Patent GB-544307-A (GB)
Title: Improvements in the production of fluoroactivating light, especially for photographic purposes
IPC: G03F3/04
Filed: 1940-06-19
Abstract: 544,307. Producing fluoroactivating light; colour photographic and photomechanical processes. KODAK, Ltd. (Eastman Kodak, Co.). June 19, 1940, No. 10598. [Class 98 (ii)] [Also in Group XL] The invention relates to the production of a predetermined mixture of, or balance between, two radiations A and B of different spectral composition, A being in the visible spectrum and B, which may or may not be in the visible spectrum, causing some substance to fluoresce in some band of the visible spectrum to which A belongs. Examples of radiations to be thus mixed or balanced are white light (e.g. from a tungsten filament at 2,700‹ C.) and ultra-violet light or radiation of shorter wave-length such as X-rays. According to the invention the radiations A and B are allowed to fall separately or in combination upon a pair of surfaces one of which is more fluorescent but less light-reflecting than the other, and the emergent radiations from the two surfaces are compared in the common spectral band, the relative proportions of the original radiations A and B being adjusted until the emergent radiations from the two surfaces in this band have a prescribed relation to one another. Since the emergent radiations from one surface contain more fluorescent and less reflected visible radiation than those from the other, the emergent radiations from the two surfaces will only have the prescribed relation when the visible radiation A and the radiation B causing the fluorescence are in the predetermined balance or mixture. The " prescribed relation " may be a visual match in brightness of the surfaces, a photographic match producing equal densities of their images on a negative, a prescribed relation between the densities of the two photographic images, or equality or other relation between the brightnesses of the illuminated patches as measured by photoelectric cells. Generally the two radiations A and B fall simultaneously or as a mixture on the two surfaces, but they may fall separately, in which case successive photographic exposures are made on the same sensitive surface, or visual matching may be performed by causing them to fall in sufficiently rapid alternation on the surfaces for persistance of vision to synthesize their effects. The invention is described in connection with the production of fluoroactivating light of a predetermined " quality," i.e. having a predetermined ratio between visible and fluoroactivating radiations comprised in it, for use in a method described in U.S.A. Specifications 2,008,290 and 2,108,503 for producing colour separation negatives from a coloured original drawing. In this method, fluorescent materials are added to those colours used in painting the original which do not normally reproduce sufficiently brightly in photographic or photomechanical reproductions made by the three-colour process, the fluorescent light from these materials supplementing the reflected light from such colours during the exposure of the separation negatives and so avoiding the necessity for subsequent colour correction. In particular, the green colour contains a material which fluoresces green. The exposures are generally made by visible and fluoractivating light simultaneously, and in order that the light from the fluorescent material shall provide the required correction effect it is necessary that the light should be of predetermined " quality ", as above defined. For the purpose of obtaining such light, two control patches are provided, the first being fluorescent but less light-reflecting than the second, which is substantially non-fluorescent, the patches being such that when (and only when) they are illuminated by light of the prescribed quality their brightnesses are equal when measured visually or photographically, or both, in a band of the spectrum common to the fluorescent and the reflected light. If the two control patches are equally reflective in some other band, or in the whole of the visible spectrum, then the light in such band or the whole of the spectrum respectively may be included in making the comparison. Desirably, two pairs of patches are used, one pair giving a visual match and the other a photographic match. If the spectral compositions of the fluoro-activating and visual components of the light employed are specified, then the control patches need not be of a colour related to that for which the light is used, since the intensity in any band will be proportional to that in the band in which comparison is made. If the spectral composition is not specified, then comparison is made in the band which is to be used; for example, for light to be used in making a red separation record, the patches need match only in the primary red portion of the spectrum ; all three colour separation records are made with the aid of fluorescent light, the patches must match over the whole spectrum. Generally, only the blue and green separation negatives are made in this way. Comparison of the brightness of the patches is generally made through filters transmitting only those parts of the spectrum for which the light is to be tested. One and the same patch may include ingredients causing fluorescence in more than one region, so that by using two different filters a single pair of patches may be used for comparison of the fluoroactivating radiation with more than one visible radiation. Alternatively, one fluorescent patch may match a second through one filter and a third through another filter, or it may match a second patch visually and a third photographically, if necessary through suitable filters. Since in photomechanical work the photographic effect of the light is of primary importance, photographic comparison of the patches is important. The Figure shows a painting 1 on a white mount 6 on which is a fluorescent control patch 9, the mount itself, which is substantially non-fluorescent, providing the companion patch or area. It is assumed that the paints used in creating the painting contain a substance or substances which fluoresce blue and green when illuminated by ultra-violet light in order to compensate for undue absorption of light of these colours by the inks or dyes used in the reproduction. It is therefore necessary that the light employed in making the blue and green separation negatives should have the correct " quality ", that is, a predetermined ratio of ultraviolet, blue, and green light. The patch 9 is made with a paint which strongly absorbs blue and green light and fluoresces strongly in the same regions when illuminated with ultra-violet light, and is such that only when illuminated by light of the prescribed quality, and observed if necessary through a blue or green filter, does it match the white area adjacent it; that is the reflection of blue and green light plus the fluorescent light from the patch 9 is equal to the reflection of the blue and green light from the white area. A number of examples of the composition and manner of use of the patch 9 are given. Example I. The patch matches the surrounding white area when photographed through primary green and blue colour filters, and also visually through stated green and blue combinations of filters. The paint used for it contains Fanchon yellow, process red, and naphthacene in a suitable vehicle such as an aqueous solution of gum arabic containing also phenol-glycerine, and Darvan or other dispersing agent, as described in Specification 542,006, [Group XL]. Example II. Chrysene, re-crystallized without naphthacene as described in Specification 542,006, is added to the paint used in the preceding example. This increases the violet fluorescence and visual comparison may be made with a less dense blue filter than in that example, but the patch appears too light when measured photographically through a primary blue filter. Example III. The amount of chrysene in the paint made as in Example II is increased to a point at which the patches match visually without filters. Example IV. An additional patch 10 containing no fluorescent ingredient is placed adjacent the patch 9 for visual comparison directly or through a weak filter, photographic comparison being made between the patch 9 and the background 6. One fluorescent patch may also be made to match a second patch visually through a suitable blue filter and photographically through a primary blue filter, and a third patch visually through a green and photographically through a primary green filter. A contrast control patch 8 is shown, which is used for controlling the colour balance in the separation negatives ; the photographic densities of which through the primary colour filters are equal to the lowest densities of the corresponding subtractive colours giving a solid tone in a correctly colour-balanced reproduction. The patch is very dark bluish-green in colour, and normally reproduces as a dark brown, which is the nearest approximation to black obtainable. Correct colour balance is obtained when it reproduces as a solid tone in all three printing-plates. A suitable paint consists of stated proportions of carbon black, burnt umber, Milori blue, Fanchon yellow, and Titanox. The necessity for control of the colour balance by the usual colour step tablets 2-5 is avoided by the process.