Week 10: Eye Protection & Anisometropic Patient Flashcards
Who Requires Eye Protection?
Mechanical Hazard:
- Sports player
- Motorcyclist
- Occupations such as; handyman
Chemical Hazard:
- Dealing with chemical solutions
Thermal/Infra-Red Hazard:
- Firefighter
- People dealing with molten metal
Radiation Hazard:
- Welding
- Laser
Microbiological Hazard:
- Preventing from a virus
- People in high risk environment/jobs (e.g. nurse)
How does lens fail due to stress?
- Elasticity is a measure of capability of stretching or deformation in response to a force, such that it will return to its original shape or size when the force is released
- Stress is a measure of force put on an object/area
- Strain is change in length of an object, original length or dimension of the object
- If strain of object increases, elastic limit of object will permanently deform or fracture
Describe Hooke’s Law
- For small deformations, object returns to original shape when force is removed
F = k△L, where △L: change in length
Smaller the k, easier it is to stretch an object
Describe Tensile stress
Rod is stretched a length △L when a force is applied parallel to its length
Describe Compressive stress
Same rod is compressed by forces with the same magnitude in opposite direction
Describe Elastic region
- Zone in which a material bends under strain and is able to return to its original shape when the force is released
- Beyond the elastic region, additional strain will result in material failing via fracture
- Charted using a stress strain curve
Describe the different Hoop Stress
- Tensile stress
- Focus per unit area which pushes the molecules apart - Compressive stress
- Force per unit area which pushes molecules together - Lens failure
- Compressive stress greater than tensile stress will occur in lens cracking
Flex Induced Back Surface Initiated typically occurs for;
- Moderate velocity particles
- Moderate mass objects
- More of an issue for minus lenses
Flex Induced Flattening Of Lens typically occurs for;
- Large mass objects
- Slow moving objects
- More of an issue for positive lenses
Flex Induced Flattening Of Lens typically occurs for;
- Large mass objects
- Slow moving objects
- More of an issue for positive lenses
How does Shock affect lens failure?
- Fracture will start at a peripheral flaw
- Flaw usually a poor quality bevel
- Large mass object
How does Plastic Permanent Deformation occur?
- Typically occurs if a lens is too thin
- It can go through the lens
How can glass be strengthened with Thermally Tempered Glass?
Production process:
1. Oven to raise to just below softening point (650o)
2. Time depends on volume (weight)
3. Air quenching so surface cools rapidly & sets hard
4. Internal body will cool more slowing & contract
5. A reinforced structure – in order to break the glass, the compressive stress skin must be neutralised
How can glass be strengthened with Chemically Tempered Glass?
- Glass is an Na+ in Si matrix
- Immerse in a K+ salt bath at temperature > 400oC by osmosis Na+ diffusion in
- Ask K+ is a larger ion than Na+, this results in compressive stresses over entire surface & edge of the lenses
- A reinforced structure – in order to break the glass, the compressive stress skin must be neutralised
- Used when heat tempering cannot be used
How can glass be strengthened with Laminated Glass?
Layer glass sheets with interlayers of plastic that holds the fragments together
What frame materials protect from mechanical hazards?
- Acrylic Frames (plastic)
- Nylon, cellulose acetate, polycarbonate
- One piece moulded with lenses
- Frame separate to lenses - Metal Frames
- Electric hazard – not recommended for people who work with electricity
- Screws work loose
