GB638333A - Improvements in or relating to apparatus for measuring the focal length of lenses - Google Patents

Abstract

638,333. Measuring focal lengths of lenses. FREEMAN, H. Feb. 3, 1948, No. 3194. [Class 97 (i)] The apparatus comprises a collimator sending parallel light from a source on to an opaque diaphragm provided with three apertures sending beams to a translucent screen, the dis. placement of the light spots on the screen which results when the lens to be tested is located between the diaphragm and the screen giving a measure of the focal length of the lens. As shown, the diaphragm apertures 15, 16, 17 are so located that light from the aperture 15 falls at the intersection of orthogonal scales a, b on the screen 18 while the light spots c, d from the apertures 17, 16 fall at the zero points of the two scales, which may be calibrated in dioptres. The lens 19, for example a spectacle lens, is inserted in the beam and is adjusted until its optical axis coincides with the beam through the aperture 15 so that the spot on the screen corresponding thereto still falls at the intersection of the scales. The displacement of the spots c or d along the scales then gives a direct measure of the focal length of the lens. If the lens be cylindrical it is oriented until the spots c and d fall on the scales when the displacements of the spots give the powers of the lens in two meridians and the orientation of the lens indicates the direction of its cylinder axes. Embodiments are described in which the axis of the system is vertical and the screen disposed either at the top or bottom of the apparatus and observed in a mirror, and in which a mirror is placed between the lens 19 and the screen and both the axis of the collimator and the plane of the screen are inclined to the horizontal and the screen viewed in an eyepiece. Means for marking the lens may be provided.