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IOL power

Since my husband has to have a cataract surgery on one of his eyes soon, we have been gathering various information on IOL.  We are also reading some medical literature, and one of the confusing things is the definition of IOL power.  We need to know the exact definition of the power of a lens in general, and the power of an IOL in particular.  We searched for the answer on the Internet, but we didn't get a clear-cut answer for this question.  Could you please tell us the exact definition of the power of an IOL in terms of IOL's focal length?

Thank you very much.

Gwenaelle
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di·op·ternoun /dīˈäptər/
diopters, plural; dioptres, plural

1.A unit of refractive power that is equal to the reciprocal of the focal length (in meters) of a given lens

The unit of all IOLs are diopters.

JCH MD
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Hi, Dr. Hagan,

Thank you very much for your answer.  Sorry it took time to respond.  We have just gotten back the Internet connection.  We have had problems since the last weekend.

As I understand, if the IOL power is, say, 10, then its focal length is 1/10 m = 10 cm.  Is this correct?

Thank you very much in advance.

Gwenaelle
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You got it.
JCH MD
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Hi, Dr. Hagan,

Thank you very much for your answer.  I'm Gwenaelle's husband.  We also believe that IOL power is defined the way you have described.

However, some of the literature on the Internet gives us an impression that some people use a different definition.  For example the following sites:

http://spie.org/x32355.xml

http://doctor-hill.com/iol-main/piggyback.htm

According to these sites, it seems that the power is defined by

power = index of refraction of the surrounding medium / focal length of the lens

For example, if IOL power is 10 diopter, and the IOL is in water, whose index of refraction is 1.336, then we have

10 = 1.336/f

Therefore, we have f (focal length) = 1.336/10 m = 0.1336 m = 13.36 cm

Thus the same IOL power 10 may correspond to the focal length of 10 cm or 13.36 cm, depending on the definition.  This is very confusing to me.

So, do you think the other definition is simply wrong?

Gwenaelle's husband

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The definition of a diopter is the same the world over and never redifined in IOL calculations.  Your formula is for determining powers of IOLs when put into the eye "piggy-back" that is using two IOLs in the eye.

JCH MD
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Let 2 definitions of IOL power (p) in diopter be

(a) p = 1 / f
(b) p = n / f

where f is the focal length of the IOL in meter (m), and n is the index of refraction of the surrounding medium.

I believe that all of us are saying that the definition (b) is wrong, and (a) is correct.

If you think I am summarizing correctly, could you please let me know?

Thank you very much.

Gwenaelle's husband

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JCH MD
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