What Is a Diopter?

Diopter is a unit of refractive power, and is represented by D, which means that parallel light passes through the refractive substance, and the refractive power of the refractive substance is 1 diopter or 1D when the focal point is 1m.

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In terms of lenses, when the unit of lens power is, for example, the focal length of a lens is 1m, the refractive power of this lens is 1D diopter and focal length or response.

Chinese name: Diopter
Foreign name: Dioptre (English), Diopter (US)

Attributes: physics
Size unit: Refractive power
Unit: D

When a light beam enters another substance with a different optical density, the propagation direction of the light beam is deflected. This phenomenon is called diopter phenomenon.

The main methods are as follows:

The stronger the refractive power, the shorter the focal length. The focal length of the 2D refractive lens is 1 / 2m or 50cm. If you want to know the focal length of the lens, divide 100cm, which is 1.00m by the refractive power, and the result is the focal length. For example, the focal length of 5D refractive power is 20 cm. (100cm divided by 5D = 20cm.) The refractive power of a convex lens is represented by a "+" sign, and the refractive power of a concave lens is represented by a "-". 1 diopter or 1D is equal to 100 degrees.

Degree of glasses = diopters × 100

On some optical instruments, such as cameras, telescopes, microscopes, etc., all users are considered when they are not convenient to watch with glasses.

First point of diopter

For a convex lens made of the same material, the greater the convexity, the greater the refractive power, and the smaller the opposite. In other words, for the same eyeball, myopia

Diopter The higher the number, the more prominent the eyeball, and the higher the degree of myopia required to wear.

Second point of diopter

The refractive system of the eyeball is an adjustable "convex lens", so the shape is variable. When a concave lens is placed in front of the eye, the eyeball still has self-adjusting function. The eye can see targets at different distances, and patients with myopia or presbyopia can adapt to wearing glasses. This speaks for itself.

Third point of diopter

Because ordinary glasses are separated from the eyeball, the image is intuitive and easy to calculate. The focus of this section is on the effect of eyeglasses on eye refraction. The discussion of eyeglasses is aimed at ordinary eyeglasses. Wearing contact lenses has the same effect on refractive power as ordinary glasses. The principle and technology of wearing contact lenses are already mature in the eyewear industry, so they will not be discussed further.

Fourth point of diopter

In refractive optics, only in some special cases, the refractive effect produced by the combination of the two lenses with the diopters P1 and P2 is the lens with the diopter P1 + P2. In the optical path composed of the eyeball and the lens, there can also be P1 + P2 in the effect or qualitative calculation. This is not the actual refractive effect after the lens combination, but a simplification and approximation, because the eye has Ability to change diopter. Although it is difficult to verify experimentally, from the perspective of the adjustment effect of the eyeball, it should have the effect of canceling the diopter of the lens, but the formula has the effect of simplifying the calculation. For a system composed of an eyeball and a lens, at most it is a refractive system composed of two lenses, so it can be calculated using the theory of refractive optics. When wearing the lens, due to the special adjustment of the eyeball, the refractive power of the lens can be added to and subtracted from the adjusted diopter, and the approximate value can also be obtained. But close. In this argument, although theoretically derived, experiments and measurements are very difficult, just like the preparation of myopic lenses requires trials, and experiments are also conducted in the process of guiding opticians.

Fifth point of diopter

According to the refractive characteristics of the eyeball, someone has measured the static refractive power of the eyeball as + 58.6D. Although this is a special case, it also basically reflects that the eyeball has a strong

The adjustment of the refractive power is relatively small. The normal eye is about 0-10D, and the near-sighted eye is n-10D (n refers to the myopic refractive power of the eyeball), and it is fixed in the orbit. It can be said that the distance of the eyeball's refractive system, the center of the "lens", to the retina does not change. In future calculations, the image distance can be considered to be constant K. For the refractive power of the eyeball, if it can be on the retina Into a clear image, the refractive system still meets the lens imaging formula

1 / u + 1 / k = P

Among them, K is a constant, P is the refractive power of the eyeball and is a variable, which means that different people look at the target at different distances and different people have different refractive powers. U refers to the distance from the target to the eyeball. The condition for this formula to be true is that at a certain time, the eye looks at a target at a certain distance, and the target is between the near and far points of the eye. From the formula, 1 / u = 0 when looking at infinity at front view, the above formula can be reduced to P = 1 / K, and 1 / k = P0, that is, P0 is the static diopter of the eyeball. When looking at a target that is L from the eyeball, the "lens" imaging formula becomes 1 / L + 1 / K = 1 / L + P0, 1 / L is the increased diopter of the eyeball, and 1 / L + P0 is the eyeball look Diopter at a distance of L target.

