GB874119A - Improvements in or relating to focimeters - Google Patents

Abstract

874,119. Lens testing apparatus. CLARKE Ltd., CLEMENT, and ROLLASON, W. A. Jan. 22, 1958 [Jan. 22, 1957], No. 2280/57. Class 97(1) A focimeter comprises a source of light, a movable graticule, optical condensing means associated with the source for directing a beam of light on to the graticule, a projection screen, a holder for the spectacle lens under test. located between the projection screen and the graticule, means for adjusting the axial position of the graticule, and an optical objective located between the spectacle lens holder and the graticule for acting in combination with the spectacle lens to project on to the screen an image of the graticule which is accurately focused by movement of the graticule to a position of adjustment dependent on the optical power of the spectacle lens, the axial separation between the spectacle lens holder and the objective being such as to give an approximately linear relationship between the optical power of the spectacle lens measured in dioptres and the movement of the graticule to its correctly adjusted position from a zero position in which an accurately focused image of such graticule is projected on to the screen by the objective alone. As shown in Figs. 2 and 3, light from a lamp A1 and condenser is reflected by a mirror B2 through a diffusing screen C<SP>1</SP> 1 to a graticule C, the screen and graticule being mounted in a tube F axially movable by rack and pinion F4 and F5 within a sealed tube E. The light from the graticule passes through an objective lens D for reflection by a mirror G to a spectacle lens under test which is held by springy fingers H on a table J carried by a bracket J 1 and vertically adjustable by a hand knob J5. Light from the lens under test passes into a housing V2 where it is reflected by mirrors. L1 and L2 on to a screen M. Mirror B2 has a bevelled edge so that some of the light from the lamp is, as shown in Fig. 2, obliquely directed on to an opaque scale P, calibrated in dioptres and disposed on the inner wall of the tube F, and thence, through windows in tubes E and F via another objective Q, to mirrors S and T whereby an image of this scale is formed on a screen U at the side of the screen M, screen U having a horizontal index mark U1. In the absence of the lens under test, the objective D forms a focused image on the screen M of the graticule C in its zero position so that introduction of the lens under test necessitates axial movement of the graticule for refocusing, the axial separation between objective D and the lens support being so selected that there is a linear relationship between the graticule movement and the power in dioptres of the test lens. Such movement of the graticule causes movement of the scale image on the screen U relative to the index mark U1, and the scale graduation registering with this index when the graticule image has been refocused on the screen M indicates the focal power of the lens under test. The graticule C consists of a set of transparent marks in the form of holes on an opaque ground, some or all of these marks being arranged in several angularly spaced rows, at least one of which when the graticule image is in focus will appear as a continuous line on the screen M to indicate the astigmatic angle of the lens under test. For measuring this angle the screen M is provided with a rotatable transparent cursor Ml, Fig. 6, bearing angle markings co-operating with an index M3 on the screen. On one side of a diametrical line M4 on the cursor is a set of parallel lines M6 while on the other side thereof is a radial .scale M7 for measuring the decentration or prismatic power of the lens under test.