An error-detecting optometer has been developed which provides high-resolution measurements of the instantaneous refractive power of the human eye. The instrument has been employed to study the dynamic and steady-state characteristics of the accommodation reflex. The error-detecting optometer utilizes a coplanar visual target and an exit slit whose position is controlled by a servomechanism. A modulated infrared beam emerges from the slit and is imaged onto the retina. A pair of lead sulfide photodetectors mounted above and below the slit are illuminated by the infrared reflected from the retina whenever the retina and the slit are not optically conjugate. The photodetectors develop a voltage whose magnitude and polarity are related directly to the degree and direction of the deviation from conjugacy. The sensitivity of the error-detecting optometer has been improved over earlier optometers by application of a null-balance technique. The optometer can be used to study open-loop accommodation responses and behavior under stabilized accommodation conditions.

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