topic 3 Light Attenuation

Question 1

what are the 3 interactions light can have with mediums?

Answer 1

1. reflection
2. transmission
3. absorption

Question 2

how to reduce the light transmittance?

Answer 2

higher RI and tint of mirror coat=lower transmittance

Question 3

what is the visible spectrum ?

Answer 3

wavelength: 380 to 780nm

consists of wavelengths of light that are able to stimulate the retina’s photoreceptors, giving rise to visual sensation of what we see

Question 4

what does the ultraviolet spectrum consist of?

Answer 4

UVC: 280nm
does not reach earth’s surface due to ozone

UVB: 300-320nm
reaches earth surface, constant exposure= cause harmful ocular condition

UVA: 340-360nm
reaches earth surface, constant exposure= cause harmful ocular condition

Question 5

what are 3 accumulative effects UV can have on eyes?

Answer 5

1. cataracts crystalline lens becomes more opaque
2. yellow spots on retina
3. pterygium
   conjunctiva extends into cornea

• cornea and conjunctiva absorb all UV. UVB causes most damage
• crystalline lens will absorb UVA of 295-400nm
• retina will have minimal UV absorption but can still be damaged. damage is permanent.

Question 6

what are the 3 occupations with high UV and IR exposure?

Answer 6

Glass blowers, furnace operators and welders.

Question 7

what are Effects of IR on Ocular Tissues?

Answer 7

When exposed to high intensity of IR, cornea will coagulate and becomes opaque. Iris will be depigmented and atrophic, lens capsule exfoliate and cataract formed. Retina will be burnt. (necrotic)

Question 8

what are 4 Types of “Sunglasses” Lens ?

Answer 8

1. Mirror coated
2. Tinted lenses (solid tint and surface tint)
3. Photochromic lenses
4. Polarizer

Question 9

what are 4 properties of mirror coat?

Answer 9

1. For glass and plastic
2. Metallic oxide is vaporized in a vacuum chamber and film is formed on the lens surface
3. Uniform layer =>Even colour appearance
4. Silvery non gradual

Question 10

what are 3 properties of solid tinted lens?

Answer 10

1. For glass lens only
2. Mineral eg iron oxide used for G15 tint in Rayban Lenses, is added during manufacturing
3. Lens appearance and light transmittance affected by lens thickness

Question 11

what happens when solid tint is used on high power lenses?

Answer 11

Uneven tint appearance

• minus lens= thicker in periphery hence darker at sides
• plus lens= thicker in center hence center is darker

Question 12

what are 3 properties of surface tinted lens?

Answer 12

1. For plastic lenses only
2. Dyed in hot bath approx 90°C.
3. The longer the soaking the darker the tint.
4. High index plastic lenses take longer time bc material is denser
5. Can be 1 tone or 2 tone / gradual tint

Question 13

what are the prescriptions for different levels of absorption?
(15-20%, 30%, 50%, 65-75%, 85% )

Answer 13

15-20%: cosmetic tint, no protection

30%: the maximum that can be worn indoors or at night

50%: max required for glare reduction (too dark for indoors)

65-75%: general purpose tint for glare reduction outdoors

85%: only 20% transmission, for pilots

Question 14

what is the general purpose of tinted lenses?

Answer 14

diff colors absorb diff wavelengths of light hence helps with certain activities

Question 15

Do all tinted sunglasses protect the eye from UV damage?

Answer 15

No. All tinted sunglasses, provide glare protection BUT The dark lenses cause the pupil to expand, letting in even more UV light. this causes more harm than good

Question 16

what is the purpose of yellow and purple tints?

Answer 16

yellow: Increase contrast in hazy conditions. Will absorb strongly at the blue end of the spectrum to improve contrast. Some says it is good for night driving

Purple: It is yellow light absorbing to enhance the discrimination of blue and red. Especially for glass blower who wants to see the heated glass against the yellow flame.

Question 17

what is the purpose of gray, dark green and red/ yellow tints?

Answer 17

1.Gray
Glare protection.
Evenness of transmission through the whole visible spectrum, allows colours to be seen in their natural state relative to one another.

2.Dark Green
transmission curve that closely approximates the colour sensitivity curve for the human eye.
Good for outdoor, golfing

3. Red/Yellow
   Increase contrast, but causes colour imbalances
   Usually for cycling

Question 18

what are 4 properties of photochromic lenses?

Answer 18

1. Photochromism: Reversible change of colour when electromagnetic energy is absorbed
2. Can be plastic(as a coating applied on front surface): Unopened Spiro-indoline compound + UV -> Opened Spiro-indoline compound (Darken)
3. Can be Glass (mixed in during manufacturing):Silver halide crystal + UV –> Free silver particles and halogen (darken appearance)
4. available in 3 colours: brown, gray and green

Question 19

how does photochromic lens work?

Answer 19

-Affected by temperature. The cooler the temperature, the darker it gets.
The hotter the temperature the faster it fades.

• Become less reactive over time.
• Does not react much in the car due to low level of UV and light

Question 20

advantages and disadvantages of photochromic lenses?

Answer 20

advantages: convenient. only need one pair of glasses for indoors and outdoors
disadvantages: costly, temp dependent, deteriorates with age, depends on UVR and can discolour. (replace discoloured lenses in pairs)

Question 21

what is the purpose of polarizer?

Answer 21

glare reduction, improve visibility

*The “blocking of light” is affected by head tilt

Question 22

how is a polorising lens made?

Answer 22

Incorporated in a lens by Lamination. A sheet of polyvinyl acetate with absorbed-iodine is embedded between the front and back layers of a lens. Can be done for glass or plastic lenses.

Prescription lenses with polarizing filter are to be custom made.

Question 23

who should use polorising

Answer 23

The activities that utilize the benefits of polarizing sunglasses include fishing, water sports, skiing, golfing, biking and jogging. For these activities, they offer a clearer view and eliminate glare.

Never prescribe polarizing sunglasses for pilots.

*Photochromic + Polarising filter can be given 2 in 1

Similarly UV protection with tinted lens and polarising filter can also be given together.

Question 24

How to increase UV protection?

Answer 24

UV coat (transparent) applied on plastic lens surface. Only CR39 wearer will need UV coating.

Mineral oxides are added into glass lens mixture to absorb UV. Different metal oxides give different colors to the glass lens at the same time.
Eg: Iron Oxide render green tint to the glass lens and it is being used for the well know G15 lenses by Ray Ban.