For the wearer, in general, the distance from the eyeball to the center of the glasses is about 1.2-2.4CM, which is expressed by h below, but the value of someone at a certain time is determined, and the diopter is set to P ' The focal length of the lens is F. When looking at a target with a distance of L, the lens imaging formula is as follows:

1 / L + 1 / V = P '==> 1 / V = P'-1 / L

At this time, the distance between the lens and the "lens" of the eyeball is | V | + h, and the refractive condition of the eyeball satisfies the formula: 1 / (| V | + h) + 1 / K = P

From the formula, if | V | is much larger than h, according to formula, formula can be approximately simplified as:

1 / | V | + 1 / K = D = | D'-1 / L | + 1 / K

Because the eye sees a virtual image through the lens, V <0, then 1 / | V | + 1 / K = 1 / L + 1 / KD '= D1 + D0-D'

From this formula, the size of | V | depends on the object distance L and the focal length of the lens. Considering the actual situation, the refractive power of myopia glasses is mostly greater than -6D, and the distance for students to read and write is mostly greater than 0.25M. The formula shows that the smaller the refractive power P '(Note D' <0, the same below) of the concave lens is, the smaller V | is, the smaller the object distance is, and the smaller | V | is. For example, when D '=-5 and U = 0.25, V | = 0.111M, still much larger than 0.02M. So as a theory

Diopter For calculation, when looking at a target that is not too close and the lens power is not too high, h can be ignored. This simplifies the calculation and facilitates qualitative analysis. In other words, for thin lenses, if the distance from the eyeball to the lens is ignored, it can be considered that the degree of adjustment of the increase in eyeball adjustment caused by wearing myopic glasses is equal to the power of the lens. In an optical system composed of an eyeball and glasses, the refractive power produced by each part can be approximately added and subtracted. This analysis can simplify the calculation and make the problem easier. In the following discussion, we will use this result for qualitative analysis and approximate calculation.

Sixth point of diopter

Error Analysis. If the formula is used as the standard, the reasons for the errors are various. Now we will analyze this.

(1) Because the adjustment of the eyeball and the deformation occur simultaneously, there is deformation when there is adjustment, and there is a change in the front and back diameter of the eyeball. There is also a "convex lens" composed of the cornea, aqueous humor, and lens due to the deformation of the lens and the cornea Change of light center. Although myopia or presbyopia itself cannot explain the change of the front-to-back diameter (for example, myopia is the eyeball imaging in front of the retina, but this can be achieved by too close adjustment or the ciliary muscles cannot relax, which cannot fully explain the front and back of the eyeball. Diameter becomes longer), but it cannot explain its invariance. The existence of these factors determines that K is only an approximation in the formula, and the larger the magnitude of the near adjustment, the greater the change in K value, which is a reason for the error. However, considering that in the adjustment of the eyeball, the diopter adjustment of the lens and the diopter of the eyeball (approximately 60 diopters) are very different, and the amplitude of the eyeball adjustment is generally less than 10 diopters, which is relatively small and the corneal refractive power changes less. The distance from the "lens" optical center to the retina is almost constant.

(2) Because the front and back diameter of each eyeball is different, for different people, K is not constant and it is difficult to measure accurately, but specific to a certain stage of an individual, the front and back diameter of the eyeball is unchanged. K is considered constant.

(3) For different people, the distance from the spectacle lens to the optical center of the "convex lens" is a difficult variable to measure, which also affects the accuracy of the calculation. It can be known from the calculation that as h increases, the error increases and vice versa.

Seventh diopter

When the lens is placed in front of the eye, compared with the normal eye, if the eye can still see the target clearly, from the perspective of the adjustment effect of the eyeball, the glasses first offset the lack of eyeball adjustment. Therefore, in future calculations, as long as it is within the normal adjustment range of the eyeball In theory, the effect used to offset the lens can be established, we do not need to pay attention to the actual change of the refractive power of the eyeball. For the eyeball, no matter how many diopters are worn, to see the target in front, you must reduce the effect of the glasses and increase the diopter adjustment.

Diopter point eight

Due to lens errors, adaptation, etc., even taking all factors into consideration, the theory is only an approximation to practice. When the eyeball adjustment range is large, this simple and idealized theory will cause errors due to its own deformation. Increase. Furthermore, the distance from the lens to the optical center of the eyeball varies from person to person, which cannot be expressed by physical formulas. Specific problems and specific analysis must be made during the specific preparation.

Ninth point of diopter

For the refractive system composed of the eyeball and the lens, the power of the lens is determined, but the power of the eyeball is a variable. Therefore, to consider the eyeball as an adjustable convex lens means that the eye can see clearly through the glasses For a certain target, the refractive power of the eyeball is determined, so it can be calculated using the refractive theory, but when the distance between the eyeball and the target changes, the refractive power also changes.

Tenth point of diopter

For a refractive system consisting of an eyeball and glasses, there are only two "lenses", which can be regarded as an equivalent lens group, and the power of the lenses can be added and subtracted. For example, a + 5D lens can be regarded as a lens. Although the (+ 2D) + (+ 3D) lens group is not established in most cases, it provides convenience for us to solve problems in theory. ^[1]