Radiation & Eye Flashcards
1
Q
What are the health effects of UV radiation according to WHO?
A
- Sunbed use or strong ground reflection from sand, water, snow
- Exposure can cause photokeratitis & photoconjunctivitis
o Comparable to sunburn of v sensitive skin-like tissues of eyeball & eyelids
o Usually appears within few hours of exposure
o V painful but are reversible - Cataracts: leading cause of blindness in world
2
Q
What are the effects of visible UV & IR radiation?
A
- Most higher energy x- and gamma rays pass completely through eye w/ relatively little absorption
- Absorption of short-ultraviolet (UV-B and UV-C) and far-infrared (IR-B and IR-C) radiation occurs principally at the cornea.
- Near ultraviolet (UV-A) radiation is primarily absorbed in the lens.
- Light is refracted at cornea & lens & absorbed at retina
- Near infrared (IR-A) radiation is also refracted and absorbed in ocular media and at retina.
- Eye transmits > merely visible light (400−700 nm), certain IR frequencies (e.g., IR-A) are also transmitted & may cause retinal injury
- For a person to receive an eye injury:
o (1) They must be looking with unprotected eye or optical sight
o (2) Laser must be oriented so it passes through the line of sight or into the eye
o (3) Central vision is affected only if person is looking directly at or near laser source - There is a likelihood that injury may be induced by light entering through the “corner of the eye,” it is unlikely that a single pulse will be too damaging.
o However, if thousands of pulses are directed into an area, one or more persons might be injured
3
Q
Describe types of UV light?
A
- X-Ray radiation and very short wavelength light (UV-B & UV-C): not much damage caused by ionisation because it passes through quickly. When it does damage it causes radiation cataract.
- Short wavelength UV-C radiation (100-280 nm): absorbed cornea and conjunctiva- but no hazardous effect.
- UV-B (280-310nm): cause conjunctivitis and keratitis because of photochemical damage imposed by photons.
- Longer wavelength UV (400 − 760 nm) and near IR (IR-A: 760 − 1400 nm) penetrates deeper resulting in blue light hazard.
- Longer wavelength IR radiation is blocked by cornea but above 750 and up to 2500 nm is transmitted through all ocular structures.
- Real hazard in glass or steel making factories because of intense heat.
- Sun also is a radiation hazard especially in both UV and IR range.
- Absorption of microwaves is small in the eye.
4
Q
What are the effects of radiation on the cornea?
A
- UV-B (280-310nm): cause conjunctivitis & keratitis because of photochemical damage imposed by photons.
o This is a painful effect also known as snow blindness or welder’s flash. It occurs because UV energy causes damage to or destruction of the epithelial cells. - Injury to epithelium is extremely painful as there are many nerve fibres located among the cells in the epithelial layer.
o However, it is usually temporary because the corneal epithelial layer is completely replaced in a day or two. - The reddening of the conjunctiva (conjunctivitis) is accompanied by lacrimation (heavy tear flow), photophobia (discomfort to light), blepharospasm (painful uncontrolled excessive blinking), and a sensation of “sand” in the eye.
- Corneal pain can be severe, but recovery usually only takes couple of days.
- Px w/ dry eye get similar feelings to these
5
Q
Effects on retina from radiation?
A
- Light (400 − 760 nm) and near IR (IR-A: 760 − 1400 nm) is sharply focused onto retina.
- When an object is viewed directly, the light forms an image in the fovea at the centre of the macula. This central area, approximately 0.25 mm in diameter for humans, has highest density of cone photoreceptors.
- Typical result of a retinal injury is a scotoma within the irradiated area.
o A blind spot due to a lesion in the peripheral retina may go unnoticed -> as brain/nervous system ‘fills in’ - However, if blind spot is located in fovea, which accounts for central vision, severe visual defects will result.
- Such a central blind spot would occur if an individual were looking directly at the laser source during the exposure.
- Size of blind spot depends upon whether the injury was near-to or far above the threshold irradiance, the angular extent of the source of radiation, and the extent of accommodation.
- The blind spot may be temporary or permanent.
