Method and apparatus for locating a principal meridian of an astigmatic optical system

Claim 1

Apparatus for use in locating a principal meridian of an astigmatic subject optical system, said subject optical system comprising imaging means of variable dioptric power in combination with a patient''s eye, said combination of imaging means and eye serving to receive light from said target means and to use said light to form a test image in the vicinity of the retina of said eye for the purpose of examination of said test image; said apparatus comprising: a target array in combination with rotational means, said target array comprising a plurality of pairs of linear images of light, each of said pairs of linear images being equivalent, with respect to said subject optical system, to an astigmatic image of a point source of light in that the two linear images constituting each pair are situated in different planes one behind the other, said two linear images being oriented at right angles to one another, and said two linear images acting together to provide light to be received by said subject optical system, said light being limited to light which is common to both of said two linear images; said plurality of pairs of linear images of light being disposed in relation to one another such that a first linear image of each pair cooperates with first linear images of all remaining pairs to form a single common linear image, and the second linear images of said pairs are separated laterally from one another as viewed by said subject optical system, the lateral spacing of said second linear images being substantially greater than the width of said second linear images, and said lateral spacing being chosen in conjunction with the distance of each of said second linear images from said common linear image such that when said target array is viewed by said subject optical system and when the imaging means of said subject optical system is varied as necessary to form said test image in the vicinity of said retina, said test image becomes composed of a linear array of linear segments of light, said segments being of sufficient length to overlap one another, said segments forming a substantially straight single line of light when the common linear image of said target array is parallel to a principal meridian of said subject optical system, and said segments forming a jagged or irregular line of light whenever the common linear image of said target array is not parallel to a principal meridian of said subjecT optical system; said rotational means enablying the orientation of the common linear image of said target array to be changed from one meridian to another of said subject optical system whereby from the appearance of said test image a principal meridian of said subject optical system may be located.