Geometrical optical analysis of photorefractive methods.

Photorefractive methods allow rapid measures of the refractive and accommodative state of infants and young children whose brief attention and co-operation limit the use of more traditional methods such as retinoscopy and autorefraction. Three methods have been defined: orthogonal, isotropic and eccentric. We provide a common geometrical optical analysis for these three methods where the photorefractive pattern is defined at the plane of focus of the camera. Since this plane is conjugate with the detector plane of the camera then the critical optical parameters can be defined without reference to the design of the camera by simply determining the relative magnification of the projected image of an object photographed at the camera's plane of focus. Specifically the pattern width CF (mm) over a photorefractor's working range can be defined for the isotropic and orthogonal methods as: [formula: see text] and for the eccentric method as: [formula: see text] where: K is a myopic refractive error of the eye (dioptres) (K Less than 0); P the distance of the flash source to the eye (dioptres); L the distance in front of the eye of the camera's plane of focus (dioptres) (L less than 0); GH the pupil diameter (mm); e the eccentricity of the flash source from the camera aperture (mm); M magnification of the image measured relative to the camera plane of focus.