Knowledge of the lens
First, lens optics
Corrective lenses: the main purpose of the application of glasses is to correct the refractive error of the human eye and increase vision. Glasses with such a function are called “corrective glasses”.
Corrective glasses are usually a single lens, made of glass or clear plastic. The simplest is a composite of two spheres containing some transparent and uniform refractive stroma that is denser than air, collectively called a lens. A scattered beam of light emanating from a point on a space object is bent by a lens to form a single image point and many image points are combined to form an image.
Lens:
According to the properties of lens, it can be divided into positive lens or negative lens.
1. Plus Lens
Also known as convex lens, light convergence, with “+”.
(2) Minus Lens
Also known as a concave lens, the light has a dispersive effect, indicated by “-”.
There are two different theories about why corrective glasses can correct refractive error of the human eye:
1. After the refractive aberration eye is combined with the corrective lens, an overall refractive combination is formed. This combined refractive combination has a new diopter, which can make the distant object image on the photoreceptor layer of the retina of the eye.
2. In far-sighted eyes, the beams must be assembled before they converge through human eyes; In myopic eyes, the beams must diverge before converging with the human eye. The proper diopter of orthotic glasses is used to alter the divergence of the beam reaching the eye.
Common term for spherical lens
Curvature: Curvature of a sphere.
ø Radius of curvature: the radius of curvature of a spherical arc. The shorter the radius of curvature, the greater the curvature of the spherical arc.
ø Optical Center: When light rays are directed at this point, no twists and turns occur.
The parallel light beams converge to a point after passing through the lens, or the reverse extension line converges to a point, which is called the Focus.
The refraction of glasses
In 1899, Gullstrand proposed to take the reciprocal of focal length as the unit of refraction force of lens, called “Dioptre” or “D” (also known as focal degree).
D=1/f
Where, f is the focal length of the lens in meters; D stands for diopter.
For example: the focal length is 2 meters, D=1/2=0.50D
The focal length is 0.25 m, D=1/0.25=4.00D
Spherical diopter
Formula: F = N ‘- (N)/R
R is the radius of curvature of a sphere in meters. N ‘and N are the refractive indices of the refractive media on both sides of the sphere. For crown glass, when R=0.25 m,
F= (1.523-1.00) /0.25=2.092D
An eye lens is a lens composed of two spheres, whose diopters are equal to the algebraic sum of the spherical diopters of the front and rear lenses.
D=F1+F2= (n1-n) /R1+ (N-n1) /R2= (N1-1) (1/R1-1/R2)
Therefore, the refraction of the lens is related to the refractive index of the lens material and the radius of curvature of the front and rear surfaces of the lens. The radius of curvature of the front and rear surfaces of the lens is the same, and the refractive index is higher, the absolute value of the lens diopter is higher. On the contrary, the lens with the same diopter has a larger refractive index and a smaller radius difference between front and back.
Two, the type of lens
Division (luminosity) by refractive properties
Flat mirror: flat mirror, no mirror;
Spherical mirror: spherical luminosity;
Cylindrical mirror: astigmatism;
3. To change the direction of light (to correct certain eye diseases).
According to the nature of the focus
Focus-free lenses: flat, prism;
Single focus lens: myopia, farsightedness lens;
Multifocal lens: dual focal lens or progressive lens
According to functional properties
Visual correction
Refractive bad
dysregulation
Amblyopia mirror
protection
Protection against harmful light;
Control visible light (Sunglasses)
Protection against harmful substances (protective goggles)
According to the material points
The natural material
Glass material
Plastic material
Third, the development of lens materials
The natural material
Crystal lens: The main ingredient is silica. Divided into colorless and tawny two kinds.
Advantages: hard, not easy to wear; Not easy to wet (fog is not easy to retain on its surface); The coefficient of thermal expansion is small.
Disadvantages: uv has a unique transparency, easy to cause visual fatigue; Density is not uniform, easy to contain impurities, resulting in birefringence; It’s expensive.
glass
1. History:
Corona glass is generally used, and the main component is silica. The transmittance of visible light is 80%-91.6% and the refractive index is 1.512-1.53. However, in case of high refractive abnormality, lead glass with high refractive index of 1.6-1.9 is used.
