Popular Science, heinäkuu 1916
By Lloyd Darling
The Brewster camera, showing the red plate, the green plate, and the perforated partition
Color photography is not new. it has been the goal of ambitious inventors ever since scientists really understood something of the nature of light. Nearly all methods of making colored photographs are long and expensive. Though beautiful results were in some cases secured, only an able scientist could manipulate the apparatus, time the exposures, and keep track of the dozens of little things all-essential to securing satisfactory results. Event the Lumière process, widely employed as it is, is handicapped by the fact that the pictures must be viewed through glass.
Most of the previous processes were of the "three-color" type. That is, they depended on the fact that from three colors of the spectrum, red, yellow-green, and blue-violet, all other hues could be made by combination. Negatives of an object were made through red, yellow, and blue filters, and positives therefrom were colored and joined in various ways to make a resultant colored picture.
Within recent years encouraging experiments have been made which involve the use of two colors only, red and green. The most recent system of color photography dependant on this method is that f Mr. Percy D. Brewster of New York.
Two Plates Are Used with the Camera
The camera employed in the Brewster system and other two-color systems differs from the ordinary photographer's mainly in that it has two plates instead of the customary one. The one directy back of the lens is known as the "green" plate; while the other at right angles is referres to as "red." This arises from the fact that light rays reaching the "green" plate must first pass through a green filter, while those falling on the "red" plate are correspondingly filtered by a red glass. The "green" plate is intended to record at the green portion of the spectrum, while the "red" is sensitive to those at the opposite end.
The manner in which the image is conveyed to both plates is interesting. Thus, Mr. Brewster mounts a few inches back of the camera shutter, a mirror called the "Swiss Cheese" plate, its surface being at a 45° angle with the plane of the lens. The mirror is thus strangely named because it is full of holes, which serve to permit parts of the image to pass through to the "green" plate; the remainder being reflected by the solid part to the "red" plate. inasmuch as images filtering through the holes overlap after passing the mirror, a complete picture is thrown on the "green" plate - and not a spotted one, as might be expected because of the holes. Likewise the solid portion throws a complete image on the "red" plate. Dividing the light between the two plates in this manner of course lengthens somewhat the time of exposure necessary; otherwise no other effects are ordinarily noticeable.
The same effect can be obtained in many other ways. Thus in what is known as the "kodachrome" process a plate is employed which, instead of being perforated with Swiss cheese holes, is thinly platinized, so that it can both reflect and transmit light.
It is understood of course that negatives obtained with the Brewster, "Kodachrome," and similar instruments are of the ordinary black-and-white variety - not colored in any way. The "green" plate differs from the ordinary negative only in the fact that it is especially dense where colors at the green end of the spectrum predominated, while the "red" plate likewise records densely roseate hues. From these two negatives positives are made on other plates by ordinary processes of contact printing. The image on the positive from the "green" plate is dyed red and that form the "red" plate green. The two positives are then placed face to face, and the image on one registered with the image on the other.
Hold the combined plates up to the light, and you can see the photographed object in its natural colors. it stands out from the background as striking as the original. The effect is startling, indeed.
Why Two Colors Must Be Employed
The reason for coloring the "green" positive red, and the "red" positive green, as mentioned in the foregoing, is rather elusive and at the same time particularly interesting. Consider for instance the case of a red rose on a background of green leaves. The "green" negative upon development will be almost black where the green leaves appear on the plate, while the rose will be almost transparent. Similarly with the "red" negative, the rose will appear dense, while the green is recorded as a transparent area.
Positives from these two plates will in each case of course be just the opposite of the negatives. That is, a positive from the green plate will show the leaves transparent, and the rose dense; while that from the "red" plate will show the leaves dense, and the rose transparent. Dyes used in this process affect only the dense places. it is obvious that if you want a red rose to bered in the resultant picture, you will have to color the positive from the "green" negative red, that being the only one showing the red rose as a dense area. Similarly, you will have to color the positive from the green negative "red," since in this case the leaves are dense. after dyeing them in this manner, the plates pass through a special process to eliminate the opaque black silver on the plates, leaving only the colored images. This process completed, the two plates are placed face to face and registered properly. Then you see the red rose in its proper place among the green leaves.
The next step is to cement these two positive emulsions together. This done, they are stripped from the glass and transferred to paper, canvas, ivory (in the case of a miniature), or any other backing. in their new positions they look not unlike an oil painting, especially when canvas is used as a mount.
For the sake of simplicity, the foregoing description of the red rose and green background referred only to these two colors. it is understood of course that almost any color which may have been present in the original object also appears in the finished picture. This is possible because red and green combined in different proportions by the process here used will give such desired colors.