In ophthalmologic examination, the unit of refraction is D, which indicates the refractive ability of the eyeball to light rays, and it is stipulated that the refraction of the normal eyeball is 0 D. According to the difference of refraction, it can be categorized into orthopia, myopia, hyperopia and other refractive states. 1. Orthopia: refers to the refractive system of the eye after the external parallel light rays converge in the macular center of the retina, and the refractive range is -0.5D~+0.5D. 2. 2. Myopia: refers to the parallel light rays from the outside world, after passing through the refractive system of the eye, converge in front of the retina. Myopia ranges from -0.5D to -3.0D for low myopia, -3.0D to -6.00D for moderate myopia, and over -6.00D for high myopia. 3. Hyperopia: refers to the parallel light rays from the outside world that converge behind the retina after passing through the refractive system of the eye. 0.5D to 3.0D is considered low hyperopia, 3.0D to 5.00D is considered moderate hyperopia, and more than 5.00D is considered high hyperopia. In general, orthokeratology is normal. If refractive errors such as myopia or hyperopia occur, timely medical correction is recommended.