ARAVIND EYE CARE SYSTEMA R A V I N D E Y E H O S P I T A L

& Postgraduate Institute of OphthalmologyMadurai, India

ARAVIND EYE CARE SYSTEMA R A V I N D E Y E H O S P I T A L

& Postgraduate Institute of OphthalmologyMadurai, India

Face Measurements

R.D.GopinathSupervisor – Optical Sales

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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 PDsThe variation between the visual axes and pupil

centres

60

61

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Step 1 Step2

Subject

Examiner

3020100

PD Rule

Subject

Examiner

30 40 50 6220100

PD Rule

RELE RELE

RE LE RE LE

Binocular Distance PDs

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31 34

Why Monocular PD important?

Wearers’ faces are not always symmetric.

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PD rulers…

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Monocular PDsPD 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 PD

Monocular PD

29mm32mm

61mm

Monocular PDs· Facial Fitting Principles PDs

- Corneal reflex pupillometer (visual axes)

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Practitioner’s side

Subject’s side

Binocular PD

Monocular PD

Fast and accurate measurement for monocular and binocular PDs

The Pupillometer

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Subject’s view

Practitioner’s view

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 )

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A pen torch will produce clear corneal

reflections

Direct Pupillary reflex marking method

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Near Centration

Distance

Facial Fitting Principles

· PDsNear centration distance

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RE LE

Centre of nose bridge to centre of

cornea

Spectacle Plane

Subject

Examiner

LE RE

40cm

Binocular Near CDs

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Near Centration Distance

Distance PD

Working distance400 mm

Centre of rotation to spectacle plane27mm

Near CD = 400 X Distance PD 427

Facial Fitting Principles

· Near centration distance

· The calculation

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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 = 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