FRISBY STEREO TEST - 3 PLATES

Manufacturer: FRISBY STEREO TESTS
Product code: ACEY026F
 

The Frisby Near Stereo Test has 3 plates of different thicknesses, providing stereo acuity measurements in the range 600-5 sec arc. They are stored in a box with a fold-down flap that provides a clear background. A tape measure is provided for controlling eye-to-plate distance.

  • First launched in 1983, it is used in orthoptic and ophthalmic clinics and by optometrists world-wide.

  • Tests for stereopsis using natural vision, with no special glasses. It has three transparent plates, which are presented to the patient one at a time against the clear background provided by the fold-down flap of the storage box.

  • Tests for stereopsis, even if reduced vision or amblyopia is present because of the coarse texture elements included in the pattern.

 

The plate is shown a few cm or inches above a clear background, such as the fold-down flap on the box as shown here. Note the tape is fixed to the flap for measuring and controlling eye-to-plate distance.

How the test works

  • The target is a cluster of randomly arranged arrowheads of differing sizes, as shown alongside.

  • It is printed on one side of the plate in one of the four quadrants; on the other side of the plate similar pattern elements are printed around the target and in the other three quadrants.

  • An observer with normal binocular stereovision can readily detect the target because it appears to stand out from the background or to recede from the background, depending which way round the plate is shown.

  • Such an observer is essentially seeing the thickness of the plate by virtue of the texture elements printed on the two sides.

  • An observer lacking normal binocular stereovision (or a normal observer viewing with one eye only) fails to detect the target as it can be distinguished only on the basis of binocular disparity cues to depth, as long as the plate is shown correctly (stationary, viewed square-on, and placed about 5-10cm in front of a clear background).

Stereopsis screening

  • For stereopsis screening the test objective is to discover whether the patient can reliably discriminate the circle-in-depth using the thickest plate.

  • Present the 6mm plate with the position of the circle-in-depth chosen randomly. The stud alongside the target has a flat surface, allowing the tester to know discretely by touch where the target is whilst following the observer’s eye movements. Hence the position of the target can be selected discretely by feeling for the stud with the flat top.

  • Patients with stereopsis usually find the target-in­-depth quickly and confidently, perhaps pointing to it if young, or staring at it.

  • If necessary, use a sequence of presentations varying the position of the target randomly. Varying plate position can be done while the tester holds the plate behind her or his back. Try making a game of the test saying “Can you find the hidden target?”

  •  Use as many trials as needed to be able to make a confident decision.  A single very speedy correct response may be sufficient but sometimes 2 or 3 presentations are needed for a cautious patient to decide if stereopsis is or is not demonstrated.

  • Patients with defective stereopsis usually make slow hesitant responses with a high frequency of errors.

 

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© Copyright Grafton Optical Company Ltd 2017, United Kingdom