Interpupillary distance (IPD) is the distance between the center of the pupils of the two eyes. IPD is critical for the design of binocular viewing systems, where both eye pupils need to be positioned within the exit pupils of the viewing system. These viewing systems include binocular microscopes, night vision devices or goggles (NVGs), and head-mounted displays (HMDs). IPD data are used in the design of such systems to specify the range of lateral adjustment of the exit optics or eyepieces. IPD is also used to describe the distance between the exit pupils or optical axes of a binocular optical system.
Pupillary distance (PD) also describes the distance between the two pupils, but is an optometric term used to specify prescription eyewear. The PD of a patient is used to specify prescriptive eyewear for that patient. The distinction with IPD is the importance of anthropometric databases and the design of binocular viewing devices with an IPD adjustment that will fit a targeted population of users.
IPD can be precisely measured with a pupilometer. This device presents a simple binocular target that can be set from a close viewing distance out to optical infinity. Closer settings will result in an IPD reduction associated with convergent eyepieces. Some pupilometers provide a separate distance readout for the left- and right-eye—taking ocular asymmetry into account.
Anthropometric databases are available that include IPD. These include Military Handbook 743A and the 1988 Anthropometric Survey of US Army Personnel. These databases express the IPD for each gender and sample size as the mean and standard deviation, minimum and maximum, and percentiles (e.g., 5th and 95th; 1st and 99th, 50th or median). Representative data from the 1988 Anthropometric Survey are shown in the following table.
Devices such as stereo microscopes have small exit pupils, and adjustment for user IPD is necessary. These devices can be designed to fit a large range of IPDs as factors such as size and weight of the adjusting mechanism are not overly critical. In contrast to microscopes, the weight and bulk of NVGs and HMDs are large factors for wearing comfort and usability. The ANVIS NVG has an adjustment range of 52 to 72 mm. The Rockwell-Collins Optronics XL35 and XL50 binocular HMDs have a range of 55 to 75 mm. The 1988 Army Survey can be used to evaluate the percentage of the Army population captured by these ranges.
Binocular HMDs can be designed with a fixed IPD to minimize weight, bulk and cost. The fixed-IPD design strategy assumes that the exit pupil will be large enough to capture the IPD range of a targeted population. An adjustable IPD design assumes that the lateral adjustment range in conjunction with the exit pupil size is required to capture the targeted population.
IPD is also used in binocular vision science. For example, a bench-top haploscope may require setting the mirror separation for each experimental subject. Other experimental presentations may require the use of IPD to control for ocular convergence and binocular depth.
Several binocular HMDs that support night vision position the sensors on the sides of the helmet, effectively extending the IPD by approximately 4x and creating hyperstereopsis. Hyperstereopsis increases ocular convergence and causes near objects to appear closer and with exaggerated depth and slant.
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