1. Field of the Invention
The invention pertains to the art of testing eyesight and more in particular to the art of testing eyesight under different conditions of the contrast ratio of a target presented to the test subject. The invention also pertains to the art of measuring visual performance of a subject on targets of varying contrast ratios, i.e. when the ratio of dark to light in the test object of the target is varied. The invention additionally pertains to the art of testing the fusion potential (binocular single image) of a subject's eyes under different conditions of the contrast ratio of the target presented to the test subject. The invention further pertains to the art of determining glare sensitivity of a subject's eyes.
2. Description of the Prior Art
U.S. Pat. No. 2,209,728 which issued to H. R. HIGLEY on July 30, 1940, discloses a device for measuring a subject's sensitivity to glare. Light from a dazzling brilliant light source is focused by a reflector directly onto the eyes of a subject who views the brilliant source in an initially totally darkened chamber. A controllable light source which is off-set and in front of the dazzling light source is then directed at objects (such as letters painted dull white) which are in the vicinity of the dazzling light source. The intensity of this controllable source is then increased until the viewer is able to see the objects in the presence of the glare of the source. The value of the intensity of the controllable dazzling source when the objects just become visible is a measure of the glare sensitivity of the viewer.
There is no disclosure in this patent of an arrangement for directing toward the subject a level of glare light extending uniformly across the field of view.
U.S. Pat. No. 3,684,355 which issued to S. F. MOLNER on Aug. 15, 1972 relates to another glare sensitivity measuring device. Light from a glare source is directed in pencil-beam fashion at a viewer through a variable density filter which permits controlling of the intensity of the glare light reaching the viewer. Out of the path of the glare source but within the field of vision of the viewer, a target is arranged. The target is illuminated by another light source arranged in back of the target. A variable density background filter arranged between the target and the viewer controls the intensity of the illumination from the target reaching the viewer. In this apparatus, the relative intensity of the light from the glare source to the intensity of the target illumination is controlled by varying the density of the background filter while maintaining the light from the glare source constant, by varying the intensity of the glare source while maintaining the density of the background filter constant, or by varying both. Here again there is no disclosed construction for directing toward the viewer a level of glare light which extends uniformly across the field of view.
In U.S. Pat. No. 1,437,809 which issued to L. A. JONES on Dec. 5, 1922 there is disclosed apparatus for measuring the visibility of an object with respect to its background such as the visibility against the sea and sky of a marine vessel which has been camouflaged by appropriate painting of its outer surface. The apparatus includes a telescope for enabling the user to view the object such as a camouflaged marine vessel. A semi-transparent mirror extends across the optical axis of the telescope in a position adapted to reflect light in the form of a veiling light from a lamp source toward the user's eye. The composite brightness resulting from the veiling light and the light reflected from the observed object is preferably maintained substantially constant by decreasing the ligh from the object and increasing the veiling light or vice versa. Since the device of this patent was not disclosed as being related to vision testing, the device was taught as operating with constant composite brightness with the stated purpose that this condition would eliminate any change of light level at the user's eyes and thereby prevent the user's eyes from interfering with the visibility condition being measured. It now has been discovered in accordance with the invention that the user's eyes will interfere with the visability condition being measured even when composite brightness is constant if the contrast ratio changes.
In U.S. Pat. No. 2,799,203 which issued to W. R. Kerr on July 16, 1957, apparatus is described for determining the degree of gloss of a finished surface. In this apparatus, the image of a mask comprising a background upon which are disposed rows of black letters of varying size is projected onto successive finished surfaces each having a different degree of gloss. A subject person observing the projected images thus sees images having different degrees of contrast between their rows of letters and their backgrounds. The surfaces of higher gloss have a higher contrast between the letters and background while those of lower gloss have a lower contrast. The different degrees of contrast give a relative measure of the different degrees of gloss of the surfaces.
It is known in the prior art to measure the fusion potential of a subject's eyes by means of a stereoscopic viewer which is adapted to direct each of the test subject's eyes to a different one of a pair of adjacent eye charts having dissimilar targets. With fused vision, the two targets are viewed by the subject as a single fused target. The stereoscopic viewer is provided with optical wedges which can be adjusted to displace the line of sight of the subject's eyes from a condition in which the targets appear fused. The ability of the subject to overcome the displacement of his line of sight by the optical wedges and then sense a fused image is indication of the subject's fusion potential. Progressively increasing the degree of displacement of the line of sight, in the measuring of the ability of the subject to again sense fused targets, enables a discrete determination of the fusion potential to be made.