Week 11: Lens as Filters, Edging Process & Australian Standards Spectacle Delivery Flashcards
What is the purpose of filters?
- Reduce light transmission
- Protect eye against excessive solar radiation
- Reduce eye strain
- Increase visual perception
Electromagnetic Radiation and the Eye for UV vs IR
UV:
- < 380nm wavelength
- Potentially damaging
- Sunburn, snow blindness, welder’s flashburn
IR:
- > 760nm wavelength
- Less damaging than UV, heating
- High exposure leads to glassblower’s cataract
- Low exposure leads to ‘dry eye’
Describe Solar Radiation
- UVB (280-315 nm) absorbed by cornea
- UVA (315-380 nm): penetrate can reach lens & retina
- 380-780 nm gives rise to the sensation of light:
discomfort within this visual spectrum = glare
What are the Effects of Too Much Light?
Disability glare
- Excessive, intense light
- Significantly reduce contrast of retinal image
Discomfort glare
- Caused by direct or reflected glare
Distracting glare – ‘ghost images’ from lens
When light hits a lens it can be…
- Reflected
- Transmitted (transmittance, τ)
*If lens perfectly clear light will be transmitted through the lens to reach the eye - Absorbed
- Absorption = loss of light as it passes through a material
*Selective filters – selective absorption over part of visible spectrum
Measuring Tint Transmission
Spectral Transmission Factor (STF):
- Fraction of the original incident light transmitted by the lens for a given wavelength
- When STF is plotted against a series of wavelengths transmission curve produced
Luminous Transmission Factor (LTF):
- Overall effect of a filter on a standard eye viewing a standard light source
- Considers spectral sensitivity of the eye
Specifying Tints
Total energy of system is constant
- Transmission + Reflection + Absorption = 100%
- Light absorbed by material converted to heat
Describe Absorption
- Light is a photon or electron – many potential outcomes as energy is conserved
- Electron returns to the ground state
- Emits the photons of light resulting in reflection or scattering
- Makes a material dark or opaque to the wavelengths which are absorbed
Describe Tinted Lenses
- Lens that has a noticeable colour in transmission
- Fixed
- Variable / Photochromic
What are the factors to consider when prescribing a tinted lens?
- Type of tint
- Colour of tint
- Transmittance characteristics
What are the basic types of tint?
a) Solid glass tints
b) Glass surface tints
c) Laminated tint
d) Plastic lens dyes
Describe Solid Glass Tint
- The tint is included in the original glass manufacturing process, such that the lens is coloured throughout the whole of the lens
- Produced by adding a metal oxide to the glass mix –type of metal determines the colour
- Tint introduced during moulding stage by adding metallic oxides
What are the advantages vs disadvantages of Solid Glass Tint?
Advantages:
- Ability to absorb radiations, most notably infra-red
Disadvantages:
- Tint cannot be removed or changed
- Lens darker as gets thicker
- Graded tints not possible
- Expensive
Describe Glass Surface Tint / Vacuum tints
- Metal oxides are heated in crucibles beneath the lenses, which are rotating on a disc above
- The crucible & lenses are in a vacuum chamber, and the vapour produced by the oxide condenses to form a film on the back of the lens
Advantages vs disadvantages of Glass Surface Tint / Vacuum tints?
Advantages:
- Evenness of tint
- Tints can be removed
- Graded tints possible
Disadvantages:
- Back surface has a bloom
Describe Laminated /Bonding Tint
- Almost exclusively used for polarising filters, with the filter laminated between a front cover and the main lens
- Allows additional features
- Contrast enhancement
- Polarisation
- Photochromic
Advantages vs Disadvantages Laminated /Bonding Tint?
Advantages:
- Can incorporate filter that cannot be applied by dye
Disadvantages:
- Requires a thicker, and hence heavier lens
- Coat can split apart after lens is bevelled
Advantages vs Disadvantages Laminated /Bonding Tint?
Advantages:
- Can incorporate filter that cannot be applied by dye
Disadvantages:
- Requires a thicker, and hence heavier lens
- Coat can split apart after lens is bevelled
Describe Polarising Lenses
- Absorb visible light & plane polarised light reflected from a horizontal surface
- Thin film of polarising material bonded, laminated or sandwiched within lens
- Glazed so polarising axis is vertical
Disadvantages of Polarising Lenses
- Looking at LCD screens
- Flying/pilots
- Low light situations
- Not always good for skiing
- Not always suitable for motor cycle visors
- Cost
Describe Photochromic Lenses
- A lens which changes colour with a change in the intensity and wavelength of light falling upon it
- Darken with exposure to ultraviolet light (transitions)
- The transmission typically ranges from a minimum of 22% (darkest) to a maximum of 88% (lightest)
Factors Influencing Photochromic Performance…
- Light Intensity – exposure to red or infra-red light will act to ‘bleach’ the lens, or speed the return to the lens’s inactivated state
- Temperature – heating will act to bleach the lens, hence photochromic lenses do not darken as much on hot days as they do on cold days
- ‘Exposure Memory’ – photochromic will only display their full range and speed of change after being ‘broken in’. If lenses have not been used for some time, they will need to be broken in again
- Lens thickness – glass photochromic become darker as they get thicker. Plastic lenses show no such change
Advantages vs disadvantages of Photochromic Lenses
Advantages:
- Convenience
- Reduced cost
- UV protection
Disadvantages:
- Take longer to clear than darken
- Most require UV activation
- Temperature dependency
- Always a compromise
Polarised vs Tinted Lenses
Polarised:
- Reduce brightness
- Reduce disabling glare
Tinted:
- Reduce brightness (ased on absorption/transmission properties)
- Alleviate some distracting & discomforting glare
Describe Plastic Lens Dyes
- Plastic lenses are tinted by dipping the lens into a dye heated to approximately 92oC
- CR-39 lenses are relatively easy to dye, however some of the newer materials are significantly tougher
Advantages vs disadvantages of Plastic Lens Dyes
Advantages:
- Inexpensive
- Easy to apply
- Graded tints relatively easy to apply
- Practices can have their own facility for tinting
- Tints can be easily changed/removed
Disadvantages:
- Can be time consuming for practice staff
Most Common Tinted Lenses on the Market
Dip-dyed plastics:
- Uniform full tints
- Graduated tints
Glass:
- Solid
- Vacuum coated
Most Common Tinted Lenses on the Market
Dip-dyed plastics:
- Uniform full tints
- Graduated tints
Glass:
- Solid
- Vacuum coated
What are the Dip-Dye Method?
