Patent GB-841125-A (GB)
Title: Electronic previewer for negative color film
IPC: G03B27/73
Filed: 1958-03-10
Abstract: 841,125. Television. HAZELTINE CORPORATION. March 10, 1958 [May 28, 1957], No. 7620/58. Class 40(3). In a photographic previewer the coloured positive image which would be obtained from a coloured negative image by a photo-chemical process is produced by means which simulate electrically the photo-chemical process and which display the positive image on a three-colour cathode ray tube. The previewer comprises three channels, one being allocated to each primary colour. Optimum picture quality is obtained by adjustment of the previewer controls and the position of these controls provides printing information for use in any subsequent negativepositive photo-chemical conversion. Signal production. The coloured negative is scanned by the flying spot scanner 30, Fig. 5, and the emerging light beam is split by the dichroic mirrors 32 and 33 into its three primary colour components. The red component is passed via the lens 34R and filter 35R to the photo-multiplier tube 36R, similarly the green and blue components are passed to their respective photo-multiplier tubes 36G and 36B. The signal produced by each tube is proportional to the associated colour component of the printing light E transmitted through the negative. Electrical circuits. In the photo-chemical process the relationship between E and the resultant positive film density D for each colour component is D=K +B log E, where B is the transfer gradient of the film. This process is simulated in the non- linear amplifiers 40 R, G and B and the amplifiers 37 R, G and B. The non-linear amplifiers each comprise three sections (Fig. 6), logarithmic amplifiers 400 R, G and B which produce output signals which are a function of the log of the printing light E, linear amplifiers 401 R, G and B which largely determine the slope of the combined characteristic and the non-linear amplifiers 402 R, G and B which provide the toe to the characteristic. Each combined characteristic is of the form Vout = P log Vin, where P is a constant gain factor, resulting in signals which are proportional to the density variations which would be produced in the positive film. The spectral absorption characteristics of the dyes used in the photo-chemical production of positive images overlap each other and thus absorb light in the adjacent colour channels. This action is simulated by the cross-over network 41 where a small fraction of each density representative signal is added to the signals in the two other channels. The density/light transmission characteristic of the positive film is of inverse exponential form. This characteristic is simulated by the non-linear amplifiers 45 R, G and B operating in conjunction with the non-linear characteristic of each section of the C.R.T. 21. The inverse relationship is achieved by applying positivegoing signals to the tube cathodes. The C.R.T. is of the 3 gun shadow mask type and it is operated in synchronism with the flying spot scanner, the deflection voltages are obtained from the circuit 80. Where the composition of the light comprising the flying spot scanner source is such that its spectral composition peaks at a particular colour, e.g. green, the signal/noise ratio in the red and blue channels is reduced with respect to that in the green channel: This can be compensated for by the provision of a taking sensitivity corrector, Fig. 14 (not shown). The spectral responses of the red and blue channels are shifted to overlap the green region, thus increasing the energy input into these channels, correction is applied by a signal combining circuit which by a signal subtracting process produces in each channel the correct colour representative signal. Operation. The intensity of each colour component of the resulting image can be varied by the potentiometers 38 R, G and B. These are adjusted to produce an optimum quality picture and their final position is indicative of the exposure necessary to produce a positive image of similar quality by means of the photo-chemical process.