Color Photography - Dufaycolor
( Originally Published 1938 )
THE Dufaycolor process falls into the general classification of an additive process, inasmuch as it utilizes the combination of the three additive primary colors, red, green and blue, in a geometric pattern or mosaic of regular design known as the screen or reseau. There is an advantage in a regular geometrical pattern, as it eliminates the grainy appearance due to "clumping'' of the color elements, which is characteristic of the irregular screen processes. The means by which this geometric pattern is applied to the film base in such microscopical units is perhaps the outstanding achievement in the perfection of the process.
The idea of applying a series of colored lines or squares to the film base, originally suggested by Vidal in 1895, was used by Dufay in the further development of the process under discussion, wherein a contiguous series of blue and green squares (or of any two of the primary colors) were placed alternately between lines of red (or the third primary color). Theoretically it would not seem to be important what order was used in the application of the three primary colors. In practice, however, it was found that because of the high visual contrast of the blue line, the screen so constructed was much more visible, when magnified to the extent necessary in motion picture work or enlargement in commercial applications, than a different arrangement. At present the screen is made with blue and green squares and a red line, which has the effect of reducing its visibility on projection or enlargement.
It is obvious that the smaller the area of each individual unit, the more perfect will be the blending of the color units by the eye, even on excessive enlargement, and the less the effect on image definition. It was formerly believed that fifteen lines to the milli-meter was the limit of practical mechanical production, but in 1935 a screen having twenty-three lines to the millimeter (i.e., approximately a million and a third color elements to the square inch) was very satisfactorily produced.
A film base of suitable thickness (acetate base has been used exclusively, but similar results are possible on nitrate) is first coated with a thin layer of collodion containing a dye, let us say blue, adjusted to the spectral hue of the blue primary. After drying, the film is passed through a highly specialized type of rotary printing machine consisting of a steel roller milled with microscopically fine lines with spaces of similar width between, engraved on the surface. The machine embodies an elaborate ink distribution device for the roller; and underneath, in contact with it, a soft rubber-covered roller capable of minute vernier con-centric adjustment for controlling pressure. The ink used on the press is a special kind, forming a moiSture resistant line printed upon the basic color.
Although the ink used in applying the resist is much thinner than would ordinarily be used for typo-graphical purposes, under suitable conditions it assumes a partially dry state, producing lines which have substantially sharp parallel edges and are free from blur or creep ; and when subsequently passed through a properly selected bleaching bath, produces perfectly clear lines between the ink-protected lines.
In the same machine, which has been especially de-signed for the process, the film is then passed into a bath containing dye of such concentration as will give the primary green on the intermediate bleached white lines. Allowing time and space for suitable washing to remove the excess green dye, the film then passes into a solution and a mechanical scrubbing action removes the protective ink.
If at this stage the film is examined under a microscope, it is found to be covered with a fine grid of alternating blue and green lines, having the same width and with perfectly contiguous but not overlapping edges, and each line representing a perfect primary filter in the two colors named.
After drying, the film is again passed through a similar rotary printing machine, but this time the lines on the printing rollers are at an angle of about ninety degrees to the original lines. The width of the lines applied in this operation is not the same as that of the lines applied in the first printing, but is so con-trolled that the imprinted area alongside two contiguous squares, so formed, is equal to the area of each of the squares. The film is then bleached in a manner similar to that already described, the bleached line is dyed the third primary color (red), and the resisting ink is then removed in the same manner as before. The film is then dried and wound up.
The next question refers to the emulsion to be applied to this screen. There are, of course, many applications for color film, and the emulsion characteristics are correspondingly numerous.
It is interesting to note that in the very early stages of color development the theorists visualized an emulsion that was truly panchromatic and had a high speed and very fine grain. As no such emulsion existed at that time they approached the emulsion makers, much as Macbeth sought the Witches of Endor, and suggested some diabolical brews that too often resulted in nothing more than toil and trouble, and never achieved results on a practical basis. For that reason it may be said that the development of color photography has had to mark time until the art of emulsion making could catch up with its theoretical requirements, and it is believed that that time has now arrived.
