Patent GB-701653-A (GB)
Title: Improvements in or relating to devices for viewing transparent objects
IPC: G02B27/02
Filed: 1951-07-09
Abstract: 701,653. Viewing transparencies. NAAMLOOZE VENNOOTSCHAP OPTISCHE INDUSTRIE DE OUDE DELFT. July 9, 1951 [July 12, 1950], No. 16236/51. Class 3(2) [Also in Group XX] A device for viewing transparent objects such as negatives, lantern slides and the like includes means for illuminating the object to be viewed by diffused light to give low contrast observation, and further means for illuminating by focused light from a source of limited area, and/or by light which passes through the object two or more times, to give high contrast observation, the ratio of the effective intensities of the rays giving high and low contrast observation being adjustable. As shown in Fig. 1, the object 20 is held between a mirror 21 with a semi-reflecting sawtooth surface and a pressure plate 22. Light from a lamp 24 is diffused by opal glass 25, passes through mirror 21, object 20 and plate 22 and is viewed through a lens 28 by the eye 23. Light from a lamp 26, focused by a lens 27, is reflected by mirror 21 and reaches the eye 23 after passing through the object twice. Reflections from plate 22 or the back of mirror 21 do not reach the eye. The relative intensities of lamps 24, 26 may be varied by means of a rheostat 30 to give variable contrast at approximately constant brightness. Mirror 21 may be replaced by a semi-reflecting concave mirror which focuses the light from lamp 26 without lens 27, and plate 22 is then replaced by a pressure plate having one face conforming with the mirror surface and the other curved so that light reflected therefrom does not enter the eye. Reflection from the other surface may be eliminated by immersing the object in a liquid of suitable refractive index. When a transparency with areas of high density is viewed with high contrast, dazzle from low density areas may be reduced by illuminating the transparency with polarized light and viewing it through an analyzer. The analyzer is arranged to prevent passage of much of the light passing directly through density areas, but light passing through denser areas is depolarized and its intensity is less reduced. Quarter-wave plates may be included in such an arrangement, as shown in Fig. 3. Light from lamps 39 passes through a polarizer 40 and a semi-reflecting mirror 41 to the transparency 42, and thence through a quarterwave plate 43 to a second semi-reflecting mirror 44. Light may be reflected between the mirrors several times, and the emergent rays pass through a second quarterwave plate 45 and an analyzer 46 to a lens which focus them on the observer's eye 47. By proper arrangement of the quarterwave olates and polarizers, only light reflected between the mirrors is allowed to pass. In all the above cases more than one eyepiece may be provided, for binocular vision or to provide for viewing by more than one observer. In another arrangement, both eyes are applied to a long slit on which light from a tubular lamp is focused after passing through the transparency. Other tubular lamps are provided, but are screened when high contrast is required. The screens are removed from these lamps progressively when contrast is to be diminished.