Abstract

Whenever the subject being reproduced has a regular pattern, the conventional type of halftone screen, because of its regular structure, may produce an objectionable beat pattern, called moiré. In multicolor printing, the regular patterns of the several structured images produce complex beat patterns whose effects are minimized, but not eliminated, through proper choice of screen angles. This paper describes a procedure in which a continuous-tone image is scanned; and the resulting photo-tube signal is used to actuate a light valve in such a manner as to produce a structured image containing dots that are, in effect, distributed at random. It is shown that this procedure avoids both the distortions in tone reproduction and the moiré patterns associated with the conventional use of halftone screens.

© 1948 Optical Society of America

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References

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  1. Extension of paper presented at the October, 1946meeting of the O.S.A.
  2. H. E. Ives, “Tone reproduction in the ‘halftone, process,” J. Opt. Soc. Am. and Rev. Sci. Inst. 13, 537 (1926).
    [Crossref]

1926 (1)

H. E. Ives, “Tone reproduction in the ‘halftone, process,” J. Opt. Soc. Am. and Rev. Sci. Inst. 13, 537 (1926).
[Crossref]

Ives, H. E.

H. E. Ives, “Tone reproduction in the ‘halftone, process,” J. Opt. Soc. Am. and Rev. Sci. Inst. 13, 537 (1926).
[Crossref]

J. Opt. Soc. Am. and Rev. Sci. Inst. (1)

H. E. Ives, “Tone reproduction in the ‘halftone, process,” J. Opt. Soc. Am. and Rev. Sci. Inst. 13, 537 (1926).
[Crossref]

Other (1)

Extension of paper presented at the October, 1946meeting of the O.S.A.

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Figures (6)

Fig. 1
Fig. 1

Conventional type of structured image for dot ratios of 0.1, 0.5, and 0.9.

Fig. 2
Fig. 2

Schematic representation of the operation of a machine adapted for flat scanning. A continuous-tone positive A is attached to a circular platen which is clamped to the carriage B. An hydraulic piston imparts a reciprocating motion to this carriage as shown by the double-ended arrow. At the end of each stroke a lead screw and ratchet mechanism causes the intermediate carriage F to advance a short distance in the direction of the single-ended arrow. Light that passes through this transparency during the scanning operation falls on the cathode of a photo-tube, and the resulting signal controls the dot-producing circuit in the relay rack C. The output of the relay rack actuates a light valve in the recording head D, producing an intermittent exposure that results in recording a structured image on the unexposed photographic plate E. When used for multicolor reproduction, the circular platens are rotated through an appropriate angle after each recording and are held in position by means of the detents shown.

Fig. 3
Fig. 3

In producing the type of structured image illustrated in Fig. 4, the slit is illuminated by a beam of light reflected from the galvanometer mirror when generating a dot (Fig. 3a) and receives no light when generating a space (Fig. 3b).

Fig. 4
Fig. 4

Type of structured image produced by the method illustrated in Fig. 3.

Fig. 5
Fig. 5

An extension of the method of Fig. 3 used in producing the type of structured image illustrated in Fig. 6. As before, the image of the aperture is in position a when generating a dot and in position b when generating a space, the width of a dot being determined by the location of the image relative to the slit in position a. A second coil of the galvanometer is supplied with a current proportional to the photo-tube signal and this moves the image of the aperture to the left as the photo-tube signal increases. The shape of the aperture is such that dots of increasing width are produced until the width of the dot equals that of the scanning line. Further increase in the photo-tube signal results in the production of spaces of decreasing width. In this illustration, positions a and b represent the formation of an image with a dot ratio of 0.1, and positions c and d represent the formation of an image of dot ratio 0.9.

Fig. 6
Fig. 6

Type of structured image produced by the method illustrated in Fig. 5. This diagram does not illustrate the change in frequency that is automatically introduced after each line has been scanned.

Equations (6)

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t c = k / i ,
t d = k / ( I - i ) .
r = i / I .
f = 1 t c + t d = r ( 1 - r ) k ,
i = a i + I ,
a = I - 2 I .