Nowadays, more and more people try to get eyeglasses online, so how to choose the right frame for them online? I will briefly introduce to you.
(1) Relevant knowledge of frame size
1. The name of each part of the spectacle frame:
The composition of the spectacle frame is divided into the following parts: ① mirror ring, ② bridge of the nose, ③ mirror legs, ④ head, ⑤ leg head, ⑥ non-slip sleeve (leg cover), ⑦ nose pad, ⑧ post, ⑨ stipple, ⑩ hinge, etc.
When choosing a spectacle frame, there are three main dimensions to consider, namely the size of the mirror ring, the size of the bridge of the nose and the size of the temples. The size of the lens ring and the size of the nose bridge should ensure that the optical center distance of the glasses is consistent with the wearer's interpupillary distance, and the size of the temples should be based on the wearer's face shape to ensure that the distance between the glasses and the eyes is about 12 mm.
2. Spectacle frame size
The standard sizes of spectacle frames stipulated by the country are as follows. The size of the mirror ring is: 33 ~ 60 mm; the size of the bridge of the nose is: 13 mm ~ 22 mm;
3. The expression method of spectacle frame size
In practice, generally speaking, there are two ways to express the size of spectacle frames: the baseline marking method and the box marking method. The details of the baseline marking method and the box marking method have been introduced in the second chapter of spectacle frames. Here, we will introduce the specific application of each method.
Under normal circumstances, the size of the spectacle frame is usually printed on the inner side of the temple. For example, the reference line marking method is marked as: 52-18-135, that is, the size of the mirror ring is 52 mm, the size of the bridge of the nose is 18 mm, and the size of the temple is 135 mm. Generally imported spectacle frames or high-end spectacle frames often use the baseline marking method.
Another example is to use the box marking method to mark 54 □ 19—140, indicating that the size of the mirror ring of the spectacle frame is 54 mm, the size of the bridge of the nose is 19 mm, and the size of the temples is 135 mm.
(2) Selection of spectacle frame size
In optometry, choosing the size of the spectacle frame is mainly divided into the following steps:
1. Check the optometry prescription of the optician and find out the interpupillary distance;
2. Select the corresponding glasses frame;
3. Check the specification and size of the selected spectacle frame;
4. Analyze and calculate, determine the amount of center shift, select the nominal size of the lens,
Among them, the following calculation formula should be followed:
Need to move the heart L1 = (mirror ring size + nose bridge size - interpupillary distance size) / 2;
Allowable shifting amount L2=(nominal size of lens diameter—pupillary distance—processing allowance)/2+△
Processing allowance--processing allowance refers to the amount of loss of spectacle lenses and the allowance of special mirror types during the assembly process of glasses. Normally, the machining allowance is 2~3 mm, and the minimum value is 2 mm;
—— is the horizontal difference of the optical center in the national standard, which refers to the unilateral deviation between the optical center of the lens and the pupil in the horizontal direction
In the actual optometry, when L2≥L1, the produced glasses are qualified;
When L2<L1, the manufactured glasses are unqualified.
Let's analyze how to choose the size of the spectacle frame through a specific example.
Example 1: A customer's optometry prescription is R: -3.50DS, L: -3.25 DS, PD=62mm, and the frame he chooses is 52-16-135. Try to analyze its feasibility.
Analysis: When analyzing the feasibility of this example, the purpose is to calculate and analyze based on the pupil distance size of the optician and the selected frame size, and find out whether there is a lens material with a corresponding nominal size that meets the requirements.
