Patent GB-1325577-A (GB)
Title: Method and system for making corrected halftones
IPC: G06G7/75
Filed: 1970-11-18
Abstract: 1325577 Half-tone image production E I DU PONT DE NEMOURS & CO 18 Nov 1970 [19 Nov 1969] 54922/70 Heading H4F [Also in Division G2] Relates to a system for producing a tonecorrected half-tone image on a photographic film from a continuous tone separation transparency in which the camera exposure conditions for main, flash, bump and flare exposure are derived by simulating a tone-corrected halftone display on a picture tube. In a first embodiment, Fig. 1, a transmission signal T N , which is a voltage proportional to the light transmission of a continuous tone separation transparency, is produced by a flying spot scanner and amplified to produce a signal proportional to the effect of the main exposure. Preferably the signal is attenuated, 10, 12, to produce a signal proportional to the main exposure and summed, 14, with an amplified (attenuated, 16, 18) flash signal K R , and logarithmically amplified, 20, to obtain a signal representative of the effects of the main and flash exposures on a particular photographic film. In detail, the transmission signal T N  is first processed by main exposure calibration voltage divider 10 to attenuate the signal to simulate the effect of main exposure upon a particular half-tone screen and photographic film combination. The main exposure calibration setting is determined as a result of an empirical photographic standardization test conducted previously with a particular screen film combination. The transmission signal is further attenuated in main exposure adjustment voltage divider 12 to determine the effect of main exposure in the halftone screen process. The main exposure adjustment voltage divider 12 is graduated into exposure settings to be used with a halftone camera. The term "exposure" refers to the latent image effect on a photographic film and is dependent upon lens aperture and period of time the shutter is open. Such settings are based on prior empirical photographic standardization tests. The twice attenuated transmission signal is proportional to the effect of the main exposure on the photographic film. The twice attenuated transmission signal is summed in main-flash operational summation amplifier 14 with an amplified flash signal which is proportional to the effect of the flash exposure on the photographic film. The amplified flash signal is obtained by producing a signal proportional to the intensity of the flash exposure K R , which is a fixed voltage proportional to the intensity of the flash exposure light source. The flash intensity signal is attenuated in two steps by means of calibration voltage divider 16 and flash exposure adjustment voltage divider 18 which perform analogous functions to the main exposure calibration voltage divider and main exposure adjustment voltage divider, respectively. The signal emitted from the main-flash log amplifier 20 is processed in adjustable screenfilm non-linear amplifier 22 which after being adjusted to the particular halftone screen produces a per cent dot area signal proportional to the per cent dot area resulting from the main and flash exposures on the photographic film. In Fig. 2 a bump exposure simulation module is added to the system of Fig. 1. This module includes a bump exposure log amplifier 24 which processes the signal from the main exposure adjustment voltage divider 12 so that it is in a form usable in dot growth non-linear amplifier 26. The dot growth signal is attenuated in two steps, first by bump exposure calibration voltage divider 28 to simulate the effect of the bump exposure on the particular photographic film and then by bump exposure adjustment voltage divider 30 to simulate the bump exposure for bump exposure conditions in terms of per cent of main exposure. A per cent dot area signal of the main, flash and bump exposures is obtained from a dot summation amplifier 32. In Fig. 3 the bump exposure module comprises a bump exposure proportional offset 34 which non-linearly amplifies the output of mainflash log amplifier 20 and establishes a threshold level for the incoming signal to reach before being further processed. A constant voltage K 0  determined by measurements with prior photographic standardization tests is used to establish the threshold level. The resulting signal proportional to the approximate increase in per cent dot area is attenuated and summed in the same manner as the increase in dot area signal in Fig. 2 to obtain au approximate per cent dot area for main, flash and bump exposures. In Fig. 4 a flare exposure simulation module for flare exposure light which is inherently produced in the camera is included. The flare exposure, since it is a non-image exposure, provides the same function as the flash exposure in the half-tone process. The transmission signal T N  is integrated, 36, and the resulting averaged transmission signal is attenuated in camera calibration voltage divider 38 and camera adjustment voltage divider 40 and summed with the flash signal in summation amplifier 42. In colour work the colour separation transparencies and black printer transparency are separately scanned and amplified by electronic simulation systems, the resulting signals being displayed on one picture tube.