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Measuring Interpupillary Distance
Indra Prasad Sharma
Optometrist, ERRH Mongar
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Anatomic pupillary distanceDistance from centre of one pupil to the centre of the
other pupilMeasured in millimeters(mm)
Sharma IP,
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Sharma IP,
Why to measure IPD?
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Instruments for measuring PDPD rulerRodenstock Interpupillary gaugePupillometer (Elissor pupillometer)Topcon Digital PD meter, Model PD 5Cal coast PD ruler
Sharma IP,
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Instruments for measuring PD
Sharma IP,
Cal coast PD Meter Digital Essilor Pupillometer
PD ruler
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We can use this
Sharma IP,
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Binocular Distance IPD
Sharma IP,
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Sharma IP,
A: Between center of two pupilB: Right temporal pupillary margin to left nasal
pupillary marginC:Right temporal limbus to left nasal limbus
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TechniqueSteps in Measuring binocular distance PD using a ruler
1.Dispenser position at 40 cm(16 inches)
2.Dispenser closes right eye , patient fixes at dispensers open left eye
3.Dispenser lines up the ruler zero point on the subject’s right pupil, left pupillary border or left limbus.
4. Dispenser closes left eye , patient fixes at dispensers open right eye
5.Dispenser reads scale directly in line with subject’s left pupil center ,left pupillary border or left limbus
6.Dispenser closes right eye, opens left; subject fixates on dispenser’s left eye
7.Dispenser checks to make sure that zero point is still correct
Sharma IP,
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Common difficulties and their solution
Sharma IP,
Problem Solution
1.Dispenser cannot close one eye Occlude the eye with free hand
2.Subject is strabismic Cover the subjects eye not been observed
3.Subject is uncooperative child Take a canthus- canthus measurement
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Sharma IP,
Monocular Distance IPD
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TechniqueSteps in Measuring monocular distance PD using a ruler
1.Measure the binocular PD using center of pupil as reference point
2.Before moving the ruler, note the reading at the center of the nose. This is the monocular PD of one eye
3.Stbtract this reading from the binocular reading to obtain the reading of the other eye.
Sharma IP,
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Near PD
Sharma IP,
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Near PDRequired for single vision reading glasses or for
multifocal lensesCan be either measured or calculated
Sharma IP,
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TechniqueSteps in Measuring near PD using a ruler1.Dispenser places his/her dominant eye in front of the subject’s nose at the subjects near working distance2.Dispenser close the non-dominant eye
3.Subject fixates on dispenser’s open eye.
4.Dispenser places zero point of the PD ruler at the center of subject’s right pupil.
5.Scale reading at the center of the subject’s left eye is read.
Sharma IP,
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Calculating near PDMost commonly used is called three -quarter ruleIt states that “for every diopter of dioptric demand, the
optical center of each reading lens, or the geometrical center of each bifocal addition, should be insert 0.75(three-quarters) mm.”
Dioptric demand =inverse of reading distance in meters
and is independent of actual bifocal addition power
Sharma IP,
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Example
A spectacle lens wearer has the following prescription:OD: -1.00 DSOS: -1.00 DS Add: +2.00DS
Distance PD is 64mm.What is the near PD at 40 cm?
Sharma IP,
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Solution:Dioptric demand=1/0.40 =2.50 DInsert per lens =2.50 x (3/4) =1.9mm = 2.00mm (approx)
We need to deduct 2 mm from each lens i.e 4 mm from the distance IPDNear IPD= 64mm- 64 mm =60mm
Easiest Solution: Deduct 4 mm from distance PD for normal working distance
Sharma IP,
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Orthoptics as a career (ZG)