GB707762A - Calculating devices - Google Patents

Abstract

707,762. Calculating-apparatus. McCORQUODALE & CO., Ltd. July 31, 1952 [Aug. 16, 1951], No. 19378/51. Class 106 (1) A logarithmic calculater for determining exposures in the production of colour separation records for photo-lithographic and photo-engraving processes, comprises fixed scales A and L of highlight and shadow densities, presettable logarithmic exposure scales B, C, D, H, I, and J, presettable logarithmic density-range scales E, F and G, and cursors P and Q covering these scales. The apparatus is intended primarily for determining the exposures in the reversal process of Specification 681,206 which, briefly, consists in converting colour-separation negatives into coloured images and reversal exposing these images to lightabsorbed by their colour. The reversal exposures are preferably effected, at least partially, to light from the original subject to enable correction to be made for colour deficiences in the inks used in printing from the records. In carrying out the process with the aid of the apparatus, the exposures for making the three colour separation negatives are found by setting the cursor line X against the measured value of highlight density of the original on the scale A and reading the scales B, C and D, for the red, green and blue respectively, against the cursor line. Corresponding flash exposures are shown by the cursor Q on an extension of scale C when set against the measured density range of the original on the scales E, F and G in turn. Negative exposures for the black printer are found by setting the cursor line X to the shadow density scale L and reading the appropriate scales B, C and D against a subsidiary cursor line Z, a further cursor line Y similarly marking the auxiliary non-screen exposures. The flash exposure for the black printer is shown by cursor Q on scale C when set to the appropriate density range scale E, F or G according to the colour of the filter used for the original exposure. Reversal exposures for the separation negatives through the red and green filters are read from the scales H and I with the cursor line X again set to scale A, and the second flash exposures similarly with the cursor line X set to " 0 " on the scale A. The black printer second exposure is read from the scale J with the cursor line X also set to " 0 " in scale A. The highlight mask exposure is read from the scale K with the cursor line X set to the highlight density on scale A. The presettable scales are adjusted in position after making preliminary tests on the batch of plates to be used ; the scales are then secured by means of locking nuts T at the rear of the base R of the apparatus. To provide for variation in the camera aperture/extension ratio, the cursor line X is supplemented by alternative lines over the scales H, I and J marked with a scale M of multiplicating factors dependent on extension. The apparatus can also be used for calculating exposures for the process described in Specification 681,212 as well as other processes.