topic 2 impact res lens

Question 1

briefly describe different methods of producing impact resistance.

Answer 1

1. lamination (put impact red material betw glass, reduce hazard of flying glass)
2. thermally toughened glass lens (undergo heat treatment, glass will shatter in larger pieces with no sharp edges)
3. chemical toughened crown glass lens
   * Higher impact resistant than heat treated glass lenses.

Question 2

what is drop ball test?

Answer 2

16 mm steel ball dropped from 1.27m height to test for impact res in toughed glass lenses

Lenses with chips to be rejected.

Question 3

what are properties of plastic lenses?

Answer 3

• All plastics are intrinsically impact resistant.

- Uncoated plastic is stronger

Question 4

how to increase impact res in plus and minus plastic lenses?

Answer 4

• Increase centre thickness of minus lens will further increase impact resistance
• Increase edge thickness of plus lenses will further increase impact resistance

Question 5

can polycarbonate lenses withstand acetone?

Answer 5

no!!! Polycarbonate is not chemical resistant

If solvent is used in the work place, then Trivex is the material to use, not poly carbonate

Question 6

what is the property of polycarbonate that allows it to have high impact res? what is a downside of this property?

Answer 6

Polycarbonate lens material is soft and requires an anti scratch coating.

However, the very softness of the material contributes to its high-impact resistance.
Instead of breaking on impact, the softer polycarbonate material is more likely to absorb a blow and just dent

Question 7

compare the RI (thickness), specific gravity (weight), abbe value, impact res, UV property and chem res of polycarbonate and trivex

Answer 7

RI: PC higher=1.59, Trivex=1.53

Specific Gravity (weight): PC heavier=1.22, trivex=1.11

abbe value: PC lower=29
trivex=44

impact res: both equally strong

UV properties: PC=variable to almost 100% UV, trivex=100% UVA UVB

chemical res: PC not chem res, trivex chem res

Question 8

list the 6 lens materials in increasing impact res order

UTCCHP

Answer 8

Untreated glass lens
Thermally toughened glass
Chemically toughened glass
CR 39 (n=1.49)
Hi-index plastic
Polycarbonate/ Trivex

Question 9

what is high velocity impact test?

Answer 9

using ¼ inch steel ball at 150ft/sec. This is meant to simulate particles that would be encountered in grinding, chipping, machining or other similar operations.

Question 10

what is High mass impact test?

Answer 10

A pointed 500 gm (1.1 lb) projectile is dropped 50 inches onto the complete protector mounted on a head form. This simulates a blow such as from a tool that slips from the work surface or when the lens collides with stationary objects.

Question 11

what are safety glasses frame requirement?

Answer 11

Only Frames that bear the Z87.1 are safe against impact.

Question 12

Identify 4 patient groups that need impact resistance lenses for dress eyewear

Answer 12

• Anyone who is monocular /with one good eye
• Children
• The elderly, particularly if unsteady on their feet
• Pseudoaphakes, aphakes and after Lasik

Question 13

Identify 2 patient groups that need impact resistance lenses for safety eyewear

Answer 13

• Patients working in dangerous industrial situations (some flying objects may hurt the eyes)
• Patients involve with dangerous hobbies or sports eg: carpentry and squash

Question 14

what are 3 things to note abt impact res lenses?

Answer 14

All lenses lose their impact strength in direct proportion to the breakdown of their polished and edged surfaces.

The greater the number and depth of scratches or the poorer the edge finishing, the weaker the lens becomes

All safety lenses require frequent replacement.