Power Specification of Ophthalmic Lens

 POWER SPECIFICATION OF OPHTHALMIC LENSES

                F=1/FL

where F is focal power of lens

FL is the focal length of the lens in meters

Power specification of Ophthalmic lenses can be based on methods are:

    1. APPROXIMATE POWER

    2.BACK VERTEX POWER

    3.FRONT VERTEX POWER

    4.EQUIVALENT POWER

    5.EFFECTIVE POWER

1.APPROXIMATE POWER

    Approximate (close to actual)

    Algebric summation of front surface power and back surface power. Fa=F1+F2

    Where F1 is front surface power, F2 is back surface power as given by curves of             respective surfaces, measured by lens measure watch.

    Respected material index. Lens thickness not considered in this method.

2.BACK VERTEX POWER

         Reciprocal of Posterior Vertex focal length multiplied by refractive index of the lens.

            BVP = [F1/(1-tc/nlens(F1))] +F2

            Where, F1 - Front surface power

            F2 - Back surface power

            tc- Geometrical Center thickness

            nlens- Refractive index of lens material

3.FRONT VERTEX POWER

        FVP = [F2/(1-(tc/nlens)(F2))]+F1

Where, F1 = Front surface power

F2 = Back surface power

tc = Geometrical Center thickness

nlens = Refractive index of lens material

4.EQUIVALENT POWER

    Many optical devices are constructed with a series of lenses separated by air or arranged with series of curve surfaces separated by a medium.

    The entire system is devised imagined to have one single focal point like a thin lens.

    The focal length produced by system is called equivalent focal length and reciprocal of focal length in metres is defined as equivalent power of the lens system.

5.EFFECTIVE POWER

                F’ = F/(1 - dF)

where , F’ = Effective power of the lens at its new distance 

F = Effective power of the lens at its initial distance

d = Change in vertex distance in meters ( d is positive if the vertex distance increases and d is negative if vertex distance decreases)

Hint: myopes need less minus and hyperopes need more plus when going from spectacles to contact lenses

Remember CAP – Closer Add Plus.

PROBLEMS:

1.A +11.00 diopter lens mounted 11mm in front of the cornea would require what contact lens power?

F’ = F/(1 - dF)

Fnew = Fcurrent/(1-dFcurrent) = +11.00/(1-0.011(+11.00)) = +12.51D

2.For a myopic eye that can be corrected with a –13.00 diopter lens mounted 13 mm in front of the cornea would require what contact lens power?

Fnew = Fcurrent/(1-dFcurrent) = -13.00/(1-0.013(-13.00) = -11.12D