A device is described that provides an electrical signal proportional to the instantaneous refractive power of the human eye. Infrared light illuminates a target whose optical distance from the subject’s eye can be changed rapidly. The position of this target is servo controlled in such a way that it remains conjugate with the retina regardless of changes of the subject’s state of accommodation. The position of the target provides a direct measure of refractive power. The device may be used on an undrugged eye and does not interfere with normal visual tasks.
TOM N. CORNSWEET and HEWITT D. CRANE, "Servo-Controlled Infrared Optometer," J. Opt. Soc. Am. 60, 548-554 (1970)