US2165407A - Color correction - Google Patents

Description

v Patented lJuly 1 l, 193.9

This invention relates to photo-mechanical processes for the reproduction of a multi-colored@ original. It relates to the makingl of color printer for use in such a process andmoref'p'ar# ticularly to a color printer which is correctedior the limitations of the best contemporarily available inks.

It is an object of vthe invention Ato aA method of making a color-correctedcolor printer.. Various methods of color correction generally rca1led"masking methods are wellknownii"Itisv an object of the present inventionto providea method of color correction which" might ybe termed "autographic and whichdoesnot require the use of special masks but employs the original itself, or more exactly a color separationimagefof the original, to act as a mask.y f Y The method provided by my` invention ist one,

in which neither the negative nor z the color printer madev therefrom is maskedl in the .usual sense, but a corrected printer is'madefby. idi'rect.

exposure through an ordinary;coloriseparation.

negative. ,f It is a special object ofy my`v invention topro-` vide a photo-mechanical -processinswhichthe:

density of the blacks and-.dark grays, ize. .areas where all three color printersfhave a relatively I vide a'method of making-.suchfa novelipicture; f

Although my invention maybe practiced with various types yof equipment; .one'fformi'of appa-1 ratus 4which `is particularly suitable for practice ing oneembodimenf my -inventionis described by F. Hahn in UISJPatent 1,576,118.41v

AOther objects and'advar'itages` of my *invention will be apparentisfrom the followingvdescription when read in Lfcnnection `v'vithithe. accompanying drawing in which: f v Fig. 1a showsalwel known manner.V of making: a halftone., color ".separationf-:negative' from .a

Vmulti-colored originali? .constitutes .theffiirsti step of my q process;y lone. lemlgaodiment iofff which 3. is. shown `in the -form of a ",flow.. cha

to l. my? invention, Ainl'imakirig f a color; printer?r from this negativa ...,1. 1. 1;;1. "ff/"'7- ..Fig. 1d illustrates a corrected color--sep'aration positive y`; '1printrresulting from my'process..y 'f p* Y Fig. '2.i11ustra'tes 'a' novel picture which can' be -5 made. according to. oneembodiment of my inven-f: tion. 'My invention. is primarily concerned vlvvith'the makinglrof acorrectedicolor printer Vfromfa pri-L.y

mary. color v'separation negative which "has `been .10 made inanyfof the usual Ways. f Afpositive madey from-i. a fcolorf separatiomnegative may comprise' the .printers itself or Vvthe printer vmay b'e madetherefrom7 by anyfsuitable duplicating process. OneVH Well-knovvn method I of introducing.` color 15 correction infthe making of fa' color printer com# prises` masking certain'of'the color 1Vseparation. negatives with .low Lcontrast vpositives madey from other color separationfnegativesf Anotherfmethl.. od-l-nam'e'ly 'that for l'which the above-mentioned.: `20 Hahn apparatus `was .intendedLintroduc'es uthek colon correctionlduring thefmaking of thenega tive,I by exposing the sensitive negative material l to acolor separation image 'from-the original; 'i rst through a negative mask correspondingto 1 ai correcting'colorie.' g: a. redflter negative may befusedftofcorrect afgreerr lter negative)`= and second, directly. i i 1: i

:According tomyinvention,.a method .offinf i troducing color vcorrection is provided which does not employ either a negativemask (in making the: negative) or a positive mask (in makingy .the printer) in theusualy senser of the fwordmask. Instead, I `employthe-.original .itself as: the mask v in ,fither` following.mannen` l` Afcolorseparation -35 tives are made. 'I'he complementary colors, i. e., the colors of the printers are called blue-green, magenta, and yellow respectively. In certain branches of press-printing work these complementary colors are called blue, red and yellow. but this latter convention and its accompanying confusion will be avoided in this specication. The term "photographic is used in a broad sense so that photographic layer includes those used as photo-sensitive resists as well as the usual silver-halide emulsions.

I will now describe the particular embodiment of my invention which is illustrated in the accompanying drawing.

