your statement is correct
JCH MD
Thank you very much for your answer. To summarize our discussion:
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
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
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?
We would very much appreciate your answer. Thank you very much in advance.
Gwenaelle's husband
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
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