1. Lens Materials, Safety and Sports Eyewear.pdf

OPHD 2604 Lens Materials Chapter 23 PART 1 7 February 2024 Lens materials: IMPORTANT!! Every practitioner needs to know the unique characteristics of each lens Main types: 1. GLASS materi...

OPHD 2604 Lens Materials Chapter 23 PART 1 7 February 2024 Lens materials: IMPORTANT!! Every practitioner needs to know the unique characteristics of each lens Main types: 1. GLASS materials to properly match Crown glass your patient’s needs to the High index glass best material 2. PLASTIC Ophthalmic lenses are made CR39 High index plastic from plastic (organic Polycarbonate lenses material) and glass (mineral Trivex lenses lenses) NXT Laminating lenses  Main Characteristics: n= refractive index: how light is bent ― Relates to thickness of the lens ― The higher the index = the thinner the minus lens edge will be Density: how heavy lens is Abbe number: indicates the amount of light dispersion ― Relates to chromatic aberrations ― Inverse relationship between the two ― The higher Abbe number = lower dispersion = less chromatic aberrations = higher image quality  Main Characteristics: NB: Go through table 23-1 on page 570 ― Make sure you understand it ― DO NOT try to learn it off by heart GLASS: 1. Crown Glass Material used for spectacle lenses for several hundred years Glass is the most scratch-resistant material Glass must be specially treated for impact resistance (glass must be hardened) The main disadvantages of glass are weight and impact resistance It may not be an option for rimless and semi-rimless materials Most commonly used clear glass lens material is made from a type of crown glass (refractive index 1.523) Low in chromatic aberration = HIGH optical quality Advantages & Disadvantages of CROWN GLASS: Advantage Disadvantage Exceptional scratch resistance (don’t need Shatter or chip easier than lenses made from scratch resistant coating) other materials Excellent optical clarity At least twice the weight of plastic or Equipment needed is inexpensive and requires polycarbonate lenses little space Require a special coating to provide 100% UV Cheap process protection About 25 – 40% thicker than polycarbonate and high index plastic lenses Impact resistance is reduced by scratches and other surface abrasions GLASS: 2. High Index Glass: High index lens material reduces lens thickness for high powered prescriptions Index 1.60 lenses are readily available in spherical and aspherical designs, as well as for segmented and progressive multifocals High-index glass lenses are composed of materials with a higher specific gravity, making them heavier PLASTIC: 1. CR-39 Refractive index of 1.498 ≈ 1.50 For years CR-39 was the most commonly used plastic material ― ‘Columbia Resin’ ― ‘39’ denotes the type of Columbia Resin used Plastic lenses are lighter than glass lenses – roughly ½ of the weight CR-39 lenses can be tinted to almost any colour and shade for a multitude of options Some CR-39 lenses come with an anti-scratch coating, making the material much more scratch resistant Do not fog up as easily as glass Advantages & Disadvantages of CR-39: Advantage Disadvantage Good optical quality About 20 to 30 % thicker than polycarbonate or Greater impact resistance than glass lenses high index lenses Lightweight = about half the weight of glass Require scratch-resistant coating for added lenses durability (ability to withstand wear, pressure or Plastic lenses accept tints easily damage) Offer greater UV protection Require a special coating to provide 100% UV Can be thinner than a glass lens of equivalent protection power Refractive indices range from 1.49 to 1.60 If the lens is fractured, the fragments tend to be larger and blunt. PLASTIC: 2. High index Plastic lenses: High index lenses are thinner and lighter They allow greater comfort and cosmetic appeal when compared to other materials Come in a variety of materials For minus lenses of equal power and center thickness, the HIGHER the refractive index, the thinner the lens will be PLASTIC: 3. Polycarbonate: Refractive index of 1.586 ≈ 1.60 Commonly used for plano safety apparatus Polycarbonate lenses have become the standard for safety glasses, sport goggles, and children’s eyewear Polycarbonate is a softer material, and thus requires an anti-scratch coating Softer material = higher impact resistance ― Softer polycarbonate material does not break on impact, it absorb a blow and just dent ― Polycarbonate lenses are less likely to fracture than regular plastic lenses Advantages & Disadvantages of Polycarbonate: Advantage Disadvantage Excellent impact-resistance (10 times more Requires scratch-resistant coating for durability impact resistant than other lenses) Peripheral vision may be slightly less clear in Thin (about 20-25% thinner than plastic or glass strong prescription powers lenses) More lens reflections than glass or plastic Lightweight (about 20% lighter than plastic lenses (AR coating recommended) lenses) Blocks 100% UV rays without