Method and apparatus for image frame length control

Abstract

Image frame length control apparatus and methods are disclosed which have particular utility in a printing or reproduction apparatus that writes one or more images on a transported photosensitive media such as a photoconducting drum or continuous-loop web. When an electronic writing system or optical exposure system is used to expose a driven media on a line-by-line basis, a variation in media thickness causes the latent image frame to be written with an inaccurate frame length. Consequently, a degraded print is produced. The latent image frame length is corrected during exposure, by adding or omitting one or more lines in the image frame. Alternatively, the exposure of an image is offet by an interval corresponding to one-half of the total length discrepancy between the uncorrected image frame length and the image frame length that would be written if the media surface speed was at a median level. The overall image length error is thereby reduced by half and distributed to the upper and lower extremities of the image frame. In another embodiment, the rotation rate of the media is increased or decreased when a respectively thin or thick portion of the media passes over the driving apparatus. The corrected image fames are written with greater fidelity to the original and, after development and transfer to a receiver, are particularly well-suited for use in forming an accurate multicolor reproduction.