GB449830A - Improvements in sight testing apparatus - Google Patents

Abstract

449,830. Sight - - testing apparatus. REASON, R. E., and KAPELLA, Ltd., 104, Stoughton Street, Leicester. April 3, 1935, No. 10312. [Class 97 (i)] In objective apparatus comprising a target system for forming an image on the retina, an observing system for viewing the image, and a fixation system for forming on the retina another image for the purpose of fixing the subject's accommodation or direction of sight, means are provided for illuminating the fixation image with light limited to a particular section of the spectrum and for viewing the target image with light limited to another section of the spectrum, in order to avoid, in the observation field, " ghosts " formed by reflection at any surface common to the observing and fixation systems. The optometer shown consists of a source of light 1 and condensers 3, 4 illuminating a target 2 from which light passes through a small aperture 5 to a mirror 7 and thence through a lens 6 and reversing prism 8 to form a target image at 9, this image being imaged by a lens 10 and the eye lens 11 on the retina. This image is projected for viewing by the lenses 11, 10, 6 through an aperture in the mirror 7 onto an eyepiece graticule 12. A fixation object 15 is illuminated by the source 1 and condensers 17, 18 and a virtual image of this object is formed on the axis of the lens 10 by a transparent mirror 19. To avoid ghosts caused by reflections at the surfaces of the lenses 10, 11, a filter 20 is mounted in the observing system and a filter 21 in the fixation system, these filters being preferably minusblue and blue respectively. Instead of mounting the observing filter at 20, a filter may be provided adjacent the graticule 12, in which case a similar filter 22 is preferably provided in the target system to reduce the brightness of the target image. Specifications 413,029, 430,761, and 437,764 are referred to.