The New Frisby Davis Distance (FD2) Stereo test

• First launched in 2003, the FD2 provides distance stereo acuity measurements using real objects as targets.
• A free space test of real depth, hence avoiding potential problems with stereograms (for example some young children are unwilling to wear red-green or polarizing spectacles.)
• A new streamlined design that doesn’t alter the test’s norms and scope. It is supplied flat-pack for easy assembly ensuring a much lower cost than the original FD2.
• Some research papers that have evaluated and used the FD2. The first-listed one is Bohr and Read (2103) who report that the new FD2 yields similar results to the original FD2.

Design Principles
The test uses four differently-shaped flat plastic objects mounted on rods set in a translucent support frame (fig. 1) . They are seen as free-floating objects in space without any visible means of support (fig.2). On each trial, one shape (the target) is set to be nearer to the observer than the other three shapes. The patient’s task is to say which one this is. Markings on the rods enable the disparity of the target with respect to the other shapes to be set in the range 50 to 5 sec arc for 6m viewing distance, 115 to 10 sec arc for 4m, and 200 to 20 for 3m .
The shape selected by the tester to be the target is varied from presentation to presentation. The door is closed when the rods are being adjusted to obscure the observer’s view. The side panels also serve this purpose. The tester reduces how far the target sticks out on successive trials, searching for the smallest disparity which the patient can reliably discriminate.

Clinical Benefits

• Suitable for young children for whom no other currently available distance stereo test seems appropriate
• Does not use stereograms and does not require special glasses
• Can be used at 3m, suiting clinics that do not have a 6m test alley

What’s New about the New FD2?

• Only the design of the unseen parts of the FD2 has been changed. The stimuli stay the same, yielding the same test norms.

In Development

• The New FD2 can be adapted to permit the presentation of a much wider range of target types (see references), such as spatial frequency (SF) filtered textures that have the potential to reveal useful, additional measures of distance stereo acuity when diagnosing and managing ocular disorders.

Monocular Cues

• The Test Protocol is to test first binocularly and then monocular. Only if the binocular result is better than the monocular result is the binocular threshold recorded as the FD2 stereo acuity.