2, optical characteristics:
(1) Refractive index: n=1.523, 1.702, etc
(2) dispersion: because there are different refractions for different wavelengths of light
(3) Reflection of light: the higher the refractive index, the greater the reflection
(4) absorption: when light passes through the glass, its intensity decreases with the increase of thickness.
(5) Birefringence: isotropy is generally required
(6) Fringe degree: due to the uneven chemical composition inside the glass, the refractive index at the fringe is different from the main body of the glass, affecting the imaging quality
3. Types of glass lenses:
(1) Toric Tablets
Also known as white plate, white plate, optical white plate
Basic ingredients: Sodium titanium silicate
Features: colorless transparent, high definition; It can absorb ultraviolet rays below 330A, and add CeO2 and TiO2 to white tablet to prevent ultraviolet rays below 346A, which is called UV white tablet. The transmittance of visible light is 91-92%, and the refractive index is 1.523.
(2) Croxus tablet
William of England in 1914. Invented by Croxus.
Characteristics: light transmittance 87%
Two-color effect: light blue under sunlight, so also known as blue. But in the incandescent lamp is light red (containing neodymium metal element) can absorb 340A below ultraviolet, part of infrared and 580A yellow visible light; It is now rarely used
(3) Croseto tablets
CeO2 and MnO2 are added into the materials of white base lens to improve the ultraviolet absorption capacity. This kind of lens is also called red sheet because it shows light red under sunlight and incandescent lamp.
Features: it can absorb ultraviolet rays below 350A; The transmittance is above 88%;
(4) ultra-thin film
Adding TiO2 and PbO to the raw material increases the refractive index. Refractive index is 1.70,
Features: about 1/3 thinner than the common white or red tablet with the same diopter, suitable for high myopia, beautiful appearance; Abbe coefficient is low, color aberration is large, easy to cause peripheral vision reduction, line bending, color; High surface reflectivity.
(5) 1.60 glass lens
Features: Refractive index is 1.60, thinner than ordinary glass lens (1.523), and thinner than ultra-thin lens (1.70) has a smaller proportion, so it is lighter, very suitable for medium degree wearers, some manufacturers call it ultra-light and ultra-thin lens.
Plastic lenses
First thermoplastic lens made in 1940 (Acrylic)
In 1942, Pittsburgh plate Glass Company, USA, invented CR-39 material, (C stands for Columbia Space Agency, R stands for Resin Resin) while preparing materials for NASA space Shuttle.
In 1954, Essilor made cr-39 solar lenses
In 1956, Essilor company in France successfully trial-produced the optical lens with CR-39.
Since then, resin lenses have been widely used in the world. In 1994, the global sales volume reached 30% of the total number of lenses.
Plastic material lenses:
1, polymethyl methacrylate (acrylic sheet, ACRYLICLENS)]
Features: refractive index 1.499; Specific gravity 1.19; Early used for hard contact lenses; Hardness is not good, the surface is easy to scratch; Now it is used for readymade glasses, such as readymade reading glasses.
Pros: Lighter than glass lenses.
Disadvantages: surface hardness as glass lens; The optical properties are inferior to glass lenses.
2, resin sheet (the most representative is CR-39)
Characteristics: chemical name is propylene diethylene glycol carbonate, is hard and transparent material; Refractive index is 1.499; Transmittance 92%; Thermal stability: no deformation below 150 ℃; Good water and corrosion resistance (except strong acid), insoluble in general organic solvents.
Advantages: specific gravity of 1.32, half of glass, light; Impact resistance, unbreakable, strong sense of security (in line with FDA standards); Comfortable to wear; Convenient processing, wide use (including the use of half frame, frameless frame); Rich product series (single light, double light, multi-focus, cataract, color change, etc.); Its uv absorption capacity is easily higher than that of glass lens; Can be dyed into various colors;
Thermal conductivity is low, and “water mist” caused by water vapor is better than glass lenses.
