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Holography Glossary


  • Rack and pinion focusing - mechanical focusing system used on copying or monorail cameras. A pinion engages a rack on a slide. Focusing is achieved by turning a knob or wheel, which moves the lens or image panel.
  • Radiography - technique of using X-rays, gamma rays and charged particles to form shadow images on photographic materials. Used in medical and industrial research because of its ability to penetrate opaque objects.
  • Rayographs - term coined by Man Ray and his friends for pictures made by placing directly on photographic paper (i.e. photograms).
  • Rear focus - refers to the focused area behind the picture's subject.
  • Rebate - margin on photographic film surrounding the image area.
  • Reciprocity failure - in photographic emulsions occurs when exposure times fall outside a films normal range. At these times an increase in exposure is required in addition to the assessed amount. This can be achieved either by increasing intensity or time.
  • Reciprocity law - states that exposure = intensity x time, where intensity is equal to the amount of light and time is equal to the time that amount of light is allowed to act upon the photographic emulsion.
  • Reconstituted image - photograph produced by translating light from the subject into electronic signals.
  • Red eye - effect encountered when light from a flash unit travels parallel to the lens axis during exposure.
  • Reflected light - light bounced off a subject, not falling on it.
  • Reflected light reading - measurement by a light meter of the amount of reflected light being bounced of the subject. The light meter is pointed towards the subject.
  • Reflections - rays of light which strike a surface and bounce back again. Specular reflection occurs on even, polished surfaces; diffuse reflection occurs on uneven surfaces, when light scatters.
  • Reflector - any substance from which light can be reflected. It also describes a white or gray card used to reflect from a main light source into shadow areas.
  • Refraction - change in direction of light rays as they pass obliquely from one transparent medium to another of different density, e.g. air to glass.
  • Refractive index - numerical value indicating the light bending power of a medium such as glass. The greater the bending power, the greater the refractive index.
  • Register - exact alignment when overlaying separate images.
  • Register punch - punched used to make alignment holes in film or paper for registering images.
  • Rehalogenization - process by which black metallic silver is converted back to silver halides. It is used in bleaching for toners and intensification.
  • Relative aperture - measurable diameter of the diaphragm divided by the focal length of the lens in use and expressed in terms of "f" numbers, marked on the lens barrel.
  • Replenishment - addition of chemicals to a processing solution to maintain its characteristics, e.g. developers are replenished with reducing agents as the old ones are exhausted through use.
  • Resin coated paper (RC) - printing paper with a water repellent base. RC Paper can be processed faster, require less washing, and dry more quickly than fiber based papers.
  • Resist - protective but removable layer applied to a surface in the form of a pattern or image. Used to prevent chemicals solutions reaching covered areas.
  • Resolving power - ability of the eye, lens or photographic emulsion to determine fine detail. In photography, the quality of the final image is a result of the resolving power of both the lens and the sensitive emulsion. Resolution is expressed in terms of lines per millimeter which are distinctly recorded or visually separable in the final image.
  • Restrainer - chemical constituent of developing solutions which helps prevent reducing agents from affecting unexposed halides and converting them to black metallic silver.
  • Reticulation - regular crazed pattern created on the emulsion surface of negatives which is caused by extreme changes of temperature or acidity/alkalinity during processing.
  • Retouching - after treatment carried out on a negative or print, in the form of local chemical reduction, local dye or pencil additions or air-brushing. The purpose is to remove blemishes on the negative or print.
  • Reversal materials - materials specifically designed to be processed to a positive after one camera exposure.
  • Rising front - camera movement enabling the front lens panel to be raised or lowered from its central position on most view cameras.
  • Rods - receptors forming part of the retina at the back of the eye sensitive only to variations in brightness, not color. See Cones.