Measuring Interpupillary Distance

Indra Prasad Sharma

Optometrist, ERRH Mongar

Anatomic pupillary distanceDistance from centre of one pupil to the centre of the

other pupilMeasured in millimeters(mm)

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Why to measure IPD?

Instruments for measuring PDPD rulerRodenstock Interpupillary gaugePupillometer (Elissor pupillometer)Topcon Digital PD meter, Model PD 5Cal coast PD ruler

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Instruments for measuring PD

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Cal coast PD Meter Digital Essilor Pupillometer

PD ruler

We can use this

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Binocular Distance IPD

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A: Between center of two pupilB: Right temporal pupillary margin to left nasal

pupillary marginC:Right temporal limbus to left nasal limbus

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

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Common difficulties and their solution

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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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Monocular Distance IPD

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.

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Near PD

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Near PDRequired for single vision reading glasses or for

multifocal lensesCan be either measured or calculated

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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.

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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

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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?

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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

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Orthoptics as a career (ZG)