Week 7- Radiation and the Eye

Question 1

Penetrating nature of higher energy, short energy and near energy:-

Answer 1

• Higher energy x-rays/gamma pass completely through eye with relative little absorption
• Near (UV-A) absorbed by lens
- Also absorbed by retina
• Short UV (UV-B, UV-C) and far infrared (IR-B and IR-C) absorbed by cornea

Question 2

Ocular Effects of UV, visible and IR:- (7 total)

Answer 2

• Visible light (400-700nm) and IR-A transmitted, therefore can damage retina
• UVC: 200-280nm; Corneal burn
• UVB: 280-315; Corneal Burn
•UVA: 315-400; Photochemical cataract
• Visible: 400-700; Cataract, retinal burn
• IR-B: 1400-3000; Corneal burn, aqueous flare, possible cataract
• IR-C: 3000+; Corneal Burn

Question 3

What are the effects of UV-B on the eye?

Answer 3

• UV-B (280-315) can cause conjunctivitis and keratitis due to photochemical dmg
- This painful effect is known as snow blindness or welders flash, due to UV damaging epithelial cels
- This is painful due to nerve endings in epithelial layer
- Only temporary due to replacement of layer is 1-2days
- Symptoms include conjunctivitis, epiphora, photophobia, blepharospasm

Question 4

What light has an affect on the retina + damage caused?

Answer 4

• Visible Light (400-760nm) and near IR-A (760-1400), is focused on retina, macula and fovea when viewed with eyes.
• Typical injury of retina is scotoma, or blind spot
• Injury to the fovea results in severe visual defects
- i.e laser source
- Size of blind spot depends upon whether injury was near threshold irradiance

Question 5

What is a retinal lesion?

Answer 5

• Haemorrhage lesion is severe eye injury characterised by severe retinal burns with bleeding, pain and vision loss
- spreading of haemorrhage will produce visual degradation + blind spot

• Thermal lesion requires less energy/intensity than haemorrhage
- still produces blind spot

Question 6

What is flash blindness

Answer 6

• Less energy than thermal lesion, but still has threshold
• Temporary decrease in VA due to brief intense exposure visible radiation
- Affect depends on length of exposure + location of focus on retina
• Flash blindness differentiates from glare by fact afterimage moves with eye, and lasts for short period of time
•Glare + dazzle is like flash blindness

Question 7

What instruments cause blue light Hazard:-

Answer 7

• Photochemical damage to photoreceptors of retina
• Can be Caused by slit lamps, indirect ophthalmoscopes, operating microscopes
- Short duration high dose risk to px with multiple exposure dose to practitioner
- Maximum threshold using indirect = 42s mid, and 23s at max
- Slit lamp fundus exam = 21s min 13s mid 8s max
- Operating micro = 2s max

Question 8

How can blue light hazard affect the practitioner?

Answer 8

• Accumulated dose due to reflection off reflex of px
• Focused by optics of eye in a collimation, small area = retinal damage

Question 9

How can Infrared light damage the eye?

Answer 9

• Cornea blocks longer wavelength light
• Light at bandwidth 750-2500 is transmitted with diminishing efficiency
• Capture of energy can result in secondary damage
- Due to heating effect
- Proteins in crystalline coagulate, resulting in a heat cataract
- Results in posterior cortical cataract

Question 10

Which industries is infrared radiation damage prevalent?

Answer 10

• Steel and glass industries
• Due to heat given off my radiation in these industries
• Remedy: heat absorbing filters
• Infrared and UV from sun can also damage retina + cataract

Question 11

Microwave exposure and eye damage?

Answer 11

• Latest study found no significant damage done to cornea by microwaves
• Study pulsed cornea specific waves, exposing caused 1-2C° increase in eyelids, at 4hrs, 3/7 for 3 weeks.
• Found no significant corneal damage after 7 days, 30 days, 90 days and 180 days

Question 12

How can radiation exposure be prevented? + spectral transmission curve

Answer 12

• Filters or tints mounted in spectacles
- Welding filters cannot be mounted in spectacles due to cut out of too much light
• Some filters may not cut it out entirely
• Need to use filters strong enough but will also specify spectral transmission curve
• Spectral transmission curve is ratio of transmitted to incident radiant flux for every visible wavelength of light
- scale has been produced to make it easier called the Luminous transmittance filter

Question 13

Luminance transmission of filter scales:-

Answer 13

• Apparent brightness of source seen through filter at given wavelength of light, depending on:-
- Radiant energy from source (CIE D65)
- sensitivity of eye at that wavelength
- Spectral transmittance if filter at wavelength
Multiplying these three values together gives apparent brightness

Question 14

Sunglasses and sun-glare filters standards:-

Answer 14

• Standards for plano, non-prescription lenses, for use vs solar radiation in general us: Annex A
• Industrial use: EN 166, 170 for Solaria, 172.
• Ski goggles:- EN 174

Question 15

What should sun-glare filters do + their categories:- (3)

Answer 15

• Main purpose : protect eye vs solar radiation, reduce: eye strain, enhance: visual perception.
- Strength measured as Tv and Tsignal (Q=Tsignal/Tv)
Sun glare filters can be categorised by:-
1. Gradient filter: gradually reduces transmittance vertical meridian
2. Photocheomic filter: reversible altered filter under influence of sunlight
3. Polarising filter: transmittance dependent on amount and orientation.