Astigmatism may be corrected with eyeglasses, contact lenses, or refractive surgery. Tunable metamaterials allow arbitrary adjustments to frequency changes in the refractive index. Silica fibers are glasses, disordered materials with microscopic variations of density and refractive index. Modern Fresnel lenses usually consist of all refractive elements. Many coatings consist of transparent thin film structures with alternating layers of contrasting refractive index. All represent advances over radial keratotomy in the surgical treatment of refractive errors of vision. Therefore, keratoconus is a contraindication to refractive surgery. For infrared light refractive indices can be considerably higher. Vision loss can occur and is associated with strabismus, refractive errors, and anisometropia. This is related to the opening angle of the lens and its refractive index. Refractometers, which measure refractive indices, often use the critical angle. A refractometer is the instrument used to measure refractive index. Concentrated photovoltaic trackers are used with refractive and reflective based concentrator systems. Temperature has a very important influence on the refractive index measurement. The scale can be calibrated in Brix or refractive index.