Disadvantages: poor wear resistance of lens, easy to scratch; With low refractive index, the lens is 1.2-1.3 times thicker than the glass lens.
Development:
(1) To overcome the wear resistance of the material, in the mid-1980s, the lens surface hardening technology succeeded; General resin lens, surface hardness surface hardness of 2-3h, after hardening treatment, hardness up to 4-5h, at present, many companies have launched hardness up to 6-7h super hard resin lens. (2) In order to reduce the lens thickness, resin sheets with different refractive indexes were developed successfully
(3) waterproof fog treatment: coating a layer of hard film, responsible for sticky moisture molecules, responsible for moisture absorption molecules, surface hardness molecules. When the humidity of the environment is lower than that of the lens, the membrane emits moisture. When the humidity of the environment is higher than that of the lens, the membrane absorbs water. When the ambient humidity is much higher than the lens humidity, the sticky moisture molecules turn a lot of water into a water film.
3. Polycarbonate (PC tablet) is also called space lens in the market.
Features: refractive index 1.586; Light weight; Especially suitable for frameless frames.
Advantages: Strong impact resistance; More impact-resistant than resin lenses.
Special lenses
Photochromic film
Features: silver halide particles are added to the raw material of the lens. Under the action of ultraviolet rays in sunlight, silver halide is decomposed into halogen ions and silver ions, thus changing color. According to the intensity of ultraviolet light in sunlight, the degree of discoloration is also different; When the uv disappears, the lens changes back to its original color.
Advantages: Corrects refractive errors for patients and doubles as sunglasses outdoors.
Can adjust the light into the eye at any time to maintain proper vision; Regardless of its discoloration state, it always absorbs ultraviolet light well;
Disadvantages: thick lens, generally 1.523 glass; When the degree is high, the color is not uniform (lighter in the middle). After a long lens time, the discoloration effect and discoloration speed slow down; The color of single sheet is inconsistent
Causes of discoloration
1, light source type: ultraviolet short wavelength light irradiation, fast color change, large concentration; Ultraviolet long wavelength light irradiation, slow color change, small concentration.
2. Light intensity: The longer the light, the faster the color changes and the higher the concentration (plateau and snow)
3, temperature: the higher the temperature, the faster the color change, the greater the concentration.
4, lens thickness: the thicker the lens, the deeper the discoloration concentration (no impact on speed)
Tips for selling photochromic tablets
1. When changing a single sheet, the color is often inconsistent. It is recommended that customers change two pieces at the same time.
2, due to slow fading, often in and out of the indoor customers, it is not recommended (students)
3. Due to different lens thickness and discoloration concentration, it is recommended not to match if the diopter difference between the customer’s two eyes is more than 2.00d.
4, high myopia feeling black, another edge and center color difference, not beautiful.
5, reading glasses center color effect is low, not with color changing lens.
6, the difference between domestic and imported lenses: domestic than imported lenses slow color, slow fade, deep color, imported soft color.
Anti-radiation lens:
In the lens material to add special substances or special anti-reflective film, blocking radiation light to relieve eye fatigue.
Aspherical lenses:
A plane of rotation (such as a parabola) having the same noncircular section on all meridians. The edge view has no distortion and is 1/3 thinner than normal lenses (prism is thinner).
Polarizing lens:
A lens with light that vibrates only in one direction is called a polarizing lens.
The purpose of using polarizing lenses: to block the glare of light reflected on a flat surface.
Precautions for use:
(1) Durability is not good, long time contact with water, the surface film is easy to fall off.
(2) when installing the mirror frame, if there is internal stress, it will affect its polarization effect.
Double light piece
Features: there are two focal points on one lens, and a small lens superimposed on an ordinary lens; Used for patients with presbyopia to see far and near alternately; The upper is the luminosity when looking far (sometimes flat), and the lower light is the luminosity when reading; The distance value is called upper light, the near value is called lower light, and the difference between upper and lower light is ADD (added light).
Advantages: presbyopia patients do not need to replace glasses when they see near and far.