Plano sunglasses:
- Mould clear plano lenses (with added UV filter)
- Use very slowly penetrating dyes
- Large tint baths, dye circulated, temp controlled
Prescription lenses:
- Fast penetrating dyes
- Lack of consistency
Describe Graduated/Gradient Tints
- Controlled variation in tint or transmittance over the whole or part of the lens
- Assumed to be in vertical direction unless otherwise stated
- Double gradient-tinted lens: a gradient-tinted lens in which tints of one or more colours are used
- one colour fading in one direction, the other in the opposite, both along the same gradient-tinted direction
The colour of the tint will effect both the;
i) appearance of the spectacle lenses
ii) appearance of objects viewed through the lenses
As a general rule, coloured lenses absorb more light of the opposite end of the spectrum to their own colour
Describe ‘grey’ appearance for colour of tint
- Often considered to be the best ‘all-round’ tint
- Absorbs all colours of the spectrum relatively equally, as grey is not a true colour
- Ideal for people who do not want the relative appearance of colours to be affected
Describe ‘brown’ appearance for colour of tint
- Offers greater contrast than grey, though they do distort colours slightly
- Absorb relatively more blue light, which is typically scattered by the atmosphere to produce haze
Describe ‘green’ appearance for colour of tint
- Transmissible to light in the area with greatest sensitivity to colour in daylight (about 550nm)
- Will transmit brilliant greens and yellows well
Describe ‘yellow’ appearance for colour of tint
- Designed as contrast filters
- Enhance the contrast in hazy conditions/shadowed areas
- Popular for skiing goggles and shooters glasses
What is Ideal for Sunglasses?
- Luminous transmittance between 50% and 15%
- Even transmission of the visible spectrum
- Q values = 1
- Complete elimination of UVA and UVB
- Impact resistance
What is transmittance characteristics?
- Can be considered a function of the tint colour, though is more accurately a function of the material, and type of tint
- A spectrophotometer will provide the transmittance of the lens for different wavelengths of light
- Visible light extends from 380nm (violet) to 760nm (dark red)
Describe the different categories for sunglasses
Category 1
- Lenses that absorb the spectrum evenly
- No more UV or IR than crown
Category 2
- Lenses that absorb UV
- Evenly transmit visible
Category 3
- Lenses that absorb UV & IR
- Evenly transmit visible
Category 4
- Lenses that absorb visible in non-uniform fashion
Category 5
- Lenses that absorb special bands
- Occupational purposes
How do we Measure Tints?
- Spectrophotometer – measures transmission at each wavelength
- Average transmission (LTF) meter
- If tinted lens has optical power, may be necessary to measure master plano
- Visually compare with master lens
Define Surfacing & Edging
- Surfacing = process of creating the lens power
- Edging = process of finishing a correctly powered lens & grinding the edges so that the lens fits the shape and required centration of the selected frame
List the Flow Chart for Edging
- Consultation
- Prescribe rx and lenses
- Select frame & adjust frame
- Facial measurements (PDs, heights)
- Select lens (check power availability, dimensions allow lens to cut out after decentration
- Order lens
- Receive and check lens order
- Power verification
- Spotting
- Centration & blocking
- Select edging parameters, material, bevel type, size
- Chuck the lens
- Rough edge then add bevel
- Add safety chamfer
- Insert lenses
- Check dimensions to Australian standards of finished spectacles
Power Verification Then Spotting of SV Lenses
Check pages 5 & 6
Power Verification Then Spotting of MF
Check page 6
Power Verification Then Spotting of PALs
Check page 6
Describe Patterns
- Are used to allow the edger to shape the lens to fit the frame
- May be made of plastic or electronic in a computer’s memory
- Mechanical centre is the point around which the lens/pattern will rotate
Describe Tracing an Electronic Pattern
Tracing data can be sent to the lens manufacturer or an edging facility if edging is not conducted within the practice
Delivery of spectacles
- Prior to delivery spectacles to your patient, the finished spectacles must be checked to check for compliance with Australian Standards
- In many countries, standards are consistent with ISO standards which is the case in Australia
List the Spectacle Delivery
On-eye verification
- PDs
- Heights
- Vision check
- Frame fit
Advice to the patient
- Demonstrate the correct use of the spectacles
- Be positive & reassure
- Inform the patient of what to expect
- Invite the patient back for adjustments
Explain Adaptation
Important for new wearer or Rx change
- Base curve changes
- Spherical SV to aspheric SV
- Modality to modality
- Bifocals and PAL spectacles may require some adaptation
What are the Factors in Adaptation?
- Motivation
- Expectations
- Education
- Eye/head movement habits
- Occupation and hobbies
- Your encouragement
