230 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1937 



The Pola-screen may be used with a color filter. If we are willing 

 to sacrifice some color rendering, we can obtain sky effects similar 

 to those in infrared photographs, on ordinary pancliromatic materials 

 by using the Pola-screen with a red filter. In some respects, such 

 photographs are preferable because the trees and grass are not ren- 

 dered extremely light as in the case of infrared photographs. 



Pola-screen offers the only loiown way of obtaining dark sky effects 

 in color photography. 



Perhaps the most beautiful results with the Pola-screen arc ob- 

 tained with the dark blue sky as background when objects are photo- 

 graphed in natural color. Wlien we look at objects, such as blossoms, 

 trees, buildings, etc., against the sky, ordinarily we cannot fully 

 appreciate their colors because the sky is very much brighter, and 

 our eyes tend to seek the lighter parts of the scene. When, however, 

 the sky is darkened, many things assume a new and strange beauty. 

 Many subjects are rendered actually lighter than the sky background, 

 so that our attention is no longer diverted from the subject but is 

 drawn to it for a full appreciation of its color and form. The Pola- 

 screen type lA rather than the type I is recommended for color 

 photography. 



So far we have discussed what can be done with the Pola-screen over 

 the lens alone. For these applications the exposure increase is about 

 four times for the type I Pola-screen, and about two tunes for the type 

 I A. These factors apply iiTcspective of the angular position of the 

 Pola-screen. When used with a yellow or red filter, the type I Pola- 

 screen factor will bo between two and four, which should bo multiplied 

 by the factor of the filter. 



POLA-SCREENS OVER LENS AND LIGHTS 



We shall now turn to effects obtainable with a Pola-screen at the 

 lens and large Pola-screeiis at the lights. To understand these 

 eflFects, let us consider for a moment the nature of the light reflected 

 by most common objects. The light reflected from most surfaces 

 consists of two parts which are technically known as the specular and 

 diffuse components. The specular component forms what we know 

 as gloss and enables us to see more or less distinctly an image of the 

 source of light. Light reflected from polished metallic articles is 

 almost entirely specular, whereas that reflected from chalk is almost 

 entirely diffuse. The diffuse component is reflected without gloss in 

 all directions. 



Now, if the ray of light which is illuminating the subject is polar- 

 ized, the reflected rays wliich form the specular component are still 

 polarized, but the rays reflected diffusely are not, as shown in figure 4, 

 If we look at the subject through a Pola-screen, we can turn the screen 

 so that practically all of the specular reflection is stopped, and see the 



