these potentially harmful frequencies. A Cochrane review of 2018 found that there is unlikely to be a significant difference in distance vision between blue-filtering and plain lenses, and were unable to identify a difference in contrast sensitivity or colour discrimination. Another type of intraocular lens is the light-adjustable IOL, which was approved by the FDA in 2017. This particular type of IOL is implanted in the eye and then treated with ultraviolet light of a certain wavelength in order to alter the curvature of the lens. In some cases, it may be necessary or desirable to insert an additional lens over the already implanted one, also in the posterior capsule. This type of IOL placement is called "piggyback" IOLs and is usually considered as an option when the visual outcome of the first implant is not optimal. In such cases, implanting another IOL over the existing one is considered safer than replacing the initial lens. This approach may also be used in patients who need high degrees of vision correction. Whichever type is used, the appropriate refractive power of the IOL (much like an eyeglass prescription) must be selected to provide the desired refractive outcome. Traditionally, preoperative measurements including corneal curvature, axial length, and white-to-white measurements are used to estimate the required power of the IOL. These methods include several formulas, including Hagis, Hoffer Q, Holladay 1, Holladay 2, and SRK/T. There are also free online calculators using similar input data. A history of LASIK surgery requires different calculations to take this into account. Refractive results using traditional power calculation formulas leave patients within 0.5D (diopters) of target (correlates to 20/25 when targeted for distance) in 55% of cases and within 1D (correlates to 20/40 when targeted for distance) in 85% of cases. Developments in intraoperative wavefront technology have demonstrated power calculations that provide improved outcomes, yielding 80% of patients within 0.5D (20/25 or better). The cost is another important aspect of these lenses. Although most insurance companies cover the costs of monofocal IOLs, patients may have to pay the price difference if they choose more expensive lenses. Preoperative evaluation An eye examination or pre-operative evaluation by an eye surgeon is necessary to confirm the presence of a cataract and to determine if the patient is a suitable candidate for surgery. The patient must fulfill certain requirements such as: The degree of reduction of vision due, at least in large