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ARAVIND EYE CARE SYSTEM Aravind Eye Hospital & Postgraduate Institute of Ophthalmology Madurai, India ARAVIND EYE CARE SYSTEM Aravind Eye Hospital & Postgraduate.

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Presentation on theme: "ARAVIND EYE CARE SYSTEM Aravind Eye Hospital & Postgraduate Institute of Ophthalmology Madurai, India ARAVIND EYE CARE SYSTEM Aravind Eye Hospital & Postgraduate."— Presentation transcript:

1 ARAVIND EYE CARE SYSTEM Aravind Eye Hospital & Postgraduate Institute of Ophthalmology Madurai, India ARAVIND EYE CARE SYSTEM Aravind Eye Hospital & Postgraduate Institute of Ophthalmology Madurai, India Vertex Distance and Calculations R.D.Gopinath Supervisor – Optical Sales

2 A R A V I N D - M A D U R A I Introduction  Vertex distance is from the back surface of the spectacle lens to the front surface of the eye.  The vertex distance affects the effective power of the lens, especially in higher powered (>4.0D) prescriptions.  The standard vertex distance is about 12mm.

3 A R A V I N D - M A D U R A I Lens Effectivity  Lens optical effect may vary with vertex distance.  Vertex distance responsible for decrease of vision.  If moves away from eye, + lens becomes stronger. -- lens becomes weaker.

4 A R A V I N D - M A D U R A I  Increasing the vertex distance of plus lens will increase the effective power of the lens.  Decreasing the vertex distance of a plus lens will decrease the effective power of the lens. VD changes & the effect in ‘+’ lens

5 A R A V I N D - M A D U R A I VD changes & the effect in ‘—’ lens  Increasing the vertex distance of a minus lens will decrease the effective power of the lens.  Decreasing the vertex distance of a minus lens will increase the effective power of the lens.

6 A R A V I N D - M A D U R A I Significance of VD  In prescription must have the same effective power as the refraction test.  The vertex distance of the phoroptor / trial frame must match the VD of the spectacle lenses.  A vertex distance becomes significant if the diopter power of the prescription exceeds 4.0D.

7 A R A V I N D - M A D U R A I Vertex compensation power A spectacle is placed at 20mm away from the eye instead of 12mm, so what is the actual prescription? To determine this we need to use the vertex compensation formula: Dc= Compensated Power Dl= Original Lens Power d= Change in Vertex Distance in Meters.

8 A R A V I N D - M A D U R A I  A spectacle power of -5.00 D Sph 6mm further from the eye than it should, then > The best sphere is – 4.85 D. Vertex compensation power

9 A R A V I N D - M A D U R A I Vertex Compensation Formula  The formula for the needed compensation per mm.of displacement, per diopter of lens power, is as follows: = D x D / 1000  The answer is multiplied by the mm of displacement. The result is added or subtracted from the diopter power according to the following set of conditions:  1) + lens moving closer - add to increase the diopter power  2) + lens moving farther away - subtract to reduce the diopter power  3) -- lens moving closer - subtract to reduce the diopter power  4) -- lens moving farther away - add to increase the diopter power.

10 A R A V I N D - M A D U R A I Example –spherical power lens Consider a -12.00 Sph refracted at 13mm. The lens in the patient's new glass will sit 10mm away from the patient's eye. (12 squared = 144), 144/1000 = 0.14 The movement is 3mm closer to the patient's eye, with a minus lens. 3 x.14 = 0.42D, so 0.5 D is subtracted from -12.00 to reduce the lens power to -11.50 D.

11 A R A V I N D - M A D U R A I Example – cylindrical power lens  If the Rx has a significant cylinder power (at least 1 D), we must perform the calculation for the primary meridians of power.  A +12.00D S/ +3.00x180 lens, refracted at 14mm & Spec’s on pt’ face 10mm away.  For +1200: 12 squared is 144, 144/1000 =0.14 mm.The movement is 4mm closer to the patient's eye, with ‘+’ lens.  4 x.14 =0.48, so 0.5 D is added to +12.00 to increase the lens power to +12.50D.  For +15.00: 15 squared is 225, 225/1000 = 0.225  4 x.225 = 0.9, so 1 D is added to +15.00 to increase the lens power to +16.00.  Our adjusted Rx is: +12.50D S /+3.50 x 180*.

12 A R A V I N D - M A D U R A I How to measure VD?  To determine VD use Distometer.  Distometer  places one arm on the eye lid while the other is placed on the back of the lens, and  a small scale attached to the device measures the distance.


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