-As shown in Fig. 1a, a multi-colored original II, which may be either a transparency or an opaque painting or color photograph, is photographed by a process camera represented by a lens I3, in the image plane of which is placed a film or plate I5. In this particular embodiment of the invention a halftone negative is produced by positioning a halftone screen It at a suitable distance in front of the sensitive layer of the lm or plate I5. A primary color filter I2 (red, green, or blue) is mounted in front of the lens I3 in any well-known manner. Any other suitable method of making a color separation negative may be employed.

After development, this lm or plate i5 comprises a color separation negative I5'. As shown in Figs. 1b and 1c, a positive I'I is made from this color separation negative I5 by contact printing. In Fig. 1b, the negative I5' is illuminated by a color separation image of the original, by means of a set-up similar to that shown in Fig. la, but employing a color filter I6 which transmits a color which is suitable for`correcting the color printer corresponding to the color separation negative I5'. For example, it has been found desirable to correct a magenta printer in proportion to the amount of red present in the original. In making a magenta printer, a green iilter is used at I2 to make a green separation negative i5'. The exposure of the positive i'I illustrated in Fig. lb is then made through a red lter I6.

In addition to this correcting color exposure,

.. the positive Il receives a direct exposure modulated by the negative I5 only. This may be accomplished in a simple manner as shown in,Fig. 1c by placing a sheet of white material I8 in front of the original II and giving a White ash exposure.

After receiving the two exposures as shown in Figs. 1b and 1c, the photographic layer I'I is developed in any suitable and well-known manner to form a corrected color separation positive or printer I'I as shown in Fig. 1d.

As pointed out above, the steps shown as Figs. 1b and 1c may be performed in either order or simultaneously. One simple method of making these two exposures simultaneously is to use a color filter at IG which is diiusing (i. e. translucent rather than transparent) over a predetermined portion of its area. This portion may be adjustable, for example, an adjustable sector of ground glass may partially replace or be superimposed on the color lter.

The step shown in Fig. 1c may be made in any well-known manner, by contact printing, as shown, or by projection printing.I In making this exposure, the illumination on the negative is uniform, preferably white. Since the sensitive layer II and the negative I5' are arranged in printing relation it is desirable to perform both exposures without disturbing this relation. However, various arrangements may be used. For example, in

more suitable form of original to be illuminated from behind. In fact, if the original is a transparency, all three elements-the original, the negative and the sensitive layer I1 may be superimposed in relative contact and illuminated by colored light.

As shown in Flg.1a, I prefer to expose the negative I 5 through the base. By doing this, the negative image I5' can be simultaneously placed in contact with the sensitive photographic` layer Il and in register with a. color separation image of the original as shown in Fig. 1b. If the original is a transparency, the negative I5 may be exposed directly and then both the original and the negative turned over in making the step shown as Fig. `117. On the other hand, if the original is not a transparency and it is not desirable to make the exposure of the negative I5 through the base as shown in Fig. la, a reflector such as a prism may be placed in front of the lens in performing the step shown as Fig. 1b. This reflector introduces the lateral inversion necessary to permit the negative image I5 to be simultaneously in contact with the sensitive surface of Il and in register with a color separation image of the original.

My invention is applicable to either continuous tone or halftone processes. In a halftone process, I prefer to introduce the halftone pattern by making each color separation negative through a halftone screen, as shown, but the halftone pattern may be introduced at a later stage such as when the positive' Il is being exposed. If the original is a transparency and no enlargement or reduction is required, it is possible to perform my invention without the use of any camera, by using contact halftone screens and making all exposures by contact as discussed above. That is, the color separation negative is made by contact printing and the positive is made therefrom also by contact printing.

Since according to the arrangement shown in the drawing, the image on I1 is laterally inverted with respect to the original, this positive may comprise the printer itself and a correctly oriented `print will result therefrom. In photogravure work, the corrected positive I1 can, of course, be used directly. However, if desired a negative may be made directly from the corrected positive I'I and any suitable type of printer made therefrom in a well-known manner.