- A haemorrhagic lesion is a severe eye injury characterised by severe retinal burns with bleeding, immediate pain and immediate loss of vision
o Requires a high intensity laser been used on eye. The spreading haemorrhage will produce long lasting (months) vision degradation/loss and ultimately produces a permanent blind spot (blind spot in visual field) at the point of haemorrhage. - A thermal lesion (caused by heat) requires less laser energy/intensity than is needed to produce a haemorrhagic lesion. However, it still produces a permanent blind spot.
- Glare/Dazzle is like flash blindness. Vision degradation occurs only during laser exposure and the glare stays in the same point in the visual field so one can move the eye to eliminate the effect.
- In summary, retinal effects are due to visible and near IR laser exposure. Retinal lesions can occur even if there is no prolonged loss of vision (i.e., at periphery of VF), however a retinal lesion is not always produced even when visual function disturbance has occurred (flash blindness, glare).
o If a retinal lesion is temporary, total visual recovery is seen within approximately 3 minutes
6
Q
Describe flash blindness?
A
- Flash blindness is a temporary degradation of VA resulting from a brief but intense exposure to visible radiation. Similar to effects of a flashbulb.
- In flash blindness, blind spot is temporary, & its size depends upon length of exposure and location of focus on the retina.
- Scatter of the laser beam through the atmosphere or an off-axis exposure may increase the blind spot size and result in an increased obscureness of the field of vision.
- There is a threshold of laser energy that can produce flash blindness, but the energy is less than that which causes a thermal lesion.
- Flash blindness is differentiated from glare by the fact that afterimage (blind spot) moves with eye movement and the afterimage lasts for a short period of time (minutes) after the laser exposure and recovery times range from a few seconds to a few minutes
7
Q
Describe radiation effects on normal eye?
A
- Longer wavelength ultraviolet radiation (UV-A, 315-400 nm) penetrates deeper into the eye as wavelength increases.
- Wavelengths of 290-320 nm the radiation can cause lenticular opacities and longer wavelengths may cause the same.
- Daylight may be a cause of cataract formation, given that the incidence of cataract is higher in countries with higher sunshine figures.
- But these effects cannot be separated from temperature effects – is it light or heat causing it?
- But the greatest effects are in high altitude regions, where the air is thin and there is snow which reflects UV wavelengths.
- The so-called blue light hazard extends from 400 nm up to around 500 nm, and peaks at 450 nm (visible range)
8
Q
Describe the blue light hazard?
A
- The mechanism of injury is photochemical, and it affects the photoreceptors of the retina.
- The extent of this risk is not clearly defined yet, but it is apparent that investigation of damaged retinas with powerful ophthalmic instruments such as slit lamps, indirect ophthalmoscopes and operating microscopes can present a short-duration high dose risk to the patient and a multiple exposure low dose risk to the practitioner.
- Corneal reflex from ophthalmic instruments is visible to user, and hence even the user of a powerful ophthalmic light source may be accumulating doses of retinal irradiance that could be hazardous. This is particularly true, considering that many thousands of episodes of such exposure will occur during the working life of the practitioner.
- An e.g. of focused visible radiation causing problems is a laser burn.
- Laser light energy need not be necessarily high, but damage can occur because the light is collimated (not diffuse)
- And it is focused by the optics of the eyes onto a very small area, which can cause retinal damage.
- Hazards may occur from specular reflections of the laser beam as well as just from the beam itself. The current safe level recommended for continuous wave lasers is 3 mW.m-2 at the cornea.
9
Q
Describe infrared light and the effect on eye?
A
- Cornea blocks longer infrared light.
- Light in bandwidth between 750-2500 nm, is transmitted with gradually diminishing efficiency.
- Some capture of energy by all ocular structures does occur resulting in some secondary damage.
- This is a heating effect caused by the absorption of the radiation.
- Proteins in crystalline lens coagulate resulting in a heat cataract.
o Cataract is normally posterior cortical – gradually spreads as a function of increasing radiation exposure - Especially relevant in industries such as steel or glass: a real hazard to the worker.
