Pupillary distance

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