L8 Lens Properties

Question 1

The Choice of lens material will influence

Answer 1

• vision
• cosmesis
• height
• safety

Question 2

Lens Properties

Answer 2

• durability
• refractive index
• density and specific gravity
• curve variation factor
• v value
• reflectance
• uv cut off

Question 3

Durability

Answer 3

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

Question 4

Refractive index

Answer 4

• 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

Question 5

Refractive Index Table

Answer 5

Standard Index = >1.48 <1.54
Mid Index >1.54 >1.64
High Index >1.64 >1.74
v high index - more than and including 1.74
CR39 = 1.498 (common plastic lens)
Spectacle crown glass = 1.523 (common glass lens)

Question 6

Density and specific gravity

Answer 6

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

Question 7

Density of finished lenses

Answer 7

• weight of the finished lens in high index glass material wont be heavier than the crown glass equivalent
• for plastics material, the weight of high index and CR39 will be at least 50% that of the crown glass equivalent
• the higher index plastics being the lightest as youre using less material and have a thinner centre substance

Question 8

Curve Variation Factor

Answer 8

= 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

Question 9

V Value

Answer 9

• 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

Question 10

V value and Transverse Chromatic Aberration

Answer 10

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

Question 11

Reflectance

Answer 11

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)

Question 12

UV Cut off

Answer 12

• 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