- Skin heat sensors sensitive to sudden high doses of radiation and not low dose exposure, which is overlooked by the body causing cataract onset.
- Remedy: use of heat absorbing filters.
- IR & UV from sun damage retina. Prolonged viewing of the sun with the naked eye can cause retinal damage
- Sun should never be viewed through focusing or magnifying lenses or with naked eye especially at eclipses because you can burn a hole in the retina
10
Q
What effect does microwave exposure have on the eye?
A
Unlikely to be much effect
11
Q
Describe Luminance transmission of filter scales?
A
- CIE standard illumination scales used
- Apparent brightness of a source seen through a filter at a given wavelength of light depends upon 3 things:
o radiant energy from source (CIE standard illuminant D65) at that wavelength (e)
o sensitivity of the eye at that wavelength (V)
o spectral transmittance of the filter at that wavelength (I). - Multiplying these three values together gives the apparent brightness of the source seen through the filter
- Luminous flux transmitted by filter
- LTF: Luminous flux incident on the filter - for illumination under standard illuminant D65.
- A third scale is used for specifying filters by British Standard 2724:1987.
- This scale is termed shade numbers and is equal to 1+ 7/3D Shade numbers vary from 1 (the lightest) to 15 (the darkest).
- Filters must fulfil the following 5 criteria:
o The absorption of harmful radiations
o The absorption of excess visible radiation producing glare
o The transmission of sufficient visible radiation to ensure good visibility
o The transmission of a wide enough band of visible radiation to retain correct colour discrimination
o Constancy of absorptive properties
12
Q
Describe sunglasses & sun-glare filters?
A
- Specifies physical properties (mechanical, optical etc.) for sunglasses & sun-glare filters of nominal plano power which are not prescription lenses, intended for protection against solar radiation for general use, for social and domestic purposes, including road use and driving.
- For sunglasses and sun glare filters for industrial use, EN 166: 1995 and EN 172: 1994 apply.
- This standard does not apply to eyewear for protection against radiation from artificial light sources, such as those used in solaria. EN 170: 1992 applies for these filters.
- This standard does not apply to ski goggles, for which EN 174: 1996 applies, or other types of eye protection used for leisure activities.
- This standard does not apply to sunglasses and filters that have been medically prescribed for attenuating solar radiation
13
Q
What should the sun-glare filters be?
A
- Protect the human eye against excessive solar radiation; to reduce eye strain; to enhance visual perception. In reduced light, sun-glare filters intended for bright daylight reduce visual perception.
- Filter requirements, Requirements for complete sunglasses, Testing procedures, Labelling, Informative appendices of sunglasses and sun-glare filters for general use.
- Defines the “relative attenuation co-efficient” (Q): T of signal, T of v
- Tv is the luminance transmittance of the sun-glare filter for CIE standard D 65.
- Tsignal is the luminance transmittance of the sun-glare filter for the spectral power distribution of the traffic signal light.
- Defines different types and category of filter:
o Gradient filter: filter in which luminous transmittance changes progressively in vertical meridian, when the filter is mounted, over some or all of the filter
o Photochromic filter: filter that reversibly alters its luminous transmittance under the influence of sunlight
o Polarising filter: filter in which transmittance is dependent on amount and orientation of the polarisation of the incident radiation
14
Q
Labelling & information that needs included in Sunglasses?
A
- Complete sunglasses
- Information to be supplied with each sunglass:
- Optom needs to double check
- In form of a marking on sunglass frame, an affixed label or packaging, or any combination thereof:
o identification of manufacturer or supplier; filter category number according to table 1 in the standard; number and year of this standard; in the case of filter category 4 and of filters the following warning: `Not suitable for driving and road use’ in the form of the approved symbol or in writing. The minimum height of the symbol shall be 5 mm. - Additional information to be drawn up by the manufacturer:
- Name and address of the manufacturer or supplier; type of the filter, e.g.: photochromic; polarizing; gradient; instructions for care and cleaning; explanation of the markings; optical class; nominal value of luminous transmittance
