Pupillary distance

Pupillary distance (PD), more correctly known as interpupillary distance (IPD) is the distance in millimeters between the centers of each pupil.^[1]

Monocular PD can be measured during an eye test.

Interpupillary Distance Classifications

Distance PD is the separation between the visual axes of the eyes in their primary position, as the subject fixates on an infinitely distant object.^[2] Near PD is the separation between the visual axes of the eyes, at the plane of the spectacle lenses, as the subject fixates on a near object at the intended working distance.^[3] Intermediate PD is at a specified plane in between distance and near. Monocular PD refers to the distance between either the right or left visual axis to the bridge of the nose, which may be slightly different for each eye due to anatomical variations but always sums up to the binocular PD.^[4][5] For people who need to wear prescription glasses, consideration of monocular PD measurement by an optician helps to ensure that the lenses will be located in the optimum position.^[6]

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.^[7] 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.^[8] In some applications, when IPD is not correctly set, it can lead to an uncomfortable viewing experience and eye strain.^[9]

Measuring pupillary distance

PD measurement using an app (PD+ by Zernike)

Pupillary distance measurement with iPad app

Binocular PD measurements are done from pupil to pupil while monocular PD measurements are from either right or left pupil to center of the nose bridge. Measurements are typically reported in millimeters.^{[citation needed]} Different methods for measuring exist but accurate measurement can usually be determined by an eye care professional (ECP) during an eye examination. This is traditionally done with a small ruler referred to as a "PD stick", but nowadays it is more likely done using a corneal reflex based instrument placed on the nose bridge and work by sighting the corneal reflection produced by an internally-mounted coaxial light source (e.g. Essilor Corneal Reflection Pupillometer^[10]). This type of instrument, which is commonly referred to as a pupillometer, albeit incorrectly,^[11] may also be used to verify PD measurements taken with a PD stick.^[12]

As an alternative to having PD measured by an ECP, a variety of web and mobile (Android and iOS) apps are now widely available. Web apps are used by a variety of online sellers of eyeglasses where an object of known size, such as a credit card, is needed to assist (size reference) the measurement process.^[13][14] Some mobile apps have eliminated the need for a reference object to make accurate PD measurements by leveraging depth imaging and advanced algorithms now available on some mobile platforms.^[15] These tools have become more prevalent as online purchasing of eyewear became more popular. Purchasing glasses online can be a potential problem if the PD measurement isn't available.^[16] In both the UK and most of Canada (excluding British Columbia^[17]), the PD measurement is categorized under dispensing rather than eyewear prescription of the person whose eyes were tested, thus there is no obligation for PD to be provided on patient request.^[18]

Viewing devices

Devices such as stereo microscopes have small exit pupils, and adjustment for user IPD is necessary.^[19] 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 night vision goggles (NVGs) and helmet-mounted displays (HMDs) are large factors for wearing comfort and usability. The ANVIS 9 aviation NVGs have an adjustment range of 52 to 72 mm.^[20] The Rockwell-Collins XL35 and XL50 binocular HMDs have a range of 55 to 75 mm. The US Department of Defense 1988 Army Survey can be used to evaluate the percentage of the US 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.

Databases

Anthropometric databases are available that include IPD.^[21][22] These include the US Department of Defense's Military Handbook 743A and the 2012 Anthropometric Survey of US Army Personnel.^[23] 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 US Army's 2012 anthropometric survey are shown in the following table.

IPD values (mm) from the 2012 Anthropometric Survey of US Army Personnel

Gender	Sample size	Mean	Standard deviation	Minimum	Maximum	Percentile
						1st	5th	50th	95th	99th
Female	1986	61.7	3.6	51.0	74.5	53.5	55.5	62.0	67.5	70.5
Male	4082	64.0	3.4	53.0	77.0	56.0	58.5	64.0	70.0	72.5

Interpupillary distance (IPD) varies with respect to age, gender and race. The stereoscopic optics industry also has to take IPD variance and its extrema into account, because optical products need to be able to cope with many possible users, including those with the smallest and largest IPDs.^[24]

Other applications

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.^{[citation needed]}

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.^[25] Hyperstereopsis increases ocular convergence and causes near objects to appear closer and with exaggerated depth and slant.

IPD application is found in stereoscopy, virtual reality headsets gaming, education and training.^[26]

See also

• Eyeglass prescription
• Ophthalmology
• Optometry
• Pupilometer

References

1. Bogren HG, Franti CE, Wilmarth SS. Normal variations of the position of the eye in the orbit. Ophthalmology. 1986;93:1072–1077.
2. ANSI Z80.1-2020. Ophthalmics-Prescription Spectacle Lenses. ANSI Electronic Standards Store.
3. ANSI Z80.1-2020. Ophthalmics-Prescription Spectacle Lenses. ANSI Electronic Standards Store.
4. Michel Millodot (2014-07-30). Dictionary of Optometry and Visual Science E-Book. Elsevier Health Sciences. pp. 101–. ISBN 978-0-7020-5188-3.
5. 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.
6. Jenean Carlton (2000). Frames and Lenses. SLACK Incorporated. pp. 33–. ISBN 978-1-55642-364-2.
7. 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.
8. J. James (2012-12-06). Light microscopic techniques in biology and medicine. Springer Science & Business Media. pp. 35–. ISBN 978-94-010-1414-4.
9. Jeff W Murray (2017-06-14). Building Virtual Reality with Unity and Steam VR. CRC Press. pp. 62–. ISBN 978-1-315-30545-5.
10. Digital C.R.P., Essilor Instruments, Retrieved 21 February 2023.
11. thefreedictionary.com, Definition of "pupilometer", Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann. Retrieved 20 February 2023.
12. 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.
13. The Pupil Meter
14. "The Pupil Meter". Archived from the original on 2015-02-01. Retrieved 2013-06-10.
15. PD+, on the App Store, Retrieved February 21, 2023.
16. "Let the Buyer Beware: A Closer Look at Ordering Eyeglasses Online". American Optometric Association. August 7, 2014. Retrieved September 28, 2017.
17. "Extra charge for B.C. eye exams 'unacceptable'". CBC News. March 22, 2012. Retrieved May 3, 2014.
18. "The Sight Testing (Examination and Prescription) (No. 2) Regulations 1989", legislation.gov.uk, The National Archives, SI 1989/1230
19. Farrell, R. J., & Booth, J. M. (1975). Design handbook for imagery interpretation equipment. Seattle WA: Boeing Aerospace Company.
20. 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.
21. 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.
22. Smith, G., & Atchison, D. A. (1997). The eye and visual optical instruments. Cambridge UK: Cambridge University Press.
23. Gordon, C. C., Blackwell, C. L., Bradtmiller, B., Parham, J. L., Barrientos, P., Paquette, S. P., Corner, B. D., Carson, 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.
24. Variation and extrema of human interpupillary distance, Neil A. Dodgson, University of Cambridge Computer Laboratory, 15 J. J. Thomson Avenue, Cambridge, UK CB3 0FD
25. 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.
26. 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.

External links

• Variation and extrema of human interpupillary distance (pdf document)
• Optical Abbreviations and Glossary (pdf document)