optometer

An instrument for determining the refraction of the eye.

[opto- + G. metron, measure]

An instrument for determining the refraction of the eye.

[opto- + G. metron, measure]

Instrument for measuring the refractive state of the eye. There are two main types of optometers: subjective and objective. Subjective optometers rely upon the subject's judgment of sharpness or blurredness of a test object while objective ones contain an optical system which determines the vergence of light reflected from the subject's retina. Electronic optometers in which all data appear digitally within a brief period of time after the operator has activated a signal can be of either type. Objective types (also called autorefractors or autorefractometers) have become very popular and several of these autorefractors are now providing both objective and subjective systems within the same instrument. Syn. refractometer. See objective accommodation; Humphrey Vision Analyser; autorefraction; infrared optometer; photorefraction; refractive error.
Badal's optometer A simple, subjective optometer consisting of a single positive lens and a movable target. The vergence of light from the target, after refraction through the lens, depends upon the position of the target. The patient is instructed to move the target towards the lens from a position where it appears blurred until it becomes clear. That point (converted in dioptric value) represents the refraction of the patient's eye. This is a crude and inaccurate instrument, in which the measurement is marred by accommodation, variation in retinal image size with target distance, large depth of focus, non-linearity of the scale, etc. Badal's improvement was to place the lens so that its focal point coincides with either the nodal point of the eye or the anterior focal point of the eye or the entrance pupil of the eye, thus overcoming the problems of the non-linear scale and the changing retinal image size (Fig. O3).
optometer of Fincham, coincidence An objective optometer which forms the image of an illuminated fine line target on the retina by passing through a small, peripheral portion of the pupil. The examiner views through a telescope with an optical doubling system, which splits the visual field into two. If the incident beam of light is not in focus on the retina, the reflected beam will not be along the optical axis and the two half-lines will be seen out of alignment. Adjusting the dioptric value of the target in order to obtain alignment gives a measure of the ametropia.
infrared optometer An optometer that uses infrared light rather than visible light. This is done so that the target used in the optometer is invisible to the patient. Otherwise when it is altered it tends to become a stimulus to accommodation. However, the instrument must be corrected for the chromatic aberration of the eye. Most modern optometers use infrared light. They are based on one of three principles: (1) retinoscopy, (2) Scheiner's experiment, (3) ophthalmoscopy (indirect).
objective optometer; subjective optometer See optometer.
Young's optometer A simple optometer consisting of a single positive lens and using the Scheiner's disc principle. The target is either a single point of light or a thread, which is moved back and forth until it is seen singly by the observer. When the target is out of focus, it is seen double and slightly blurred. See Scheiner's experiment.