Disadvantages: see far and see near conversion when jumping phenomenon (prism effect); It is obviously different from ordinary lenses in appearance. The field of vision is smaller.
According to the form of the light part under the bifocal lens, it can be divided into:
A gleam of light
Features: maximum visual field under light, small image jump phenomenon, small color aberration, large edge thickness, beautiful impact, large weight
Flat double light
Dome double Light (invisible double light)
Characteristics: boundary line is not obvious; The edge thickness does not increase with the increase of near-use degree; But the phenomenon of image jump is obvious
Progressive multifocus lenses
Features: Multiple focal points on the same lens; The degree of the progressive band in the middle of the lens changes point by point from top to bottom.
Advantages: the same lens can see far, medium and close distance; The lens has no obvious boundaries, so it is not easy to be noticed. From the vertical direction of the central part of the eyes do not feel jumping phenomenon.
Disadvantages: High price; The test is difficult; There are blind areas on both sides of the lens; Thicker lens, generally 1.50 resin material (new 1.60)
Comparison of characteristics between bifocal lens and asymptotic multi-focus lens
Double light:
(1) There are obvious differences between different regions. The appearance is not beautiful, giving people the impression that the wearer is old
(2) the middle distance fuzzy, such as: playing mahjong, etc.
(3) Due to the existence of two focal points, resulting in visual obstacles: image staggered or jump, so that the user has a feeling of stepping on the empty, no confidence to walk on the stairs or between the streets.
(4) The use and development prospects of materials are limited.
Steps:
(1) From far to near uninterrupted line of sight, the middle distance becomes clear.
(2) Beautiful appearance, no visible interval.
(3) Jump without image, walk confidently on stairs and between streets.
(4) Both design and materials are evolving.
(5) thinner than the same single lens.
(6) Relieve eye fatigue and improve visual health.
Multi-focus lenses are suitable for objects
(1) Presbyopia, especially early presbyopia.
(2) Those who are dissatisfied with wearing two pairs of glasses (seeing far and seeing near).
(3) Those who are dissatisfied with wearing traditional bifocals.
(4) Adolescent myopia patients.
Professionally:
Suitable for: frequent eye shifters, professors (lecturing), supervisors (meeting), store owners, card players.
Unfavorable: a dentist, electrical or mechanical maintenance personnel (often must close strabismus or look up), close work time is too long, if you need regular fast moving head, whether to need to near vision when looking up, such as look at the table or shelf on the wall (the pilot and hydropower workers, large instrument operators), whether or not to look down to distant vision (construction workers, etc.)
Physiologically:
Suitable for: eye position and convergence normal person, two glasses degree difference small person, myopia glasses family
Unfavorable: strabismus or hidden strabismus, eyelid hypertrophic obstructs the line of sight, high astigmatism, high upper brightness and ADD high degree of people.
By age:
Suitable for: early presbyopia patients around 40 years old (easy to adapt due to low degree of ADD)
Unfavorable: At present, the ADD of the first match in China is relatively high. If the ADD exceeds 2.5d, whether the physiological state is good or not should be considered.
From the history of wearing mirrors:
Suitable for: previous wearers of bifocals, myopic presbyopia (myopic progressive multi-focus lenses are the easiest to adapt to)
Unsuitable: the original does not wear astigmatism lens, now the astigmatism degree is higher or has the history of wearing lens but the astigmatism is too high (generally more than 2.00d); Anisometropia;
How to explain usage instructions to guests
(1) Introduce the lens degree distribution and aberration distribution
(2) When the customer puts on the eyes, guide the customer to find the best visual area by moving the head position (move the eyes up and down, move the head left and right)
(3) generally 3-14 days of adaptation period, so that the brain formed a conditioned reflex, gradually adapt (adding degree, adaptation period is long).
Symptoms of problems with progressive lenses
Reading area is too small
Blurred near vision
Dizziness, undulating feeling, wandering feeling, shaking feeling
Blurred far vision and blurred objects
Turn or tilt your head to see when reading
Possible causes of problems with progressive lenses
Incorrect distance between one eye pupil
The height of the lens is incorrect
Incorrect diopter
Incorrect frame selection and wearing
A change in the base arc (usually flattening)
Instruct customer to use progressive lens
(1) Use of remote area
“Please look far away and focus on clear vision” demonstrates changes in blurred and clear distant vision as the chin moves up and down.
