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Face Measurements. R.D.Gopinath Supervisor – Optical Sales. 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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Face Measurements R.D.Gopinath Supervisor – Optical Sales
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
Principles in Measuring PDs • PD rule (visual axes or pupil centres) • Pupillometer (visual axes) • Near centration distance
PD Measurement devices • PD ruler. • Pupillometer • Pen torch
60 61 Binocular PDs The variation between the visual axes and pupil centres
Binocular Distance PDs Step 1 Step2 PD Rule PD Rule RE RE LE LE 0 10 20 30 0 10 20 30 40 50 62 Subject Subject Examiner Examiner RE LE LE RE
31 34 Why Monocular PD important? Wearers’ faces are not always symmetric.
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
Monocular PDs • Facial Fitting Principles PDs • Corneal reflex pupillometer (visual axes) 61mm Monocular PD Monocular PD 32mm 29mm
Fast and accurate measurement for monocular and binocular PDs Binocular PD Practitioner’s side Monocular PD Subject’s side The Pupillometer
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 )
Direct Pupillary reflex marking method A pen torch will produce clear corneal reflections
Facial Fitting Principles • PDs Near centration distance Near Centration Distance
Subject Spectacle Plane Centre of nose bridge to centre of cornea 40cm Examiner LE RE Binocular Near CDs RE LE
Centre of rotation to spectacle plane 27mm Working distance 400 mm Near Centration Distance Distance PD Facial Fitting Principles • Near centration distance • The calculation Near CD = 400 X Distance PD 427
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
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
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
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.