The size of the frame is indicated by the reference line marking method
The lens ring size is 52mm,
Nose bridge size is 16mm,
The geometric center distance of the spectacle frame is 52+16=68mm,
The interpupillary distance of the optician is 62mm,
The ideal state of spectacle fitting should be: the optical center of the lens is exactly located in the pupil center of the spectacle wearer, then at this time, the wearer's vision will not produce prism effect, and the interpupillary distance of the spectacle wearer is 62mm, the pupil centers of the left and right eyes of the spectacle wearer deviate from the geometric center of the frame to the nose side,
Amount of shifting according to needs L1=(mirror ring size+nose bridge size-pupillary distance size)/2
Need to move the heart amount L1=(52+16—62)/2 =3mm
According to the requirements of the ideal glasses matching state, the optical center of the lens should not be placed at the geometric center of the frame, but needs to be moved to the nasal side, that is, the center should be moved inward by 3mm.
Then, according to the required shifting amount of 3mm, select the nominal size of the lens diameter. At this time, if the nominal size of the lens diameter is 56 mm,
That is, when the shifting amount is 3mm, consider the processing allowance to be 2mm, and the △ value at -3.50DS is 4mm, using the formula
Allowable shifting amount L2=(nominal size of lens diameter—pupillary distance—processing allowance)/2+△
L2=(nominal size of lens diameter—62—2)/2+4
Nominal size of lens diameter = 62 mm,
The nominal diameters of lenses stipulated by the state are: 56, 60, 65, 70, 71, 72, and 75. Because there is no such file, 65mm can meet the needs.
Therefore, it is feasible to choose a lens material with a nominal size of 65mm to meet the requirements of the frame selected by the optician.
Example 2: A customer's optometry prescription is R: -3.50DS, L: -3.25 DS, PD=66mm, and the frame he chooses is 58-20-140. Try to analyze its feasibility.
Analysis: This customer has a typical face shape with a slightly wider face, and prefers or needs large frames. In this case, it is easy to make mistakes in size selection.
Spectacle frames chosen by the optician
Lens size = 58 mm,
Nose bridge size = 20 mm,
The geometric center distance is: 58+20=78mm.
The interpupillary distance of the glasses wearer is 66mm.
Need to move the heart L1 = (mirror ring size + nose bridge size - interpupillary distance size) / 2
L1=(58+20—66)/2=6 mm
Allowable shifting amount L2=(nominal size of lens diameter—pupillary distance—processing allowance)/2+△
That is, 6=(nominal size of lens diameter—66—2)/2+△
Nominal size of lens diameter = 72 mm,
Let's take a look: the lens of glass material has a nominal diameter of up to 70 mm, which cannot be processed in this case. At this time, the customer's frame selection requirements cannot be met.
If you choose a lens made of resin material, the maximum nominal size of the lens made of resin material is generally 70mm, which cannot be processed in this case.
Solution: Advise customers to choose a slightly smaller spectacle frame to ensure the optical quality of the glasses.
There are three specifications of 71, 72, and 75 among the nominal lens sizes stipulated by the state. Usually, resin lenses with a diameter of more than 70 mm are rarely used because of the high production cost. If the optician insists on using the spectacle frame, we can choose 72mm and 75mm resin spectacle lenses for him.
Improper handling: When the optician insists on insisting that the frame must be made of glass, and the optician insists on using glass material, why not do it from a commercial point of view? So what are the consequences of such glasses? Please see another treatment of the previous example.
The optician chose a 65mm glass lens for this customer. To ensure that the glasses are qualified, it must be ensured that △≤4mm,
Need to move the heart amount L1 =6 mm, △=6—(65—66—2)/2=7.5>4mm
Therefore, the horizontal deviation of the glasses produced in Example 2 exceeds the optical level tolerance of 4 mm in the national standard, so the glasses prepared in this way are unqualified glasses.
In optometry, to evaluate whether the level deviation of glasses exceeds the standard, it is necessary to carefully compare the national GB13511-1999 quality standard for fitting glasses. The optical level tolerances for glasses with different focal powers are also different, and we will introduce them in detail in the following courses.
Advice: Although the glasses are small, they have a great impact on people.
Wearing a pair of comfortable glasses will bring people a clear world; but wearing a pair of unqualified glasses will not only make people unable to see clearly, but will also cause great harm to people's psychology and body.
