum. Then if there are 707 repeating lengths in the loop, the number of repeating lengths exposed on to the drum in 1 revolution of the latter is 10601/2. Appropriate choice of the ratio of strip length to drum circumference would enable correct phase delays to be obtained for other screen angles.
FIG. 5 shows a modification of the optical system in which the screen loop is interposed in the light path at a different conjugate point, before the light beam reaches the aperture 15, the point being so chosen that an image of the screen element through which the light beam passes is formed at the aperture 15.
It will be appreciated that the invention is not confined to application to scanners of the rotating drum type. It can also be applied to other kinds of scanners, for example the traversing table scanner; it is of course then necessary to match the speed of traverse of the table to the peripheral speed of the loop. In addition the invention can be applied to any screen having a repeating pattern, for example to screens having dot shapes other than circular and to screens in which the dot axes are not mutually perpendicular. An enlarging scanner can be used in place of the scanner shown, and the scanner may also be of the kind suitable for color reproduction. An enlarging color scanner is described in detail in our British Patent Specification No. 1,166,091.
The separate analyzing and reproducing light source may be replaced by a single light source, used either with the exposing head of the analyzing head, and with a light guide to convey light to the other of the two heads.
It would be possible to use a disc carrying the screen elements around a marginal track, in place of the loop of screen element shown and described.
It would also be possible to make the screen element move across the aperture itself but this requires the screen loop to be located very close to the aperture and is more difficult to put into practice.
1. In image-reproduction apparatus including a lght source, an optical system for deriving a light spot from said light source, means for obtaining relative motion between a light-sensitive sheet placed in the apparatus and the light spot derived from the light source, so that in an exposure operation the sheet is exposed element by element to the light spot, and means for modulating the light spot in accordance with tonal values of successively scanned elements of an image to be reproduced on the light-sensitive sheet, the improvement comprising a light-transmitting half-tone screen element, means for moving said screen elements through the light path between the light source and the light-sensitive sheet at a point in the light path conjugate with the light spot on the light-sensitive sheet, so that an image of the moving screen element is formed by the light spot at the lightsensitive surface, each portion of said screen element passing repeatedly through the light path during said exposure operation, and means synchronizing the movement of the screen element through the light path with the movement of the light spot over the light-sensitive sheet.
2. Apparatus in accordance with claim l, in which the screen element moves across the aerial image of an aperture in the optical system.
3. Apparatus in accordance with claim 1, in which an aerial image of the half-tone screen moves across an aperture in the optical system.
4. Apparatus in accordance with claim 1, in which the light-transmitting half-tone screen element is in the form of a loop of film having an integral number of screen elements along its length and which is driven at a rate related to the speed of the said relative motion.
5. Apparatus in accordance with claim 1, further comprising a drum around which the light-sensitive sheet to be exposed is wrapped, means for rotating the drum to provide relative motion between the light spot and the said sheet to provide line scanning of the sheet, and means for Obtaining relative axial movement of the drum and the light source to provide frame scanning of the sheet.
References Cited The following references, cited by the Examiner, are of record in the patented tile of this patent or the original patent.
UNITED STATES PATENTS 2,282,337 5/1942 Mies. 3,461,229 8/1969 Oppenheimer 9645 JOHN M. HORAN, Primary Examiner U.S. C1. X.R. -36, 45