It is of interest to review some of the problems of the photographic chemists. The art of emulsion making formerly depended upon controlling the balance of silver haloid and gelatin characteristics, together with delicate manipulation of heating, digestion, washing, ripening, etc., with more or less crude equipment. Today those factors have been so well correlated as to establish a science ; emulsion-making equipment has become practically standardized, with automatic mechanical controls that assure an accuracy permitting duplication of results within remarkably narrow limits.
Considering also the great strides that have been made within the past few years by all the large photo-graphic manufacturerS, both in this country and abroad, in controlling the size of grain of superspeed emulsions without increasing the fog or impairing the keeping quality; and further, in developing new sensitizers by means of which emulsion can be made selective to any portion of the spectrum, visible, infra-red, and ultra-violet, it can be understood that problems in the reproduction of color that seemed in-surmountable a few years ago have now been solved by the progress that has been made in the black and white field of the art. The application of these advances to color photography now realizes satisfactory results, where before the same efforts met only with failure.
So now, for the requirements of this process, emulsions may be selected practically according to specification, provided the basic characteristics of the process are known. These factors are well known, and are the same as those now applied in black and white technique. They may be briefly mentioned as speed, color-sensitivity, grain size, gradation, and gamma. With slight modification it may be said that a good panchromatic emulsion, rich in silver as compared with its gelatin content, with a color-sensitivity that will pro-duce, as nearly as possible, the same density with all three of the primary colors, with a fine grain, and high speed will serve admirably for the process. Such an emulsion is now being used.
A system or process of color photography which utilizes a film screen must make a selection of dyes which will divide the entire spectrum into the three primary colors, red, green and blue. To give truly scientific reproduction, these dyes would have to be so chosen that they transmitted only within the limit of their own spectral region and were of sufficient saturation to eliminate the passage of any white light. These are what might be called narrow cut dyes. However, the use of such dyes in a degree of saturation to meet the above specifications would produce a resultant screen or reseau whose light transmission would be too low to make it practical for use in the existing camera equipment and under the ordinary light conditions in which the various types of photographs must be made, without a prohibitive increase of exposure.
It has been found that the use of dyes having a much lower degree of saturation and a wider spectral band of transmission will increase the over-all light transmission of the screen and still give a satisfactory color result in the transparency. In other words, this means that by sacrificing some depth or intensity of color it is possible to increase the speed of the result-ant film by a very large percentage. However, the handicap which is encountered in accepting this compromise becomes evident when reproduction either in the form of duplicate film or three-color separations for photo-mechanical work are to be made, because the overlapping portions of the transmission bands of the three colors tend to dilute and degrade the resulting reproduction.
It is therefore necessary when making reproductions to use special so-called narrow-cut filters. In the case of separation negatives for photo-mechanical reproduction, two types are available, the "P" series for optical work and the "S" series for contact work. Duplicate transparencies are best made by the Dufaycolor company, a special filter technique being used to ensure the best color reproduction. These filters will transmit only that portion of the color which is within the true band of the primary color and eliminate the over-lapping portions.
For this purpose the special narrow-cut separation filters referred to above have been developed; so that for the use of Dufaycolor transparencies in press, illustrations, or advertising work, separation negatives can now be made that will give true color balance in the three black and white panchromatic negatives so made, and from these the proper halftone plates can be produced to give a duplication in the finished prints having a remarkable fidelity to the visual impression of the transparency as viewed by transmitted light.
The development of a number of methods for making color photographic prints from transparencies is progressing very rapidly, and several have already been introduced on the market. The use of Autotype carbon tissue or so-called Autotype Trichrome Carbro has long been known, and Dufaycolor film fits admirably into this process. Excellent prints can also be obtained from Dufaycolor separations by the Eastman Wash-Off Relief and by the Chromatone processes.
