Apparatus for reproducing an image of variable tone density

Claim 1

Apparatus for reproducing an image on a photo-sensitive medium, the image being defined by binary data representative of different tone levels of pixels defined as elementary points constituting the image, the apparatus comprising: a source of light providing a beam for printing the pixels; means for moving the beam over the medium in an X direction to scan the pixels of an image line; means for moving the medium in a Y direction substantially perpendicular to the X direction to scan the pixels of different image lines; a screen located close to the said medium on the path of the beam, said screen being opaque to said beam and including n rows of diaphragms that are transparent to the beam and are arranged in N columns extending substantially parallel to the Y direction, for an image having N pixels per image line, with each one of the n diaphragms of any given column being of a different size from the others in said column to define different half tone dot sizes, the diaphragms of each row being disposed at the spacing of pixels along an image line, and the rows of diaphragms being disposed at a spacing of m times the spacing of the image lines, where m is an integer equal to unity or a small number; conversion means for converting said data into binary words, each representative of the rank r of the diaphragm to be used to define the tone of each pixel, where the rank r of a diaphragm is its position in the Y direction along its column counting from the first row of diaphragms encountered by the moving medium, and where 1<r<n; processing means for processing said binary words delivered by said conversion means to apply shifts corresponding to the time taken by the medium to move m(r-1) image lines relative to the screen; and means for deflecting said beam under the control of said processing means to direct the said beam onto those diaphragms of each column whose respective ranks are indicated by said words as delivered by said processing means.