Improvements in or relating to illuminating devices

摘要

1,005,273. Lamps. EDNALITE CORPORATION. Dec. 11, 1963, No. 48842/63. Heading F4R. [Also in Division G2] A light table for illuminating transparent or like objects comprises a circular plate 22 of translucent light dispersing material, a concentric circular light tube 19 spaced from and parallel to the plate, and a reflecting surface 12, 25 extending from the periphery of the plate and enveloping the tube, the reflecting surface being in the form of a surface of revolution generated by the rotation about the axis Z-Z of a generatrix including an outer part P 1 -P 4 conforming substantially to the arc of an ellipse 30 and an inner part P 4 -P 9 extending from the arc to the axis Z-Z and comprising an arc P 8 -P 9 of reversed or opposite curvature to that of the outer part, the central portion of the surface being a mound 31 directed towards the plate 22. The ellipse 30 has its major axis extending radially from the axis Z-Z and its outer focus F 1 located at the centre of the section of the light tube 19. Between the points P 2 and P 3 the outer part of the generatix deviates from the elliptic arc along lines which converge on the central plane of the light tube either outside (Fig. 3) or inside (Fig. 5) the ellipse, so that light which would otherwise be reflected back into the tube is caused to by-pass the tube and is therefore available to illuminate the plate. The construction of the reflecting surface is such that the light table compensates for the " vignetism " of photographic objective lenses, which produce an image the intensity of illumination E of any point of which is equal to the intensity of illumination E 0 of the corresponding point on the object multiplied by the fourth power of the cosine of the angle u between the optical axis and the point on the object. If a photographic negative placed on the table is to have a uniformly bright image, then to compensate for the law E = E 0 cos4 u the illumination of the table must obey the E c = E r COS4 u where E c is the intensity of illumination at the optical axis Z-Z and E r is the intensity of illumination at any point at the angular distance u from the axis. The Specification tabulates measurements relating to three light tables having plate diameters of 6, 9¢ and 13¢ inches, respectively, which show that the invention provides intensities of illumination differing by only small amounts from the theoretically desired values. The part 12 of the reflecting surface preferably comprises the upper part of the housing of the device, the part 25 of the surface being a circular reflector. A lower housing part (not shown) contains the control equipment for the tube 19. Stepped support means (not shown) for illumination filters, glass slides and the like may be secured to the upper part of the housing at intervals around the periphery of the plate 22.