Face Measurements
R.D.Gopinath
Supervisor – Optical Sales
ARAVIND EYE CARE SYSTEM
A RAV I N D E YE H O S PI TAL
& Postgraduate Institute of Ophthalmology
Madurai, India
Why Measure PDs ?
 Spectacle lens optical centres (OCs) should
correspond to the wearer’s PD
 To avoid unwanted prismatic effect due to
decentration especially in higher Rx powers
 Advanced lens designs must be positioned accurately
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Principles in Measuring PDs
 PD rule (visual axes or pupil centres)
 Pupillometer (visual axes)
 Near centration distance
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PD Measurement devices
 PD ruler.
 Pupillometer
 Pen torch
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How to do it…
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Binocular PDs
The variation between the visual axes and pupil
centres
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Binocular Distance PDs
Step 1
RE
0
PD Rule
Step2
LE
10 20 30
PD Rule
RE
0
10 20 30
40 50
Subject
Subject
Examiner
Examiner
RE
LE
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LE
C A R E
LE
62
RE
S Y S T E M
Why Monocular PD important?
Wearers’ faces are not always symmetric.
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PD rulers…
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Monocular PDs
PD Ruler Method
Sit directly opposite the patient (arm’s length)
Place ruler on the bridge of the patient’s nose
Hold pen torch under your left eye directed at
the patient’s RE
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Monocular PDs
 Facial Fitting Principles PDs
 Corneal reflex pupillometer (visual axes)
61mm
Monocular PD
Monocular PD
32mm
29mm
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The Pupillometer
Fast and accurate measurement for monocular and
binocular PDs
Practitioner’s
side
Binocular PD
Monocular PD
Subject’s
side
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Pupillometer
 Ability to vary working distance from 20cm to infinity
 Procedure:
 Internal hairline moved until line and corneal reflection are coincident
 Corneal reflection typically located nasal to pupil centre (2 to 5 )
Subject’s view
Practitioner’s view
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Direct Pupillary reflex marking
method
A pen torch will produce clear corneal reflections
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Facial Fitting Principles
 PDs
Near centration distance
Near
Centration
Distance
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Binocular Near CDs
RE
LE
Subject
Spectacle Plane
Centre of nose
bridge to centre of
cornea
40c
m
Examiner
LE
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RE
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Facial Fitting Principles
Centre of rotation to spectacle plane
27mm
 Near centration
distance
 The calculation
Near
Centration
Distance
Distance PD
Near CD = 400 X Distance PD
427
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Working distance
400 mm
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Sources of error taking PDs with
the PD ruler
 Parallax:
 Practitioner does not close the non-observing eye
 practitioner and patient are not directly in line
 Incorrect ruler position: not straight or close enough
to the spectacle plane, especially at near
 Significant differences in PDs of subject and examiner
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Aspherics and Near CD
 Aberration or prismatic effect
 The principal axis should pass through the eye’s centre of
rotation
 For near, this requires a distance PD
 Traditional setting for the OC for reading is the near CD
Eg. Rx = +6.00D both eyes Add = +1.00D both eyes
 Readers set at distance PD = 2.8 base out
 Readers set at near CD = 0
 Prism at near with bifocals
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= 2.4 base out
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Face measurements
To select the appropriate size frame.
 Temple length
 Length of drop / Ear piece.
 Bridge size.
 Intra temporal distance.
 Pantoscopic angle
 Vertex distance
 Segment height
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Summary
Frame & Face measurements are necessary to
 Avoid unwanted prismatic effect.
 Minimize the optical inherent defects.
 Better reading area (in bifocal)
 Correct fit on the face.
 Accurate PAL dispensing.
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Thank You
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