ABO Parts 4&5: Lens Materials and Coatings/Multifocal Designs

Plus lenses

Converge light, thicker in center, magnify objects, correct hyperopia

More thickness nasally, problems w nose pad adjustments

Minus lens

Diverge light, thinner center/thicker edges, minify objects, correct myopia

Reduced peripheral vision

Near vision

14 to 16 in

Flat top bifocal

Half moon, straight top, D segment

Round bifocal

Round segment or kryptoc lenses, limited near vision

Executive Bifocal

Segment line goes straight across, widest reading area available, thicker and heavier with noticeable set line

Blended bifocal

Segment line is blended to hide outline

Trifocal lenses

3 pts of focus: dist, int, near.

Intermediate vision

18 in to arms length

Progressive lens

Naturally smooth, continuous transition through all powers, no visible line

Occupational lenses

Double d segment, 14 mm

The higher the index of refraction

The more the material can bind light

Refractive index

How much the material will refract or bend light

Specific gravity

Density of material

ABBE value

Chromatic aberrations in a material, occur because color components have different wavelengths

Reflectance

Percentage of light reflected from the surface

Crow glass ABBE value

59

CR 39 ABBE value

58

Trivex ABBE value

46

1.7 glass ABBE value

35

Polycarbonate ABBE value

29

1.8 glass ABBE value

25

Transmittance

Amount of light that will pass through a material

CR 39 reflectance

4.0

Crown glass reflectance

4.3

Trivex reflectance

4.4

Polycarbonate reflectance

5.2

1.7 glass reflectance

6.7

1.8 glass reflectance

8.2

Crown glass UVA tranmittance

84.3%

1.7 glass UVA Transmittance

24.6%

1.8 Glass UVA Transmittance

19.5%

CR 39 UVA Transmittance

10.3%

Trivex UVA Transmittance

0%

Polycarbonate UVA Transmittance

0%

Glass lenses block

UV below 290 nm

Sunglass lenses should have between

15% and 30% transmission

1 and a

Lighter tints

2 and b

Medium tints

3 and c

Darker tints

Gray tints

Reduce overall brightness, doesn’t distort colors

Brown tints

Reduce glare, absorb higher frequency colors, increase contrast, good for emerging cataracts and hazy days

Green tints

Filter some blue light, glare, great contrast, good color balance, good for sports, best absorption of infrared lights

Yellow, gold, orange tints

Reduce blue light, good for skiing, cycling, provides maximum contrast good for overcast days

Purple and rose tints

Good contrast against green or blue background, good for hunting, waterskiing, snowmobiling, pink is good for computer use

Red tints

Increase contrast, good for fishing in early morning or late evening, low light conditions