Polarized light remains polarized after reflec tion on a polished surface, but it is de-polarized by diffusion on a matt surface or by passing through a ground glass.
If two polarizers are placed one behind the other, the light polarized by the first passes freely through the second if the planes of polarization are parallel. It is totally extinguished if these planes are mutually perpendicular, and it is par tially extinguished in all the intermediate posi tions, and to an increasing extent as the angle between the two planes approximates to a right angle.
Some attempts had been made to utilize in photographic practice the particular properties of polarized light, but these applications were restricted by prohibitive cost and by the very narrow angle of field of effective polarizers. Very numerous practical applications have been made possible by the recent placing on the market of polarizing screens of fairly large dimensions, reasonable efficiency in the visible extent of the spectrum and accessible cost (Eastman Pala Screens, Land ; Zeiss Herotar Screens, Herapath).
The fact that these polarizers extinguish all components of vibration other than those orientated in the plane of polarization would, in the case of colourless polarizers, cause the exposure to be doubled. It has to be quadrupled owing to the grey-brown colour of the polarizers used in photography.
For some purposes it is only necessary to use one polarizer, mounted on the lens. Others re quire at least two polarizers, one on the lens and one in front of each of the sources of light illuminating the subject.
By using one polarizer (the suitable orienta tion of which must be found by examining the image on the ground glass screen of the camera while turning the polarizer on itself in its own plane), it is possible at will to decrease the luminosity of a blue sky without modifying the brilliance of other parts of the subject, and to decrease very considerably reflections on all brilliant non-metallic surfaces (glassware, water, earthenware, lacquered metal and other var nished objects) by directing the axis of the lens at an angle of about 35° to the surface to be photographed, thus causing to appear objects placed behind this (transparent) surface (Figs.
90.A. and 90B) or the actual texture of the subject.
By photographing through a polarizer a num ber of objects illuminated by polarized light one decreases at will the reflections on all brilliant surfaces (including metals), whatever their posi tions may be. These reflections can even be totally extinguished if the planes of polarization are crossed this orientation is that especially adopted for the photography of varnished paint ings or of all documents under 223. Prisms and Mirrors. We will consider here only the inaccurately named total-reflec tion prisms and the mirrors (at an angle of 451 as used in commercial photography. They are employed either for obtaining a picture the right way round direct (with certain methods of printing, reversed pictures would otherwise be obtained), or for the photography of ceilings, articles arranged on a horizontal table (jewels, natural history specimens), and, more particu larly, for immersed objects.
The ideal reflector is a metal mirror, but unfortunately such articles are very costly ; in their place, optically worked glass mirrors, which are silvered on the surface, are employed, it being possible to protect the silver, to a certain extent, by a very thin coat of celluloid varnish A mirror has the following advantages over a prism : It absorbs less light ; does not cause the slightest aberration, and does not limit the angle of view as does a prism. On the other hand, a mirror possesses the disadvantage of being extremely fragile in so far as the reflecting surface is easily The one advantage of a prism is the perfect stability of the silvering, which is applied externally on the hypotenuse (and which ex cludes all possibility of total reflection) without risk of any doubled image. Prisms, however, do not permit of an angle of field greater than about 30° being used without other reflections creeping in. The definition is often slightly inferior at the margins of the field from aberra tions, which are the same as for a cube of glass with the side equal to the length of one side of the prism.