needing a special coating No age-related warping, chipping or discolouration PLASTIC: 4. Trivex: Refractive index of 1.53 Trivex lenses are thin, lightweight, and much more impact resistant than regular plastic or glass lenses The lens material was originally for military use as a plastic material to provide excellent safety for windows and good optics This is the lens of choice for drill-mounted lenses because it does not crack or split at the drilled hole ‘TRI-’ performance lens material ― Superior optics ― Impact resistant ― Ultra light weight PLASTIC: Polcarbonate & Trivex: 1. When eye safety is a concern, polycarbonate or trivex lenses usually are the best choice for your eyeglasses, sunglasses and sport eyewear 2. Both are thinner and lighter than regular plastic lenses 3. They offer 100% protection from the sun’s harmful UV light 4. Up to 10 times more impact-resistant than plastic or glass lenses 5. This combination of lightweight comfort, UV protection and impact resistance also make these lenses an excellent choice for children’s glasses and safety glasses  PLASTIC: Polcarbonate vs Trivex: Polycarbonate Trivex Thickness Polycarbonate has a higher refractive index than trivex = thinner (minus lenses) Weight Trivex has a lower specific gravity than polycarbonate Optical clarity Trivex lenses have less internal stress and may produce (center of lens) sharper central vision than polycarbonate lenses Optical clarity Trivex lenses have a higher Abbé value and may (periphery) produce sharper peripheral vision with less chromatic aberration than polycarbonate lenses Impact resistance Trivex rivals polycarbonate in impact resistance Availability Polycarbonate lenses are available in a wider variety of lens design (e.g. progressive lenses and other multifocals) Cost The cost of polycarbonate and Trivex lenses can vary, but many optical stores charge more for Trivex lenses than polycarbonate lenses PLASTIC: Choosing the perfect frame for polycarbonate and trivex lenses: When it comes to eye safety, polycarbonate and trivex eyeglass lenses are only part of the solution For the best eye protection at work and during sports, be sure you also invest in high-quality safety frames or frames designed specifically for sport eyeglasses Regular eyeglass frames are not rated for use as safety and typically don’t provide the type of eye protection needed for sports Therefore, playing sports while wearing an eyeglass frame that is not rated for sports eyewear is dangerous and can result in a serious eye injury if the frame snap, dislogging the lenses Frames for children: ―Choose a sturdy frame and lightweight polycarbonate or trivex lenses ―Even if he /she does not participate in organized sports, choosing impact-resistant eyeglass lenses and frames is an important step to protect the child’s eyes PLASTIC: 5. NXT Material: Refractive index of 1.53 This lens is light-weight material that is extremely strong Compatible with photochromic pigments and polarization It has been used in sun and sport eyewear, helmet visors for motorcycles, airline cockpit door view ports, ballistic police shields and vehicle door armor It has a density of 1.11, an Abbé value of 45 Compatible with low-powered sphere and cylinder powers Highly flexible PLASTIC: 6. Laminating Lenses Lenses that are made from two or more layers of material are called laminated lenses Polarizing lenses have a stretched polarizing film sandwiched between two layers Lamination can also be used to increase impact resistance Example? ― The wind screen of a car ― When it shatters the glass will stick to the inner plastic layer Effects of lens coatings on impact resistance: When a plastic lens is either scratched resistance coated or anti- reflection (AR) coated, the impact resistance of the lens decrease ― Opposite of what is expected? Both scratch resistance and anti-reflection coating are harder than the plastic lens material to which they adhere ― When the lens breaks, the break starts at the weakest point ― If an object hits a lens, the lens flexes, but the harder (more brittle) coating cracks first ― Because the coating is bonded to the lens, the energy released by the crack is concentrated and may lead to the lens breaking If a plastic lens is hit by an object, the lens may flex, but not break, however if the coating is harder than the lens, it may break Effects of surfaces scratches on impact resistance: A scratched surface reduces impact resistance The scratch introduces a weak spot on the lens, creating a weakness (‘fault line’) The scratch creates an easy area for stress to build during impact, making breakage more likely Back surface scratches will reduce impact resistance MORE than front surface scratches ― Front surface scratches reduced impact resistance by 20% ― Back surface scratches reduced impact resistance by 80% ᴥ Coatings: 1. ARC: anti-reflective coating Reduces reflection, glare and ‘halos’ around light sources that you may experience at night HOW: reduce the amount