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Vertex Distance and Calculations

Vertex Distance and Calculations. R.D.Gopinath Supervisor – Optical Sales. Introduction. Vertex distance is from the back surface of the spectacle lens to the front surface of the eye.

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Vertex Distance and Calculations

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  1. Vertex Distance and Calculations R.D.Gopinath Supervisor – Optical Sales

  2. 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. 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. VD changes & the effect in ‘+’ lens • 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.

  5. 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. 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. 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 PowerDl= Original Lens Powerd= Change in Vertex Distance in Meters.

  8. Vertex compensation power • A spectacle power of -5.00 D Sph 6mm further from the eye than it should, then > The best sphere is – 4.85 D.

  9. 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. 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. 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. 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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