Week 7 - Eye protection + british standards Flashcards
How can hazards be contained at source?
• Using screens or guards around machines or processes to reduce radiation hazards, impact resistant screens to reduce mechanical + chemical hazards
• Transfer process of chemical substances using enclosed pipes, and lubricants for mechanical hazards
• Replacing potential hazards, i.e glass with laminated glass
• Training workforce to recognise risks, and ensure there is good lighting
Function of eye protectors:-
• Any form of eye protective equipment covering at least the region of the eyes
- Optic radiations (0.1m to 1000m)
- molten metals and hot solids
- Droplets and splashes
- Dust; Gases
- Short circuit electric arc or any combination of these
Types of eye protectors:- Spectacles
• Spectacles: Eye protector with ocular mounted in frame, with or without side shields, i.e safety glasses
- Some protection vs chemical hazards + radiation hazards. NOT suitable for medium-high energy impact, dust, any liquid etc.
- One piece eye protectors:- usually moulded from polycarbonate
- Cup type: Mainly used in grinding flexible bridge, excellent eye closure, lenses easily changed, however bridge adjustment may not be small enough, poor ventilation = mist, uncomfortable; fov restricted.
Types of eye protectors:- Goggles first type
• Eye protector that sightly encloses orbit + sits on face, can be :
- Box type - single moulding covering both eyes + bridge of noise.
Advantages: full fov, good fitting, good ventilation, worn over specs, grade 1 impact, dust, chemical, gas and molten metal protection
Disadvantages: no adjustment of nasal fitting; limited ability to tint+change lens; easily scratched; difficult with multifocal; may impact spectacles underneath
Types of eye protectors:- Goggles second type
• Face shields: eye protector covering substantial part of face, normally suitable headband, helmet, protective hood or other appropriate mounting devices incorporated. Used for motorcycling, cricket etc
Advantages: whole face protection; worn over specs, full FOV; impact grade 1; chemical splashes, gas welding protection, possible tint
Disadvantages: Easily scratched, heavy
Types of eye protectors:- Welding hand shield:
• Welding hand shields: Hand held device covering face and neck
• Welding helmet: Worn on head, protects face, neck, eyes etc. Has glazed eye compartment for seeing in welding
- Opaque material of mask proof vs mechanical, chemical and radiation hazards
• Glazed filter can he changed to provide protection for both electrical, gas welding due to different energy wavelengths produced
Materials for eye protectors:- Thermally toughened glass
• Lens heated to 600°C for few mins, cooled rapidly creating compression envelope - improving impact resistance
• Advantages: Cheap and quick process
• Disadvantages:
- Prescription over +5D require prolonged heating, may cause warping which degrades optics.
-Heated lens always thicker than unheated of equivalent power.
- They do not retain same level of impact resistance over time due to scratches.
- They also reduce range if transmission of photochromic lenses.
- Heat-toughening process induces shadow or strain patterns
Materials for eye protectors:- chemically toughened glass
• Compression-tension coat produces by swapping smaller sodium ions in glass with larger sodium ions in solution - producing very thin compression coat. 470°C or 16hr
• Advantages:
- Thinner than thermal toughened
- Greater impact resistance
- Suitable for wide range of lens thickness
- uses lower temperature than that used for heat toughening reduces warping
• Disadvantages:
- Requires expensive equipment
- 5Ds+ require prolonged heating which can warp
- Thin compression coat damaged by scratches, reducing impact resistance
Materials for eye protectors:- Laminated Glass
• Composed of two glass layers with intermediate plastic layer.
• If breaks, glass splinters expected to stick to plastic
- Impact only slightly higher than crown glass
Materials for eye protectors:- Plastic lenses (advantages - 8)
• Advantages:-
- Greater impact resistance vs high velocity
- Scratches on lens surface do not reduce impact resistance
- If fractured, fragments are large + blunt
- 50% lighter than glass
- Thinner than glass of equivalent power
- Withstand molten metal splashes and hot sparks better than glass
- Less condensation
- Greater protection vs UV radiation
Materials for eye protectors:- Plastic lenses (disadvantages - 2)
• Soft and easily abraded, may need additional abrasion resistance coat to reduce impact resistance
• Refractive index for plastic lenses range of 1.49- 1.60 which result in higher chromatic abberations
Plastic materials :
• PMMA - first plastic used in UK for Rx
• CR39 - Greater impact resistance, easily tinted. If broken parts are sharper.
• Polycarbonate - High impact resistance compared to other lenses, very soft + easily abraded. Quartz coating used to avoid abrasion. Lightest material. Absorbs UV.
• Cellulose acetate - Poor impact resistance, but good chemical resistance.
BS EN 166 applies to:-
• All eye protectors where hazards likely to damage eye in: Labs, Education, home
- Includes those fitted with Rx lenses
- Doesn’t protect vs gamma and X-rays, laser beams, infrared radiation
• Excludes eye protectors for which separate and complete standards exist; laser eye protectors, sunglasses for general use
BS EN 166:- Deals with which requirements:-
• Designation
• Classification
• Basic requirements
• Particular requirements
• Testing and applications
• Marking
- Marking should contain information regarding material, manufacturer etc
Filter Marking on BS EN 166:-
• A scale number: Comprised of code number and shade number