of light reflected onto your lenses, allowing more light to be transmitted through the lens without the glare Green reflection? AR coatings have a peak reflectance at some point on the visible light spectrum and it is the colour associated with that peak, which is reflected more than other colours 2. Blue control Marketed as lenses specifically designed to reduce the amount of blue light that reaches the eye from digital screens HOW: Filter blue light rays (certain wavelength) to help prevent them from entering your eye and causing potential damage (many on going studies about potential damage) Blue reflection? Interestingly – the sunlight is the biggest source of blue light in our everyday lives ― Blue light helps to regulate the body’s natural sleep and wake cycle ᴥ Coatings: 3. Photochromic Tint Optical lens that darkens on exposure to light of sufficiently high frequency, most commonly ultraviolet radiation (UV – direct sunlight) HOW: Tiny molecules of silver halide and chloride are embedded within a photochromic lens which is invisible and clear until exposed to sunlight/UV rays ― When exposed to UV - chemical process takes place ― The molecules effectively move, change shape and absorb the light - changes the molecules and they darken to a sunglasses shade/tint 4. Polarization Reduce glare from surfaces such as water, snow, and glass HOW: Blocks horizontal light waves—the type that most surfaces reflect when causing a glare ― Only vertical waves make it through the filter in these lenses How to test? OPHD 2604 Sport and Safety Eyewear Chapter 23 PART 2 7 February 2024 General eyewear catagories: 1. Dress Eyewear – Every day use 2. Safety Eyewear: ― Designed to meet higher standards of impact resistance since it will be worn in situations that could be potentially hazardous to the eyes 3. Sport Eyewear: ― Designed to protect the eyes and/ or enhance vision in specific sports situations ― What is appropriate will vary dramatically, depending upon the sport Dress eyewear: Safety eyewear: Designed to meet higher standards of impact resistance since it will be worn in situations that could be potentially hazardous to the eyes Protection in dangerous hazardous environments Reduces injuries Different levels of safety: Basic or High Important safety tests: 1. Drop ball test Ball is dropped from 1.3 meter (50 inches) The size of the ball varies Impact test 2. High velocity impact test 6mm steel ball travelling at 164km/h 3. High mass impact test Large pointed slow moving object with a diameter of 25mm is dropped from a height of 1m through a tube onto the lens 4. Thermodyne test 5. Acid durability test  4. Thermodyne Test: Simulates the conditions of natural atmospheric exposure such as variation in GRADING SCALE: temperature and humidity S1: Excellent resistance - likely to remain Invented by Chance-Pilkington free from attack indefinitely. S2: Good resistance - dull patches may Exposed lenses to variations in temperature appear on the surface after approximately and humidity and obtained results two years. S3: Fairly good resistance - patches of Results compared to lenses that were stain may appear within a two year period. exposed to tropical atmosphere for S4: Fair resistance - tarnish spots may prolonged periods = similar results appear within two years. S5: Poor resistance - general tarnish may appear within one year. S6: Very poor resistance - liable to tarnish badly within a few months 5. Acid Durability Test: Resistance to chemical damage is tested ― Involves the visual assessment and coding of the lens corrosion following exposure to a chemical ― eg. Nitric acid High refractive index materials and acid durability tests ― LESS resistance when coating with ARC Performance of Protective Lenses: Factors of the lens which are assessed are: Impact resistance Surface hardness Chemical resistance Thermostability Flammability Resistance to hot particles Radiosensitivity Performance of Protective Lenses: 1. IMPACT RESISTANCE 2. HARDNESS Influenced by the size and Best to coat plastic lenses speed of the particle and with a hard coating lens thickness and material especially polycarbonate Scratches and abrasion lenses as these are soft reduce impact resistance The greater the thickness 3. CHEMICAL RESISTANCE the greater the resistance Glass is resistant to most chemicals CR39 is high chemical resistance Performance of Protective Lenses: 4. THERMOSTABILITY 6. RESISTANCE TO HOT PARTICLES Welding and grinding etc Glass is more resistant to extremes in temperature Fuse into the glass lens compared to polycarbonate Not embed easily in plastic lenses 5. FLAMMABILITY 7. RADIOSENSITIVITY All plastic material are X-rays will be able to locate the particle flammable within the eye X-ray techniques can perceive glass particles greater than 0.5mm Plastics are not easily perceived by X-ray techniques Safety goggles vs Protective specs: Safety goggles ― Directly on the face, seals the eyes (no side particles can enter) ― Protects from