Patent GB-883670-A (GB)
Title: Improvements in or relating to colour photography
Inventor: NEALE DENIS MANKTELOW
IPC: G03B27/73
Filed: 1959-05-01
Abstract: 883,670. Photo-electric control of colour photographic printing. ILFORD Ltd. April 11, 1960 [May 1, 1959], No. 15004/59. Class 40(3). [Also in Group XX] A method of making a photographic colour print from a colour negative comprises forming an image of different parts of the negative each on a different screen, masking to a greater extent those screens on which falls the image of part of the negative having an integrated transmittance greater than those parts of the negative which are imaged on other screens, combining the light fluxes from the unmasked areas of the separate screens and controlling the relative times or intensities of exposure of the print material to the red, green and blue components of the negative image in inverse relation to the intensities of the red, green and blue components respectively of the total light flux received from the unmasked areas of said screens. The invention has the advantage over the whole negative integration method in that the colour balance is not seriously effected by low density saturated colour areas and a more satisfactory print results. In Fig. 1, light from a source 1 passes through condensing lens 2, 3, and a colour negative 4, to form an image of the light source in the objective lens 5. The objective lens 5 projects an image of the colour negative 4 on to the colour print material 6. An inclined partial reflector 7 diverts part of the light transmitted by the negative to form an image of the light source in the lens 8. The lens 8 forms an image (preferably a reduced image) of the negative 4 on to two translucent screens 9, 10. Photocells 11, 12 receive light emerging from the edges of the translucent screens and differentially operate a moving coil instrument to swing a mask 13 to a position tending to equalize the light fluxes falling on the two screens 9, 10. Light transmitted by the screens 9, 10 is integrated by known means, e.g. a perspex rod 14, before reaching photocells 15, 16, 17 used to assess the colour balance and control exposure. If for example a low density part of the negative is imaged on screen 10, the moving coil instrument is energized to swing the mask 13 so that it obscures more of the screen 10 and allows more of the negative image to fall on screen 9. Equilibrium is reached when the total flux received on screen 9 is approximately equal to that received on screen 10. The photocells 15, 16, 17, instead of assessing the colour balance largely from the low density area of the negative, take this area and the remaining area of the negative equally into account. In a second arrangement, Figs. 2 and 3 (not shown), the two screens 9, 10 are replaced by four quarter screens. Between all the screens are opaque sections arranged so that light incident on one screen cannot reach another. Four photocells are disposed adjacent the four edges of the screens. The pairs of photocells on opposite sides of the assembly of screens operate together differentially to control moving coil instruments to move forkshape masks to assume positions allowing substantially equal light fluxes to fall on each of the quarter screens. The assessment of colour balance affected by the photocells 15, 16, 17 then corresponds to a first approximation to an impartial integration of individual measurements of colour balance in each quarter of the negative. According to a modification, the inner margins of the masks are reflective and inclined so that incident light is reflected towards the adjacent photocell. The light flux reaching each photocell is in this manner approximately equal to the additional light which would have reached the same photocell through the translucent screen had the reflective edge of the mask been removed. Thus the mask moves as if the reflective edges had been removed and, if a low density area is present to one side of a negative, the mask will move to cause the image of most of the low density area to fall on the non-reflecting part of the mask. Since the light fluxes reaching the photocells must be approximately equal, part of the light density area will fall one one reflected edge of the mask. Such light does not reach the translucent screens, however, and thus in this modification the light fluxes passing through the screens are not, in general, equal. The assessment of colour balance effected by photocells 15, 16, 17 is thus biased in favour of that part of the negative having the lowest integrated transmittance, since the photocells 15, 16, 17 will receive a greater light flux from this quarter of the negative.