1. Field of the Invention
The present invention relates to an apparatus for measuring refractive characteristics such as spherical power, cylindrical power, prism power etc. of an optical system, said apparatus being applicable for example to a so-called automatic lens-meter for automatically measuring the refractive characteristics of a spectacle lens or the like.
2. Description of the Prior Art
Recently there have been proposed various apparatus for use as an automatic lens-meter. These apparatus generally rely on a principle of introducing a parallel light beam into an optical system to be inspected, photoelectrically detecting the deviation of the beam deflected by the refractive characteristics in passing said optical system and determining the refractive characteristics from said deviation.
For example, the U.S. Pat. No. 4,275,964 discloses an apparatus in which a parallel light beam is guided through the optical system to be inspected and a diaphragm having annular apertures and positioned close to said optical system into a photosensor array, the refractive power of said optical system in a radial direction corresponding to the direction of the photoreceptor elements of said array being determined from the distances of the light images falling on said array.
However, such apparatus is unable to measure said distances accurately since the light images falling on the photosensor array mutually overlap in case the light source of the apparatus is approximately conjugate with the photosensitive plane of the photoreceptor elements with respect to an optical system composed of the optical system belonging to the apparatus and the optical system to be inspected.