How to calculate the refractive power of crystal

1.

The eye is an organ that can sense light. The simplest eye structure can detect the light and shade of the surrounding environment, and the more complex eye structure can provide vision. In many vertebrates and some mollusks, the eyes image by projecting light onto the light sensitive retina, where light is received and converted into signals that pass through the optic nerve to the brain

make the divergent light from the near object bend and image on the retina, then the near object can be seen clearly, which is called regulation. The distant point of emmetropia is more than 5 meters, and the distant point of myopia is less than 5 meters. The formula of D = 1 / F (diopter = 1 meter / focal length) can be used to calculate the distant point of myopia, that is, the distance that myopia can see the target. For example, the distance that 4.00d myopia can see the target is 0.25 meters

extended data

in the process of eye adjustment, abnormal adjustment occasionally occurs, the main reasons are as follows:

1, insufficient adjustment, accompanied by exophora, mostly caused by long-time close work

2. Hyperadjustment was more common in children. When looking at close objects, the regulatory response was greater than the regulatory stimulus

However, the response was slow when the distance and distance were alternately fixed

3. In terms of accurate diopter correction, lens implantation is not as good as spectacle correction, because once the implanted lens is put into the eye, it will be "dead card" and can not be exchanged or fine tuned, and the progressive value of lens diopter is larger than that of lens. This is the reason why there is a certain deviation in the correction of myopia with intraocular lens. The problem of seeing things in double images depends on whether they are monocular or binocular. ① Monocular diplopia shows that there is astigmatism in the eyes, which is the combined effect of crystal and eyes in optics. In order to overcome this astigmatism, glasses can only be used to correct it, otherwise visual fatigue will occur. ② If it is binocular diplopia, it may be binocular vision dysfunction - cryptostrabismus. High myopia itself has exotropia, which can be better corrected by glasses. When the condition of spectacle correction is removed after implantation of intraocular lens, esotropia may reappear. As for the recovery of "close look" (if you "close look" for a long time, you will be more prone to visual fatigue), it may be that the current refractive state of the eyes has the tendency of hyperopia. You might as well understand the current refractive state of the eyes through optometry.

4. Hello, the total refractive power of cornea is about + 43D, which accounts for more than 2 / 3 of the total refractive power of eyeball optical system.

5. Unknown_Error

6. Astigmatism refers to the fact that the focus of each meridian of the parallel light entering the eyeball is not in one section. There are many causes of astigmatism, the more common are:

(1) curvature astigmatism: caused by abnormal changes in corneal curvature. If the meridian with the largest refractive power and the meridian with the smallest refractive power are perpendicular to each other, regular astigmatism is caused, which is mostly congenital, and the degree of astigmatism is large. If the surface of the cornea is irregular deformation, uneven bending, unable to form a focus on the retina, it is called irregular astigmatism, such as corneal traumatic scar, keratoconus, corneal degeneration and so on< (2) eccentric astigmatism: it used to be seen in lens displacement, such as congenital deviation and subluxation; In recent years, e to the increase of refractive cornea, astigmatism caused by PRK and LASIK facula deviation from the optical center can also be seen in clinic< (3) refractive rate astigmatism is caused by the change of refractive index of lens. Astigmatism degree is generally small, such as senile cataract, lens cortex water gap, feathery opacity, resulting in changes in the refractive index of the lens, can form astigmatism< According to the refraction, astigmatism can be divided into irregular astigmatism and regular astigmatism. Irregular astigmatism refers to the curvature of the meridian is not consistent, which can not be corrected by ordinary cylindrical mirror; Regular astigmatism means that the meridian with the largest curvature is just perpendicular to the meridian with the smallest curvature, and better vision can be obtained by correction with cylindrical lens. Among them, regular astigmatism can be divided into:

(1) simple myopic astigmatism: the parallel light on one main meridian is imaged on the retina, and the parallel light on another meridian perpendicular to it is imaged in front of the retina

(2) simple hyperopia astigmatism: the parallel light on one main meridian is imaged on the retina, and the parallel light on another meridian perpendicular to it is focused on the posterior retina< (3) compound myopic astigmatism: on two main meridians which are perpendicular to each other, the parallel rays are imaged in front of the retina, but their refractive power is not equal< (4) compound hyperopia astigmatism: on two main meridians which are perpendicular to each other, parallel rays are imaged behind the retina, but their refractive power is not equal

(5) mixed astigmatism: on two main meridians perpendicular to each other, one of the parallel rays is imaging in front of the retina, and the other is focusing imaging in the back of the retina.

7. The corneal diopter is about 4300 degrees, and the lens has the corresponding diopter, so that the object image can be projected to the retina.

8. There is a conversion formula for the power of contact lens. The diopter of contact lens = the diopter of frame lens ÷ 1－0.012 × Frame glasses diopter), 1 diopter is equivalent to the usual said 100 degrees, myopia diopter is negative, hyperopia diopter is positive
in general, if the astigmatism degree is below 50 degrees, it can be converted into myopia degree, and only use myopia lens to correct. For astigmatism above 100 degrees, the effect of ordinary contact lens correction is not ideal, and special astigmatism contact lens is generally required. Generally speaking, the degree of astigmatism of the eye is halved and added to the degree of myopia
low astigmatism below 0.75D, high astigmatism below 0.75D

9. It is difficult to measure the diopter of lens in living eyes. Even the specimen has to be calculated.