Application Note:
Two-Dimensional (X-Y) Stiles-Crawford Effect

Dr. Philip Kruger at the State University of New York, College of Optometry, used 100 W Quartz Tungsten Halogen Oriel® Light Sources and Optical Accessories to build a system to measure the two-dimensional (X-Y) Stiles-Crawford Effect. The Stiles-Crawford Effect results from the fiber-optic nature of retinal cones. Cones have very small acceptance angles, and as a result, they only capture light from a relatively small area near the center of the pupil. Thus, light entering near the center of the pupil appears brighter than light entering near the edges of the pupil. As a result, the pupil is effectively smaller than the real pupil.

The measurements taken with the system illustrated in Fig. 1 provide a two-dimensional map of the luminous efficiency across a patient's pupil, which indicates the position in the pupil toward which the retinal cones are aligned.

A "Background Channel" presents a 7° monochromatic (552 nm) background field, and a "Target Channel" presents a monochromatic (552 nm) spot target (10 min arc) at the center of the background field. A chopper in the target channel provides a spot target flickering at 8 Hz. There is also an accommodation target for fixation. An infrared channel provides a magnified view of the patient's pupil, Purkinje Image I, and a superimposed grid pattern (0.5 x 0.5 mm graticule).

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Figure 1: Diagram of system developed by Dr. Philip Kruger at SUNY, State College of Optometry, used to measure the two-dimensional (X-Y) Stiles-Crawford effect, using 100 W QTH Oriel Light Sources.

The Experiment

Two highly stable Oriel® QTH Light Sources provide illumination for the "Background" and "Target" Channels. Pinholes in the "Background" and "Target Channels" are imaged in the subject's pupil plane as 0.5 mm sources. A pair of motor-driven stages move Source 1, the pinhole and an ND filter wedge as a unit, in the x and y directions. The background enters the eye through one of 49 positions in a 7 x 7 matrix that measures 3.6 x 3.6 mm in the pupil, and is centered on the visual axis of the eye.

Stepper motors move Source 1 in steps of 0.02 mm, which result in 0.01 mm steps in the patient's pupil. The spot target is always imaged through the center of the pupil while the background enters through 37 different positions. Background luminance remains the same for all pupil entry positions. Luminance of the spot target is altered by computer via a calibrated light valve to determine the threshold for detecting the flickering target spot on the background. The investigator via an infrared camera and telephoto lens monitors the position of the pupil and Purkinje Image 1 continuously.

The Purkinje image is used as a landmark for accurate positioning of the eye during each 2-second trial. Digitized images are used to determine the position of the Stiles-Crawford Peak with respect to the pupil edges, pupil center and visual axis. The light valve allows two-second presentations of the flickering test target on the background, while the patient indicates by button press whether the target is visible or not. Data is used to create images that simulate the effects of defocus, aberrations of the eye and the Stiles-Crawford Effect. Simulations are used to drive the focus of the eye in accommodation experiments to determine the nature of the stimulus for accommodation.