Editing Corrective lens (section)

== Refractive index ==
{{More citations needed section|date=January 2015}}
In the [[United Kingdom|UK]] and the [[United States|US]], the [[refractive index]] is generally specified concerning the yellow [[Helium|He]]-d [[Fraunhofer line]], commonly abbreviated as n<sub>d</sub>. Lens materials are classified by their refractive index, as follows:
* Normal index: 1.48 ≤ {{mvar|n}}{{sub|d}} < 1.54
* Mid-index: 1.54 ≤ {{mvar|n}}{{sub|d}} < 1.60
* High-index: 1.60 ≤ {{mvar|n}}{{sub|d}} < 1.74
* Very high index: 1.76 ≤ {{mvar|n}}{{sub|d}}

This is a general classification. Often for marketing purposes, materials with {{mvar|n}}{{sub|d}} values that are at or above 1.60 can be referred to as "high-index". Likewise, Trivex and other borderline normal / mid-index materials may be referred to as mid-index.

=== Advantages of higher indices ===
* Thinner, sometimes lighter lenses ([[corrective lens#Cosmetics and weight|See below]]).
* Improved [[Ultraviolet|UV protection]] over CR-39 and glass lenses.

=== Disadvantages of increased indices ===
* Lower [[Abbe number]], meaning, amongst other things, increased [[chromatic aberration]].
* Poorer light transmission and increased backside and inner-surface reflections (see [[Fresnel equations|Fresnel reflection]] equation), increasing the importance of [[anti-reflective coating]].
* Manufacturing defects have more impact on optical quality.{{Citation needed|date=December 2008}}
* Theoretically, off-axis optical quality degrades (oblique astigmatic error). In practice this degradation should not be perceptible – current frame styles are much smaller than they would have to be for these aberrations to be noticeable to the patient, the aberration occurring some distance away from the optical centre of the lens (off-axis).