DISP Flashcards
What are the refractive indices for plastic complex lenses?
1.6, 1.67, 1.74 with MAR
What are the refractive indices for glass complex lenses?
1.7, 1.8, 1.9 with MAR
How does refractive index affect density and aberration in lenses?
As refractive index increases, density and amount of aberration increases, while weight decreases for plastic and increases for glass.
What effect does reducing lens diameter have?
It reduces weight and thickness.
Why is it important to match HCD to PD?
To reduce the need for decentration.
What is the influence of high refractive index materials on lens thickness?
In full aperture lenses, they have much more influence on thickness than aspheric lenses.
What are the benefits of aspheric lenses?
Better optics over +7.00D, better cosmetic appearance due to flatter front surface, increased FoV. Better cosmetic appearance for minus lenses too.
What are lenticular lenses?
Lenses where the effective optical aperture is smaller than the frame aperture.
What are the uses of lenticular lenses?
Used to reduce weight and thickness of high powered lenses, but have poor cosmetic appearance and reduced FoV.
What types of lenticular lenses are there?
Standard lenticular and blended lenticular.
What are the types of bifocal and multifocal forms for negative lenses?
Bifocal and progressive up to -12.00D.
What characterizes classic progressive power lenses?
Stable distance power in the top half, stable reading area in the bottom central area, and a progressive power corridor joining the two zones.
What are the characteristics of soft multifocal lenses?
Longer progression lengths, aspheric distance curves, small stable distance and near zones, low surface astigmatism.
What are the characteristics of hard multifocal lenses?
Shorter progression lengths, spherical distance curves, larger stable distance and near zones, higher surface astigmatism.
What is prism thinning in lenses?
Incorporated vertical prism to reduce thickness and weight of lens with no relative prismatic effect.
What factors are important in lens fitting?
Mono PDs, heights required for progression, and pantoscopic tilt.
What are specialist multifocal lenses?
Occupation Varis.
What are some developments in multifocal lenses?
Optimizing for different additions and Rx, flatter form for better cosmetic appearance, using aspheric and atoroidal designs to improve optical performance.
How are tints applied to glass lenses?
By mixing metallic oxides into glass or vacuum coating.
How are tints applied to plastic lenses?
By dipping into a lens bath.
How are photochromic properties achieved in glass lenses?
By adding AgCl and CuCl during manufacture; UV exposure causes oxidation and reduction reactions.
How are photochromic properties achieved in plastic lenses?
Using spiroxazines that rotate to reduce light transmission.
What influences photochromic performance?
Temperature dependence and enhanced by MAR.
What is the purpose of hard coating on plastic lenses?
To increase surface hardness and resist wear and tear.
What is MAR in lens coatings?
Phases reflected rays by ½ wavelength causing destructive interference; multiple coatings for different wavelengths improve performance.
mirror coating
Coating causes constructive interference so doubles the reflects light
polarising
light reflected off surfaces become plane polarised, polarising filter will reduce the effect of this glare
vertical differential prism
Vertical differential prism is the difference in the amount of vertical prism in each lens of a pair of glasses. It can occur when the lenses have different powers, such as in the case of anisometropia
- usually not an issue with SV lenses - as px’s can move their head to look through OCs
Prism induced according to prentice rule
P=cF
o c = decentration in cm
o F = power in DS
