Working distance detecting device for ophthalmic instruments

Claim 1

An ophthalmic instrument comprising an observing optical system for observing a patient's eye, said observing optical system having objective lens means adapted to be located at a working distance from the cornea of a patient's eye, an illuminating optical system having an illuminating optical axis for projecting beams of illuminating light to the patient's eye through the objective lens means, said illuminating optical system including a first annular aperture adapted to be located at a position substantially conjugate with the cornea of the patient's eye with respect to the objective lens means, a second annular aperture spaced by a predetermined distance along said illuminating optical axis from said first annular aperture, at least one light-shading portion provided in one of said annular apertures and having a selected width in the circumferential direction, light source means for projecting the beams of illuminating light to the eye through said apertures and the objective lens means, and a working distance detecting device comprising a light-receiving portion disposed around said objective lens means at a position corresponding to said light-shading portion of an image of said one annular aperture formed by beams of the illuminating light reflected from the cornea of the eye, whereby the working distance between the objective lens means and the cornea of the eye is detected based on the width of the light-shading portion of the image of said one annular aperture at said light-receiving portion.