The Cine Technician (1935-1937)

10 The Journal of the Association of CineTechnicians May, 1936 Polarised Light and the Kodak "Poia-Screen" IAN DENIS WRATTEN (Motion Picflure Film Dept., Kodak Limited, London) The term "polarise" does not refer to colour or brightness, but to a third property invisible to the unaided eye, which is concerned with the way a light ray vibrates. Ordinary light rays vibrate through space in the form of waves, but unlike sound waves (where the vibrations are along the direction of the wave) the vibration of a light wave is at right angles to the ray and generally in all possible directions. (Fig. 1). Now it is possible by using a polarising device to change the light ray so that it vibrates only in one direction (Fig. 2). This one vibration is not only composed of the one originally vibrating in this same direction, but is also composed of parts of all the others, except the one vibrating at right angles to it. The result is that almost half the light is allowed through, even though there is only one direction of vibration. A light ray in which only one direction of vibration exists is called "plane polarised." Although the Nicol prism has been used as a polarising device by scientific workers for many years, it is unsuitable for ordinary cinematography, because its length is very much greater than its free aperture. The introduction of a highly efficient polarising device in sheet form, the Kodak Pola-Screen, will enable the motion picture cameraman to investigate the possibilities of polarised light. The PolaScreen incorporates between glass plates a material in wliich are countless minute rod-like crystals, which are all jxirallel to one another. It should be mentioned at this point that the vibration plane of the Pola-Screen is in line with the handle (Fig. 3), and that a ray of light already polarised will pass through the Pola-Screen if the vibration of the ray is in line with the screen's vibration plane. The desired effect is, therefore. PL ANC POLAR1Z.ELD RAN' SOUND VIBRATIONS % % "^ UN POLARIZE-D WAV plane! polarizing device: LIGHT VIBRATIONS Fig. 1. Fig. 2. obtained by viewing the subject through a Pola-Screen and rotating the screen (through an angle of 90°) until the correct position is found. Plane polarised light, or light that is partially plane polarised, is very common in nature, so that the cameraman who is equipped with a Pola-Screen on the lens of his camera will find tliat he has considerable control o\-er contrasts in his subjects, even though he is unable to change the lighting of his subjects. There are two sources of polarised light in nature — clear blue skylight, arriving at right angles to the sun's rays is strongly polarised, and light reflected at about 30" from non-metallic surfaces is polarised by the action of reflection. For a clear blue sky, the Pola-Screen acts as a variable depth filter. Any sky effect, from no filter to red filter, may be obtained by rotating the screen to the desired position. An important advantage over the use of colour filters for this type of work is that the use of a Pola-Screen will produce the dark sky effect without giving colour distortion of any objects in the foreground. In this connection the Pola-Screen offers the only known method of obtaining dark sky effects in, colour photography. The most effective position is when the sun's rays are at an angle of 90° to the direction in which the camera is pointed. The Pola-Screen has no effect when shooting directly at the sun, or directly away from it. When the camera lens axis is at about 30 to the surface, reflections can be removed from glass or water by rotating the Pola-Screen to the recpiired position. Other camera angles are less effective and at 90" no control is obtained. For architectural work a Pola-Screen over the camera lens makes possible considerable control of the relative brightnesses of the walls and roof of a building. It two Pola-Screens are placed together, so that their vibration planes (in line with the handle) are at an angle of 90°, practically no light is allowed through, but when the vibration planes of the two screens arc parallel, all the light A