(2) Use of near use area
“Please look at the newspaper and look where you can see clearly.” Demonstrate changes in vision when moving your head from side to side or moving a newspaper.
(3) Use of mid-range area
“Please look at the newspaper and look where you can see clearly.” Move the newspaper outward to increase reading distance. Demonstrate how blurred vision can be restored by adjusting head position or moving the newspaper. Demonstrate changes in vision when moving the head or newspaper side to side.
Five, some important parameters of the lens
The refractive index
The refractive index of a lens is determined by the material used. Other parameters being the same, a lens with a high refractive index is thinner.
Lens diopter (vertex focus)
In units of D,1D is equal to what is commonly called 100 degrees.
Lens center thickness (T)
For the same material and luminosity, the center thickness directly determines the edge thickness of the lens. Theoretically, the smaller the center thickness, the thinner the lens appearance, but too small center thickness will cause.
1. Lenses are fragile, unsafe to wear and difficult to process and transport.
2. Center luminosity is easy to change. So national standard has corresponding regulation to lens center thickness, real qualified lens may be thicker instead. Safety center thickness of glass lens >0.7mm Safety center thickness of resin lens >1.1mm
The lens diameter
Refers to the diameter of a rough round lens.
The larger the lens diameter, the easier it is for the fabricator to get the customer’s pupil distance right.
The larger the diameter, the thicker the center
The larger the lens diameter is, the higher the corresponding cost is
Six, anti – film technology
(1) interference of light; So that the coating reflected light and lens reflected light crest and trough coincide.
(2) Conditions for making the reflection amount of the lens zero (monolayer film) :
A. The refractive index of the coating material is the same as the square root of the refractive index of the lens material. When n=1.523, n1=1.234.
B. The coating thickness is 1/4 of the wavelength of incident light, the yellow wavelength is 550nm, and the coating thickness is 138 nm
(3) Coating materials and methods
Material: MgF2, Sb2O3, SiO2
Methods: Vacuum under high temperature steaming
(4) Characteristics of coated lens
Advantages: improve transmittance, increase clarity; Beautiful, no obvious reflection; Reduce lens vortexes (vortexes are caused by light reflected from the periphery of the lens reflecting off the front and back of the lens multiple times); Remove the illusion (the inner surface of the lens accepts the reflection of the incident light behind it into the eye, which is easy to produce visual fatigue); Increased resistance to harmful light (best demonstrated by contrast with membraneless lenses).
Disadvantages: oil stains, fingerprints reflect clearly; The color of the film is obvious from the side Angle
Seven, lens selection
Customer demand for lens: beautiful, comfortable and safe
Beautiful and thin: refractive index, mechanical strength
Durability: wear resistance, no deformation
Non-reflective: add film
Not dirty: waterproof film
Comfortable light:
Good optical properties: light transmittance, dispersion index, dyeability
Safe uv resistance and impact resistance
How to help customers choose lenses:
1. Select materials according to requirements
Impact resistance: meet the SAFETY test of FDA standard, the lens is not easily broken.
Lens white: excellent polymerization process, low yellow index, not easy to aging, beautiful appearance.
Light: the specific gravity is low, the wearer feels light and comfortable, and there is no pressure on the nose.
Wear resistance: the use of new silicon oxide hard technology, its wear resistance close to glass.
2. Select refractive index according to customer luminosity
3, according to customer needs to choose the appropriate surface treatment
4. Select brands according to customers’ psychological price
5. Other requirements
The inventory of all kinds of lenses must be understood based on the actual situation of the store, including:
1. Inventory of existing products
2, can be customized to the factory factory piece range, cycle
3. Lenses that cannot be made
Disadvantages: processing is difficult; Surface easy to scratch, poor thermal stability, 100 degrees Celsius change
Post time: Sep-01-2021