GB1314007A - Optical aligning device - Google Patents

Abstract

1314007 Optical aligning device BRITISH RAILWAYS BOARD 24 April 1970 [25 April 1969] 21274/69 Heading G2J An externally applied device for checking the alignment and focusing of a light-projector 10, 12, Fig.1 comprises a lens 16 adapted to focus a beam 22 on to a translucent screen 14 and an optical system 18, 20 which forms an image of the distant field on the screen such that to an observer both images appear superimposed. The system comprises a convex lens 18 and an erecting lens or prism assembly 20, and the observer would see an upright image of the distant field overlaid by a "beam-image" 15. The beam 22 is brought to best parallelism by moving source 10 axially relative to the aspect lens 12 to find that location where the beam-image is smallest; it may be aimed along a selected observation path by lateral adjustment of source 10 until the beam-image centre overlies a target point on the field-image. The beam-image brightness at any point of the field image represents that brightness which would be observed if the projector were viewed from the corresponding location, and the angular spread of the beam may be measured by means of a scale on the screen 14. In a second embodiment, beam 22 is focused by partially reflecting paraboloid mirror 26, Fig. 2 on to a screen 28: the beam-image 15 is seen through the mirror 26 to be superimposed on a virtual field-image. For weak beams in bright weather, the screen surface 28 may be diffusely reflecting or it may be formed in a regular fine pattern of small curved mirrors; for strong beams in dark weather, an aria of the mirror 26 may have a reflection co-efficient different from the rest of the area. To indicate the angular spread of the beam, the screen may be marked by an array of small light-sensors or by small apertures exposing a comparison light field of controllably variable luminance. Screen 28 may be disposed adjacent and in front of lens 12, when one side will be light absorbing Fig. 2b (not shown). In a third embodiment Fig. 3 (not shown) the paraboloid 26 is replaced by a partially reflecting plane mirror (32) and a lens (33), in which case both images are virtual and at infinity. In a fourth embodiment lens (33) is replaced by a lens 34, Fig. 4 between the aspect lens 12 and the plane mirror 32. A cross-wire 36 is positioned so that its image in the mirror coincides optically with the nodal point of the lens 34; with the crosswire aligned with the centre of image 15 seen through mirror 32, the line of observation is along path 24. The angular spread of beam 22 may be ascertained by adapting the cross-wire in the form of a radial arrangement of identical transparent lenticular strips 39 Fig.4a (not shown). The device may be rearranged in the same manner as described with reference to Fig.2b (not shown).