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# 澳洲代考|光学代考Optics代考|Crystalline Lens: Shape & Optical Properties

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## 澳洲代考|光学代考Optics代考|Lenticular Toricity and Asphericity

Like the cornea, the lens has both toricity and asphericity.
Toricity: The lenticular optical power varies along different meridians. Typically, the largest optical power in the lens is along the horizontal meridian. ${ }^{107}$

Asphericity: The asphericity of the lens in young people (up to 40 years) has the same magnitude and the opposite sign (it is positive, i.e., under-corrected) compared to the cornea (which is negative, i.e., over-corrected), thus contributing to a reduced ocular spherical aberration. ${ }^{108}$

With increasing age (e.g., 60 years), there is a reversal in the spherical aberration algebraic sign from negative to positive: The lenticular spherical aberration gradually changes to about 0 by the age of 40 , then becomes positive $\left(+0.25 \mu \mathrm{m}\right.$ RMS). ${ }^{109}$ Thus, on average, at this age, the total ocular spherical aberration RMS is about $+0.50 \mu \mathrm{m}:+0.25 \mu \mathrm{m}$ from the cornea and $+0.25 \mu \mathrm{m}$ from the crystalline lens.

The clinical relevance of the above is important in the selection of an aspheric intraocular lens (IOL) in cataract surgery and in achieving corneal asphericity via laser vision correction procedures. In the first case, the aim is to maintain crystalline lens asphericity. Aspherical IOLs today such as the Tecnis (Advanced Medical Optics) $-0.27 \mu \mathrm{m}$, SofPort AO (Bausch + Lomb), and the AcrySof IQ (Alcon) $-0.20 \mu \mathrm{m}$, offer negative spherical aberration, contributing to zero total ocular spherical aberration. In the second case, the aim is to maintain corneal asphericity, i.e., a custom-designed ablation profile aims to preserve the corneal asphericity, the objective being to preserve total ocular spherical aberration.

## 澳洲代考|光学代考Optics代考|Refractive Indices in the Eye

Water is the main component of all of the ocular tissue, so the majority of the refractive indices of the different parts of the eye are around 1.33; individual differences in tissue composition account for the different indices. Thus, the value of the refractive index for the tear film is $n \approx 1.333$, for the cornea (mainly, the stroma) is $n \approx 1.376$, for the aqueous is $n \approx 1.336$, and for the vitreous is $n \approx 1.336$.

The crystalline lens refractive index varies from a peripheral (capsule) $n \approx 1.39$ to the center (nucleus) $n \approx 1.42$. This is called a gradient index (gradient index lens – GRIN). In the Gullstrand model (§ 1.5.2.2) that integrates six refractive surfaces, an additional pair of surfaces represents an added lens ‘in the lens’ with n=1.406, while the capsule has n=1.386.

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