4.1.5: Dispenses range of lens forms Flashcards
What are common lens forms?
- Concave/Biconcave: Minus Lens
- Convex/Biconvex: Plus Lens
- Planoconcave/Planoconvex
- Meniscus Lens: Spherical
- Toric Lens: Cylindrical + Spherical
Describe aspheric lenses and -ves?
Thinner, Flatter, Lighter
o Eliminates spherical aberrations, as causes all light that hits the lens
surface to converge onto one focal point
o Better Cosmetic Appearance due to flatter front surface
o Reduces centre thickness for High Plus and Edge thickness for High Minus
o Better optics in patients >+7.00D
Disadvantages of Aspherics:
* No useable zone in periphery
* Distorted vision is notorious in minus lenses à the only real use for a minus
aspheric is to reduce edge thickness
What are common lens materials? What happens as refractive index increases?
o Plastic – 1.6, 1.67, 1.74 with MAR
o Glass – 1.7, 1.8, 1.9 with MAR
CR-39: 1.498
* Crown Glass: 1.523
* Polycarbonate: 1.586
o As refractive index increases density and amount of aberration increases and weight decreases (plastic) or
increases (glass)
o Reduce the lens diameter will reduce weight, thickness
o Match HCD to PD to reduce need for decentration
o In full aperture lenses, high refractive index materials have much more influence on thickness than
aspheric.
Describe Freeform lenses?
o Generated precisely to 0.01D to allow for all light penetrating the lens to
come to a particular focal point.
o Frametised Freeform Lenses: Modified to fitting, frame and adjustment characteristics including patient’s fitting heights and monocular centration.
o Customised Freeform Lenses: Created specifically to the prescription and
the individual viewing habits of the user, including back vertex, pantascopic tilt and faceform wrap.
Describe lenticular lenses?
o The optical aperture is smaller than the frame aperture
o Reduces the weight and thickness of high powered lenses (especially plus lenses)
o Poor Cosmetic Appearance
o Reduced FoV
o Made from plastic
o Standard Lenticular: Carrier and Corrective portion are visibly separate
o Blended Lenticular: Carrier and Corrective portion are blended.
How do you work out total power of a spec lens?
The total power of a lens is a sum of the front and back surface of the lens
F1 + F2 =FT
e.g.
+4 + +2 = +6.00 D
+7 + -1 = +6.00D
Describe multifocal lenses?
o Classic progressive power lenses have stable distance power in the top half of the lens, stable reading area in the bottom central area of the lens, a progressive power corridor joining the 2 zones.
o Soft – longer progression lengths, aspheric distance curves, small stable distance and near zones, low surface astigmatism
o Hard – Shorter progression lengths, spherical distance curves, larger stable distance and near zones. Higher surface astigmatism
o Prism Thinning – incorporated vertical prism to reduce thickness and weight of lens (Equal R+L – no relative prismatic effect) (base down prism applied to reduce central thickness)
o Fitting – Mono PDs, Heights (Height required for progression), Pantoscopic tilt,
o Specialist – Occupation Varis
o Developments – optimising for different additions and Rx, flatter form for more cosmetic appearance, uses aspheric and atoroidal improve optical performance, lenses for different progression length
What are the bifocal segments?
- D-Segment (‘Half Moon’)
- C-Segment
- R-Segment (‘Round’)
- Executive
Number denotes the diameter of the reading portion
Describe the anti reflection coating? +ves and -ves?
Decreases surface reflections inside the lens by phasing
the reflected rays into ½ their wavelengths, causing destructive interference.
Multiple coatings are applied for different wavelengths. Ultimately this improves
the optical performance and removes glare for the patient.
o Advantages of MAR:
§ Improved visual performance
§ Reduced Glare
§ Increased Contrast
§ Improved Cosmesis
§ Mandatory for High Index
o Disadvantages of MAR:
§ Easily smeared due to hydrophobic coating
§ Expensive due to vacuum coating process
§ Prone to chemical damage from everyday products (e.g. hairspray)
Describe scratch-resistant? UV400? Glass? Plastic?
- Scratch-resistant: Increases the surface hardness of the plastic, resisting general
wear and tear due to cleaning. - UV400: Blocks UVA & UVB rays up to 400 nanometers. Good for patients with
cataracts & AMD.
o Glass – mixing metallic oxides into glass or vacuum coated
o Plastic – Dipped into lens bath (high index plastics variable ability to be tinted)
Describe photochromic/reactions? For glass? For plastic? What affects their performance?
Darken under light conditions when exposed to UV light.
May not work behind windscreens with a UV filter.
o Glass: AgCl and CuCl is added to the glass at manufacture, and UV exposure causes the chlorine to oxidise, causing the darkness. This reaction reverses under low levels of UV.
o Plastic: Spiroxazines rotate under UV exposure and form a shape which reduces light transmission. Several different Spiroxazines with different transmissions and temp
o Photochromic performance – Temp dependent (darker and faster in cold), Enhanced by MAR (increased UV transmission)
Describe tints, mirror tints, polarising?
- Tints: Can aid a patient with light sensitivity.
o Mirror – Coating causes constructive interference so doubles the reflects light
o Polarising – light reflected off surfaces become plane polarised, polarising filter will reduce the effect of this glare. Eliminates Horizontal and Vertical reflected light, causing surfaces to become plane polarised. This allows for enhanced detail and contrast and glare reduction – like MAR for a sunglass
What is recommended regarding the frame when dispensing multifocals?
- Do not pick a frame too deep:
o the reading portion will be further down and may become harder to access - Do not pick a frame too shallow:
o the differentiation between the Dist/Int/Read will become very close and
the px may accidentally access the wrong portion with a slight movement.
o The corridor lengths may not allow for a small eye size, and the reading
portion may be cut off in production
What is blank size? What is calc? What is decentration calc and why needed?
- A lens blank must be large enough to be cut to a specific frame and allow for some decentration:
Blank Size Calculation:
Frame PD – Px PD + Effective Diameter + 2mm.
Decentration calculation:
Frame PD – Px PD / 2 = X (in)
This is important as if the lens is not decentred in to fit the Px’s PD, a prismatic effect will
be induced
What do you need to measure for high Rxs? What levels of thinning are recommended for which rxs?
Measure:
* PDs
* Back Vertex (-/+5.00):
o lenses become more positive if moved away from the eye.
o Choose a frame that sits at this vertex distance
* Heights
Levels of Thinning:
o 1.6: 20% thinner than a standard lens (2.50 to 4.00)
o 1.67: 33% thinner (4.00 to 6.00)
o 1.74: 42% thinner (6.00 +)
On all thin lenses an anti-reflection coating is recommended as high-index lenses refract
the light at a quicker rate and therefore create more reflections