splashes and dust and other particles ― Generally made from PVC Protective specs ― Sit across eyes ― Air and particles can entire from the sides ― Good for occupation where danger comes from front Safety Frames: Requirements are performance based Withstand certain impact tests (velocity and mass) ― High velocity impact test ― High mass impact test: lens must not break, nor come out of the frame Frame must not break and the lens or frame must not be in contact with the eye Do not put normal lenses into safety frames ― Gives a false sense of security Safety Frames: Markings 1. Size 2. Manufacturers trademark 3. Z87 or Z87-2 markings on both temples Z87 vs Z87-2: Safety frames that have passed impact testing by ANSI (American National Standards Institute) ― Z87: for basic impact requirements only ― Z87-2: Frames tested with 2mm thick lenses in place ― These frames Z87-2 also expected to retain 2mm high impact and 3mm basic impact ALL SHOULD THEREFORE HAVE Z87-2 Safety Frames: Marking Requirements: 1. FRONTS A – Dimension (eye size) DBL (distance between lenses) Z87 or Z87-2 Manufacturers ID trademark 2. TEMPLES Overall length Z87 or Z87-2 Manufacturers ID trademark Safety Frames: Metal Frame: CONCERNS Screws become loose quickly Lens might fall out if cut too small Metal frames have narrower rims than plastic frames (NB minimum thickness of lens) Safety Frames: 1. Side Shields: Injury from side Removable or permanent Must not restrict field of view Transparent material Polycarbonate Safety Frames: 2. Face Shields: Head worn To protect entire face or large portions can also cover the neck Different materials and thickness Protection from flying debris, metal, molten metal, chemical splashes, large chips Worn over prescription spectacles Good field of view  Safety Frames: 3. Goggles – to protect against flying debris Power tools Goggles ― Clear and plastic ― Covers front of eyes doesn’t have nose coverage Welding Goggles ― Not to look at UV light when welding ― Special filter to allow to look through the goggles while welding ― Can also be welding helmets ― Without = may lead to ARC eyes! An inflammation of the cornea, caused by ultraviolet radiation from the arc during welding Safety Frames: 3. Goggles – to protect against flying debris Cut-type goggles ADVANTAGES DISADVANTAGES Adjustable nasal fitting Cannot be used over prescription glasses Lenses can be replaced individually Poor ventilation – leading to fogging of the lenses Safety Frames: 3. Goggles – to protect against flying debris Box-type goggles ADVANTAGES DISADVANTAGES Can be worn over prescription specs Nasal fitting is not adjustable Good ventilation – usually have little Lenses cannot be replaced holes on the side of the goggles Lightweight Wide field of view Sport eyewear: Designed to protect the eyes and/or enhance vision in specific sports situations What is appropriate will vary dramatically, depending upon the sport ASTM: American Society for Testing and Materials American Society for Testing and Materials (ASTM) ― Develops standards for testing and materials Evaluate the ability of the eyewear to withstand and protect from impact of common equipment used in sports There are several ASTM standards that apply to ophthalmic dispensing ― Refer table 23-5 page 582 Most common code is F803: standard specification for eye protectors for selected sports ― Applies to common sports: baseball, basketball, soccer and tennis ASTM: American Society for Testing and Materials ASTM Product Marking 1. Marking on eyewear (applicable to all F803 approved eyewear) Manufacturers ID marking The eye protector model identity 2. Label or tag Week and year of manufacture Protector size Guidance concerning the age and gender that the product is designed for Must indicate which sport/sports the product is designed for ASTM: American Society for Testing and Materials ASTM Product Marking 3. Specific warnings (include but not limited to) Lenses should be replaced when scratches become troublesome or if cracks appear at edges Replace the product if damaged, even if it is not completely broken after it has been impacted If the lens pops out due to impact during play, replace the protector If protector is stored in a cold environment, let it return to normal room temp before use Include anti-fog and cleaning agents that may be used Themes in Sport Eyewear: 1. Virtually ALL sports demand highly impact-resistant lenses made from materials like polycarbonate 2. Helmets are required when there is danger of head injury 3. Outdoor sports call for UV protections, and when sunlight is a factor, sun lenses are appropriate 4. Most sport using round balls call for ASTM F803-approved protectors 5. Underwater sports for those with dependent on a prescription need special in mask or in goggle prescription adaptions 6. Golf, flying and shooting may require relocation of multifocal segments and/or optical centers 7. Bicycling and billiards may require changes to the positioning of the frame front END