At the present time Dufaycolor film is available in 35 mm width for miniature cameras, 50-foot and 100foot rolls for 16 mm cine cameras, in standard cut sizes from 6/ by 9 cm to 8" by 10", and in the most popular roll film sizes.
The speed is about one-half that of portrait pan-chromatic film and one-fourth that of supersensitive panchromatic film. The Weston meter factor is about eight and the Scheirer number is 18.
No filter is necessary for the roll film in daylight, but a filter is available for cut film sizes. Filters are necessary for all types of artificial lighting.
MANIPULATION.—In handling, as far as loading, developing, reversing, etc., is concerned, Dufaycolor follows the general procedure with Agfacolor Ultra Plates and Lumiere Filmcolor. The film is loaded face down in the holder so that the exposure is made through the filter lines on the film base.
Although the film can be handled with the regular Series No. 3 Wratten green safelight, it is best to develop by time and temperature for three minutes at 65° F. (i8° C.) with the formula given below. If the green safelight is used, do not expose the film directly to it.
Development by desensitization may be employed and the Dufaycolor Company can supply full information regarding this.
After the development is completed, wash for a minute in running water and then bleach in a bath of
Potassium permanganate 48 gr. 3 g
Agitate frequently, bleaching until the image is clearly visible, which will take four minutes, after which you must wash again for two minutes. You are then ready to clear the film for two minutes in a bath of
Sodium bisulphite (or potassium metabisulphite) 400 gr. 25 g Water 35 oz. 1000 ccm
Again wash for three minutes to remove clearing solutions and proceed to the second development, which will bring out the natural colors.
After clearing and washing as mentioned above, expose the film to a 100-watt bulb, about twelve inches away, for one to one and a half minutes. Over-exposure here will do no harm. This exposure to the white light acts on the remaining silver salts so they will be completely reduced in the second development.
Any regular M.Q. developer will do for the second development. The following has been found to develop well in four minutes at 68° F. (20 C.) :
Metol 16 gr. 1 g
The first developer may also be used for this operation but cannot then be used again for first development of other films.
After the second development the film is rinsed well and fixed for three minutes in any acid hypo bath or the following:
Sodium hyposulphite (hypo) 13 oz. 360 g
Chrome alum 154 gr. 10 g
Dissolve the alum and add solution B to A.
A final washing of fifteen minutes is necessary, after which the color film iS ready to dry.
A certain amount of correction can be made to compensate for over- or underexposure by the use of standard intensification or reduction baths. The following have been found entirely satisfactory:
BLEACH BATH FOR INTENSIFICATION
Ammonium chloride 24 gr. 360 g
The image should be bleached to completion, which takes approximately 4 minutes. Wash for 15 minutes and redevelop in either of the formulae below according to the degree of intensification needed.
For Slight Intensification
Sodium sulphite, dry 50 g 1 3/4 oz.
For Considerable Intensification
Ammonia 50 ccm 1 3/4 oz.
Note: Avoid putting fingers in these solutions, especially the ammonia bath, which stains badly if there is a trace of the bleach bath on the skin.
Dufaycolor transparencies may also be reduced if they have been underdeveloped or underexposed. Most reducers are suitable, but the following is very convenient:
Sodium hyposulphite 50 g 1 3/4 oz.
Potassium ferricyanide 5 g 80 gr.
For use mix equal parts of A and B. (The carbonate increases the life of the bath, which will keep for hours.)
Great improvement is sometimes effected by intensifying pictures which have been slightly reduced in order to increase brilliance. Having washed the reduced transparency, intensification is carried out as outlined above.
MAKING SEPARATION NEGATIVES FROM DUFAYCOLOR FILM.—As mentioned previously, when making separations from Dufaycolor, one must make use of what are known as narrow-cut filters, which only transmit narrow bands of blue, green and red light, so that negatives so made will be suitable for the various methods of making the color positives and prints such as Auto-type Dyebro, Eastman Wash-Off Relief Process, Auto-type Carbro, Defender Chromatone, etc.