Lens Materials Flashcards
The Choice of lens material will influence
- vision
- cosmesis
- safety
Lens Properties
- durability
- refractive index
- density and specific gravity
- curve variation factor
- v value
- reflectance
- uv cut off
Durability
Impact resistance = the ability of a material to withstand a high force or shock applied to it over a short period of time
- materials are tested with the drop ball test and graded in terms of impact resistance
- affected by the elasticity of the material - if alens is very elastic and a force is applied to it it can manoeuvre slightly and take some of the shock without cracking
- elasicity can be affected by glazing
- term can also be used in relation to how hard/soft a material is - tends to influence how well a material resists scratching
- can also be in relation to chem resistance
Refractive index
- higher n means the denser the material - more denser the material the more light will reduce in speed and the more light will refract as it enters the material
- to achieve the same effective surface power, less curvature is required and the lens material required
- flatter the curvature the thinner the lens will be
- less high index material is required than a lower refractive index lens of the same power
Refractive Index of plastic and glass lenses - common
CR39 = 1.498 (common plastic lens)
Spectacle crown glass = 1.523 (common glass lens)
Density and specific gravity
weight in g of 1cm3 of the material
1cm3 of glass weighs 2.5g
glass is 2.5x heavier than water
- ideally lenses should be as light as possible and density of material should be as low as possible
specific gravity = density of material/density of water
Curve Variation Factor
= ratio of the curvature of a materials surface compared to the surface providing the same power in spectacle crown glass
CVF = (1.523-1)/(n-1)
- can also convert the power of the lens into its crown glass equivalent
crown glass equivalent = F x CVF
V Value
- when a refractive index is given for a lens material, it is quoted for yellow light
- the other spectrum colours all have their own index
- manufacturers also give indices of the ends of the spectrum and give a ratio of the indices known as the v value
= a measurement of how much light is dispersed by the lens material - lower v value = more dispersion
- tolerance 0.1 prism dioptres
V value and Transverse Chromatic Aberration
TCA = P/Vd
Vd is wavelength and p is prismatic effect
The aberration is noticed by the eye when viewing through spectacle lenses esp when the v value is low
- its more significant in higher power lenses
- more significant in lower v value materials which are often high index lenses
Reflectance
reflectance at each surface of a lens is given by
p= (n-1/n+1)squared x 100%
- surface reflections dramatically reduce the transmission of white light with high refractive index materials
- some form of anti reflection coating is therefore essential when dispensing higher index lenses (1.67 or above)
UV Cut off
- light that is not reflected from the lens surface passes through - here some is absorbed by the lens material instead of being transmitted
- lenses have protection from some wavelengths even if there is no added UV protection
- UV ligght is found bet 100- 380nm