If we look at a spectrum we shall see that the brightest part of it is the yellow-green and yellow (the position of the yellow in the spectrum being between the yellow-green and the orange) so that the eye is most sensitive to the yellow, yellow-green and red rays and least sensitive to the blue and violet rays. (Fig. 111.) But if, instead of looking at the spectrum, we use a piece of bromide paper so that the light of the spectrum may fall on it, and then make a positive print from this negative image, we shall find that the photographic action on the print is not produced in the region that is bright to the eye, but in the region which the eye can scarcely see, and, indeed, there is a strong action in the part of the spectrum beyond the visible spectrum, showing that there are waves which are shorter than the violet waves, which were discovered when the spectrum was first photographed and are called the ultra-violet waves. (Fig. 112.) This explains at once why when we photographed an orange on a blue cloth the orange was dark in the photograph and the blue cloth was bright, which is the opposite to the way they appear to the eye. The bright orange absorbs the blue light to which the film is sensitive and the blue cloth reflects it, so that although the cloth looks dark to the eye, it is bright in the photograph, and the orange which reflects very little blue and violet light is dark in the photograph. Fortunately, this defect, for defect it is, of photographic materials can be remedied to a considerable extent.

Invisible Ultra-Violet

Limit of Visibility

Violet

Blue

Blue-Green

Green

Yellow-Green

Orange

Red

Deep-Red

Limit of Visibility

Fig. 112.

If dyes are incorporated with the emulsion the dyes sensitize the emulsion for the part of the spectrum which they absorb, so that if we put a pink dye of the right kind in the emulsion the film will not only be sensitive to the blue light, to which it is naturally sensitive, but will also become sensitive to the yellow-green light, which the pink dye absorbs, and if we take a photograph of the spectrum on this sensitized film we shall get a photograph which appears as is shown in Fig. 113. Film made sensitive in this way is called orthochromatic, and in photographing colored objects the use of an orthochromatic film is a great advantage.

Invisible Ultra-Violet

Limit of Visibility

Violet

Blue

Blue-Green

Green

Yellow-Green

Orange

Red

Deep-Red

Limit of Visibility

Fig. 113.

The orthochromatic film is still not sensitive to red, which to the eye is a bright color, and so red objects are still rendered too dark in a photograph, but this is not a great disadvantage for most work, and we have the very great advantage that the film can be developed in a red light.

Emulsions can be treated in such a way as to make them panchromatic, that is, sensitive to all colors, but such panchromatic materials cannot be handled by the light of an ordinary dark room lamp; they have to be used either in total darkness or by means of a very faint green light. For amateur photography it is therefore better to be content with orthochromatic film unless special subjects are to be photographed. A full account of photography with panchromatic plates and of the use of light filters in general is given in "The Photography of Colored Objects," published by the Company.

Fig. 114. Made Through a Yellow Light Filter.

Great care is taken to make Kodak NC film as ortho-chromatic as will confer satisfactory color sensitiveness upon it without sensitizing it so far that it will be difficult for the user to handle or that there will be danger of fog when developing it.

While the sensitizing with dye makes the film sensitive to the yellow and green light, it is still much more sensitive to the blue and violet waves, as is shown in Fig. 113, and consequently it will still photograph blue objects much lighter than they appear to the eye. This is a disadvantage in some photography, and especially in landscape photography where we have blue sky with white clouds. White clouds are much brighter to the eye than the blue sky, but if they are photographed on the film in the ordinary way the blue sky appears too light and the clouds are lost against it. In order to overcome this and to enable orthochromatic film to represent most of the colors in their correct tone values light filters are used which absorb the excess of blue light and prevent it from reaching the film.

These light filters are, of course, yellow in color, since yellow absorbs blue light and thus, by the use of yellow light filters, which are sometimes called color screens, the excess of blue light can be absorbed and a much improved rendering of sky and clouds can be obtained. (Fig. 114.)

When light filters were first introduced it was thought that any yellow glass would be satisfactory, and light filters were made of brownish yellow glass, which really are of no advantage at all. The reason for this is that they transmit the ultra-violet light, which lies out in the spectrum beyond the violet. This ultra-violet light is quite invisible, but produces a strong impression upon the photographic plate, and in order to get satisfactory action from a filter it is very important to remove the ultra-violet light as completely as possible. The ultra-violet light is far more easily scattered by traces of mist in the atmosphere than visible light, and since it is this mist which so often makes objects in the distance invisible in photographs that are taken without a filter (Fig. 117a) it is necessary to use a filter that will cut out this ultra-violet light in order to show the distance well. (Fig. 117b.)

Modern light filters are made by dyeing gelatine with carefully chosen dyes and then cementing the dyed gelatine between optically prepared glasses.

Some yellow dyes, while removing violet light quite satisfactorily, transmit a great deal of the ultra-violet light and only a few dyes cut out the invisible ultra-violet satisfactorily. One of the best of these dyes is the dye used in the Wratten K filters and the Kodak Color Filters. In Fig. 115 are shown two photographs of the spectrum - the one taken through a filter made with a dye of a type often used for filters, but not cutting out the ultra-violet, and the other the same spectrum taken through a K filter.

Fig. 115

Photograph of the spectrum, through two yellow Filters, which are of almost the same color to the eye, showing (A) that the K Filter cuts out the ultra-violet, while (B) the other Filter does not.

The K filters were made with a dye produced in Germany, and during the war the requirements of the aerial photographers in the army made it necessary to prepare a new dye which could be made in America and which would cut the mist even more sharply than the K filters. This presented a problem which was solved in the Kodak Research Laboratory by the discovery of an entirely new dye which was named "Eastman Yellow," with which special filters are prepared for aerial photography.