Pupillary distance (PD) or interpupillary distance (IPD) is the distance measured in millimeters between the centers of the pupils of the eyes. This measurement is different from person to person and also depends on whether they are looking at near objects or far away. Monocular PD refers to the distance between each eye and the bridge of the nose which may be slightly different for each eye due to anatomical variations. For people who need to wear prescription glasses consideration of monocular PD measurement by the optician helps to ensure that the lenses will be located in the optimum position. Purchasing glasses online can be a potential problem if the PD measurement isn't available. In both the UK and most of Canada (excluding British Columbia), the PD measurement is classed as a dispensing tool rather than a part of the actual prescription of the person whose eyes were tested, thus there is no obligation for a PD to be provided on patient request. Whilst PD is an optometric term used to specify prescription eyewear, IPD is more 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. 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. Because instruments such as binoculars and microscopes can be used by different people, the distance between the eye pieces is usually made adjustable to account for IPD. In some applications, when IPD is not correctly set, it can lead to an uncomfortable viewing experience and eye strain.
Measuring pupillary distance
Different methods for measuring exist but accurate measurement can usually be determined by an ECP during an eye examination. This is normally done with a small millimeter ruler referred to as a "PD stick" or with a corneal reflex pupillometer, which is a machine calibrated to help the optical professional more accurately measure the pupillary distance. There are also mobile phone and web apps that can measure one's pupillary distance.
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.
Anthropometric databases are available that include IPD. These include Military Handbook 743A and the 2012 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.
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.
- Michel Millodot (2014-07-30). Dictionary of Optometry and Visual Science E-Book. Elsevier Health Sciences. pp. 101–. ISBN 978-0-7020-5188-3.
- David McCleary (2009). The Optician Training Manual: Simple Steps to Becoming a Great Optician. Santa Rosa Publishing. p. 120. ISBN 978-0-615-19381-6.
- Jenean Carlton (2000). Frames and Lenses. SLACK Incorporated. pp. 33–. ISBN 978-1-55642-364-2.
- "Let the Buyer Beware: A Closer Look at Ordering Eyeglasses Online". American Optometric Association. August 7, 2014. Retrieved September 28, 2017.
- "Extra charge for B.C. eye exams 'unacceptable'". CBC News. March 22, 2012. Retrieved May 3, 2014.
- "The Sight Testing (Examination and Prescription) (No. 2) Regulations 1989", legislation.gov.uk, The National Archives, SI 1989/1230
- Moffitt, K. (1997). Designing HMDs for viewing comfort. In J. E. Melzer & K. Moffitt (eds.), Head mounted displays: Designing for the user. New York: McGraw-Hill.
- J. James (2012-12-06). Light microscopic techniques in biology and medicine. Springer Science & Business Media. pp. 35–. ISBN 978-94-010-1414-4.
- Jeff W Murray (2017-06-14). Building Virtual Reality with Unity and Steam VR. CRC Press. pp. 62–. ISBN 978-1-315-30545-5.
- David McCleary (2009). The Optician Training Manual: Simple Steps to Becoming a Great Optician. Santa Rosa Publishing. pp. 116–. ISBN 978-0-615-19381-6.
- Farrell, R. J., & Booth, J. M. (1975). Design handbook for imagery interpretation equipment. Seattle WA: Boeing Aerospace Company.
- Rash, C. E. (2001). Introductory overview. In C. E. Rash (ed.), Helmet-mounted displays: Design issues for rotary-wing aircraft. Ft. Rucker AL: US Army Aeromedical Research Laboratory.
- Dodgson, N. A. (2004). Variation and extrema of human interpupillary distance. In A. J. Woods, J. O. Merritt, S. A. Benton and M. T. Bolas (eds.), Proceedings of SPIE: Stereoscopic Displays and Virtual Reality Systems XI, Vol. 5291, pp. 36–46. San Jose CA.
- Smith, G., & Atchison, D. A. (1997). The eye and visual optical instruments. Cambridge UK: Cambridge University Press.
- Gordon, C. C., Blackwell, C. L., Bradtmiller, B., Parham, J. L., Barrientos, P., Paquette, S. P., Corner, B. D., Carosn, J. M., Venezia, J. C., Rockwell, B. M., Murcher, M., & Kristensen, S. (2014). 2012 Anthropometric Survey of U.S. Army Personnel: Methods and Summary Statistics. Technical Report NATICK/15-007. Natick MA: U.S. Army Natick Soldier Research, Development and Engineering Center.
- Temme, L. A., Kalich, M. E., Curry, I. P., Pinkus, A. R., Task, H. L., & Rash, C. E. (2009). Visual perceptual conflicts and illusions. In C. E. Rash, M. B. Russo, T. R. Letowski, & E. T. Schmeisser (eds.), Helmet-mounted displays: Sensation, perception and cognition issues. Ft. Rucker AL: U.S. Army Aeromedical Research Laboratory.
- Anthony Lewis Brooks; Sheryl Brahnam; Lakhmi C. Jain (2014-01-28). Technologies of Inclusive Well-Being: Serious Games, Alternative Realities, and Play Therapy. Springer. pp. 294–. ISBN 978-3-642-45432-5.