Orthokeratology (also referred to as Ortho-K, Overnight Vision Correction, Corneal Refractive Therapy and CRT), refers to the creation of gas permeable contact lenses that temporarily reshape the cornea to reduce refractive errors such as myopia, hyperopia and astigmatism. This method can be used as an alternative to eyeglasses, refractive surgery, or for those who prefer not to wear contact lenses during the day. Orthokeratology is most often used for candidates with up to -6.00 diopters of myopia.