Figure 106 (K1 and K2) shows the spectral transmission curves of the Wratten K1 and K2 filters, intended for correct color rendering with orthochromatic plates. The absorption increases gradually toward the blue. In the same figure is shown on an arbitrary scale the spectroscopic character of typical haze illumination, increasing in brightness inversely as the fourth power of the wave-length, that is, with great rapidity in the blue and violet. It is evident from this that a much more abrupt absorption than that of the K1 or K2 filter is desirable, because in the green of the spectrum the haze light is comparatively weak, and more will be lost by any absorption in this region through decreasing useful photographic action than will be gained by cutting out the haze. This latter consideration is important. The use of any filter means an increase of exposure; the use of yellow filters multiplies it several times. Careful experiment has shown that no filter of depth less than K 1½, to use the Wratten filters as a basis for discussion, are of real value in haze piercing. The filter ratio, or ratio of exposures with and without filter, is 4.7 for the K 1½ with the Cramer Isonon plate—a figure which shows the importance of securing the necessary haze-piercing character with the minimum absorption of useful photographic light.
Fig. 106.—Characteristics of various filters.
Practical Filters.—Since the character of the absorption of the “K” filters is not all that could be desired, new filters, both of dyed gelatin and of glass, have been produced. The glass, a Corning product having a very sharp-cut absorption, has not yet been produced on a commercial scale with the high transparency in green, yellow and red that selected samples have shown. The United States Air Service has adopted filters of a new dye, called the EK, from the name of the company in whose laboratory it was produced. These filters are standardized in two depths of staining, called the “Aero No. 1” and “Aero No. 2.” Their spectral transmission curves appear in Fig. [106], along with those of certain darker filters useful only with panchromatic plates for exceptionally heavy haze. The characteristic of these Aero filters is their great transparency through all the spectrum except the blue, whereby the greatest haze-cutting action is attained together with a low filter factor. The filter factors of the Aero No. 1 and No. 2 with Cramer Isonon plates are 3 and 5, respectively.
Effects Secured by the Use of Filters.—The efficiency of yellow filters for haze-cutting is best shown by photographs taken at high altitudes with filters and without. Such illustrations are given in Figs. [107] and [108], where the first photograph is one taken at 10,000 feet without a filter, the second taken at the same altitude under the same conditions, but with an orange filter. Both are on panchromatic plates, and it will be seen that even with these plates the filter makes all the difference between a useless and a useful picture. But it must be clearly understood that the difference here lies between a plate sensitive chiefly in the blue and violet, and a plate affected only by the yellow, orange and red. The difference is not between what the eye sees and what a plate with a filter sees, as is sometimes supposed. As shown in Fig. [108], a filter enables the plate to photograph through the haze between clouds, but not through the clouds themselves. In general, no filter and plate combination which is feasible for aerial exposures is capable of showing more than the eye can see if yellow or orange goggles are worn. To do this it would be necessary for the photographic action to take place by deep red or infra-red light, which would demand exposures now out of the question.
Fig. 107.—A photograph taken at 10,000 feet, without a filter.
Filters are almost always necessary in photographing from high altitudes or in making distant obliques. At times, particularly after a heavy rain, the air is clear enough so that filters may be dispensed with. Clearing weather was therefore chosen whenever possible for making obliques of the battle front.
Fig. 108.—Photograph taken at same time and over same neighborhood as Fig. [107], but with an orange filter.