Diplopia or doubling of vision occurs when there is sufficient binocular disparity present so that the bounds of Panum's area (the area of single vision) is exceeded. This condition arises whenever one object is significantly closer (or farther) than another so that looking at one will cause the image of the other to appear double. This can be easily demonstrated: Close one eye and look at a clock (or other object) on a distant wall. Now place your thumb to one side of the image of the clock. Now open both eyes. If you look at the clock, you should see two thumbs. If you look at your thumb, you should see two clocks.

Complaints from pilots flying aircraft equipped with wide field of view head up displays (HUDs) such as the LANTIRN HUD indicated that they were experiencing discomfort (eye fatigue, headaches, and so forth.) or seeing either two targets or two pippers (aiming symbols on the HUD) when using the HUD. Subsequent investigations revealed that the problem arose from the fact that the aircraft transparency and the HUD significantly changed the optical distances of the target and the HUD imagery so that binocular disparity, which exceeded Panum's area was induced. Use of this test method provides a procedure by which the amount of binocular disparity being experienced by a human operator due to the presence of a transparent part in his field of view may be easily and precisely measured.

1.1 This test method covers the amount of binocular disparity that is induced by transparent parts such as aircraft windscreens, canopies, HUD combining glasses, visors, or goggles. This test method may be applied to parts of any size, shape, or thickness, individually or in combination, so as to determine the contribution of each transparent part to the overall binocular disparity present in the total “viewing system” being used by a human operator.

1.2 This test method represents one of several techniques that are available for measuring binocular disparity, but is the only technique that yields a quantitative figure of merit that can be related to operator visual performance.

1.3 This test method employs apparatus currently being used in the measurement of optical angular deviation under Method F 801.

1.4 The values stated in inch-pound units are to be regarded as standard. The values given in parentheses are mathematical conversions to SI units that are provided for information only and are not considered standard.

1.5 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility of the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatory limitations prior to use.