Furthermore, according to my invention, I am able to provide a novel picture of a multi-colored original, in which all visual density gradations (i. e. all blacks, grays, and Whites) are reduced to the same average gray tone, but in which the color variations are reproduced substantially correctly. Since all three color photography is based on the sensitivity characteristics of the human eye, this novel picture is perhaps more accurately defined as one having only color variations and substantially uniform -visual density. 'Ihis is shown in Fig. 2. In the more general case, the relative visual densities may be controlled within limits to any desired value. In a special case, the neutral densitiesi. e. whites, grays and blacks are reduced to an average gray, but the density variations of certain colors are not so reduced.

One useful application of a novel picture having only color variationsis as a test chart for the investigation of color'blindness. As is known, color blindness test charts comprises areas which have practically identical visual densities but which differ slightly in hue. Indeed, it may bev desirable in some cases to introduce a slight known difference in visual density as well as the difference in hue.

According to a special embodiment of my invention, a novel color picture of the above'type 'having uniform visual density, is made up from shown in Fig. 1b, wherein the color of the filter I6 is practically complementary to that of the primary color filter l2. No flash or exposure corresponding to that shown in Fig. 1c is given in this case. I have`found that useful results are obtained when the' color of the lter I6 is any spectral range not transmitted by the primary color filter i2.

By choosing certain color filters and/or giving a small ash exposure such as indicated in Fig. 1c a certain predetermined amount of density variation of the colors and/or the neutral tones, may be included as well as the color variation. Obviously, this permits many various types of color blindness test charts to be made.

Furthermore, this effectnamely the reduction of blacks to a\ grayprovides another important advantage to the rst described embodiment of my invention. Those skilled in this art are aware that a reduction o f the density of blacks and dark grays is desirable in many cases, and this reduction is inherently provided by a. process which employs my invention to obtain color correction. At the same time, by choosing a suitable total exposure the highlights or white areas of the original are made to appear as zero printing density on each of the printers, in the usual way.

Having thus described one embodiment of my invention in detail andseveral ways in which it may be employed, I wish to point out'that it is not limited to' the speciiic arrangements shown but is of the scope of the appended claims.

What I claim and wish to protect by Letters Patent of the United States is:

1. In a'photo-mechanical process for the reproduction of a multi-colored original, the method of making a corrected color printer which comprises making a color separation negative and making a positive therefrom by exposing a sensitive photographic layer to two separate exposures, one in printing relation to the negative uniformly illuminated and the other in printing relation to the Anegative illuminated by a color production of a multi-colored original,

separation positive image of the original, said last mentioned color being a correcting color and different from that corresponding to the negative.

2. In a photo-mechanical process for the reproduction of a multi-colored original, the method of making a corrected color printer which comprises making a color separationnegative and making a positive therefrom by exposing a sensitive photographic layer in printing relation to the negative illuminated by uniform illumination and by a color separation positive image of the original, said last mentioned color being a correcting color and different from that corresponding to the negative. i

3. In a photo-mechanical process'for the reproduction of a multi-colored original, the method of making a corrected color printer which comprises making a color separation negative and making a positive therefrom by positioning a sensitive photographic layer in printing relation with the negative, illuminating the negative with uniformly distributed illumination and with illumination comprising a color separaten positive image of the original, the/amounts of said illuminations being in predetermined relation and said last mentioned color being a correcting color and ldifferent from that corresponding to the negative, and developing the layer.

4. In a photo-mechanical process for the reproduction of a multicolored original, the method of making a corrected color printer which comprises making a color separation negative of the original and making a positive therefrom by positioning a sensitive photographic layer in printing relation with the negative and illuminating the negative with light whose distribution over the negative is made up of predetermined relative amounts of a uniform portion and the portion corresponding to a color separation positive image of the original, the last mentioned color being a correcting color an digerent from that corresponding to the separation negative.

5. In a photo-mechanical process for the rethe method of making a corrected color printer which comprises making a color separation negative, placing a sensitive photographic layer substantially in contact with said negative, and exposing the layer through the negative to a uniform beam of light and to a color separation positive image yof the original, said last mentioned color being a correcting color and different from that corresponding to the negative.

6. In a photo-mechanical process for. the re-v production of a multi-colored original,` the method of making a corrected color printer which comprises making a color separation negative by .through the negative to a uniform beamof light and to a color separation positive image ofthe original, said last mentioned color being a correcting color and different from that corresponding to the negative. Y

ALEXANDER MURRAY.

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