Contact Lens Power Calculation- How to convert spectacle power into contact lens power.

Introduction of Contact Lens Power Calculation:

• Generally, we wear spectacle 12-14 mm(vertex Distance) away from our eye.
• But there is no significant gap between the cornea and contact lens, due to which contact lens power is different from spectacle power.
• Spectacle power may be only Spherical, Only Cylindrical and Sphero-Cylindrical.
• The formula to identify Contact lens power is:

CL = SP / 1 – (SP X VD)

• Here,
  • CL = Contact Lens Power.
  • SP = Spectacle Power.
  • VD = Vertex Distance in Meter.

Contact Lens Power = Spectacle Power / 1 – (Spectacle Power X Vertex Distance).

How to convert Spherical Power into Contact Lens Power:

Example: Suppose Spectacle power in both eye is -5.0D and Vertex Distance is 12 mm. What will be Estimated Contact Lens Power?

• Here,
  • SP = -5.0D
  • VD = 12 mm or 0.012 m (12 / 1000)

• According to Formula:
  • CL = SP / 1 – (SP X VD)
  • CL = -5.0 / 1 – (-5 X 0.012)
  • CL = -5.0 / 1 – (-0.06)
  • CL = -5.0 / 1 + 0.06
  • CL = -5.0 / 1.06
  • CL = -4.7169
• The nearby available power is: – 4.75DS

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How to convert Sphero-Cylinder Power into Contact Lens Power:

• Conversion of Sphero-Cylinder Power into contact lens power is bit different. We can convert sphero-Cylinder power into contact lens power in 3 steps:
  • Step-1: Make an optical cross & distribute the power meridian wise.
  • Step-2: Convert the power of each meridian using formula: CL = SP / 1 – (SP X VD)
  • Step-3: Now convert the optical cross into Sphero-cylindrical form.

Example: A Patient both eye power is – 5.0/- 3.0 x 180 and Vertex Distance is 12 mm. What will be the contact lens power?

Step-1: Make an optical cross & distribute the power meridian wise:

• If we distribute power in an optical cross, then we have:
  • In One meridian (90 deg): -8.0DS
  • In Other Meridian (180 deg): -5.0DS

Step-2: Convert the power of each meridian using formula: CL = SP / 1 – (SP X VD).

• In One meridian (90 deg), we have:
  • SP = -8.0DS.
  • VD= 12 mm or 0.012m.
• So, Contact Lens power will be:
  • CL = SP / 1 – (SP X VD)
  • CL = -8.0 / 1 – (-8.0 X 0.012)
  • CL = -8.0 / 1 – (-0.096)
  • CL = -8.0 / 1 + 0.096
  • CL = -8.0 / 1.096
  • CL = -7.29927
• Nearest available power: -7.25DS

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• In Other meridian (180 deg), we have:
  • SP = -5.0DS.
  • VD= 12 mm or 0.012m.
• So, Contact Lens power will be:
  • CL = SP / 1 – (SP X VD)
  • CL = -5.0 / 1 – (-5.0 X 0.012)
  • CL = -5.0 / 1 – (-0.6)
  • CL = -5.0 / 1 + 0.06
  • CL = -5.0 / 1.06
  • CL = -4.7169
• Nearest available power: -4.75DS

Step-3: Now convert the optical cross into Sphero-cylindrical form.

• Now, if we convert the optical cross power into Sphero-cylindrical power then we got: -4.75/ – 2.50 x 180.
• So, our initial spectacle power was – 5.0/- 3.0 x 180. After converting into Contact Lens Power, we got: : -4.75/ – 2.50 x 180.

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