Absorptive lenses and lens coatings

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Absorptive lenses and lens coatings
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The optical spectrum
From Leroy Davis
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The optical spectrum

• We are regularly exposed to some UV radiation,
  the visible spectrum, and the IR portion of the
  electron-magnetic spectrum.
• Although exposure to radiation bordering on the
  visible spectrum does not cause the sensation of
  vision, these bands of radiation can exert
  harmful effects on the eyes.

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The optical spectrum

• UV radiation extends approximately from 100 to
  380 nm.
• The certain bands of UV radiation are associated
  with particular biological effects, the UV
  spectrum is arbitrarily subdivided into three
  bands
• UV-A extends from 380 to 320 nm.
• UV-B extends from 320-290 nm.
• UV-C extends from 290-200 nm.

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The visible spectrum

• The visible spectrum, extending from
  approximately 380 to 760 nm.
• The range varies with the level of illumination,
  the clarity of the crystalline lens of the eye,
  and other factors relative to the observer.
• Within the specified boundaries, radiation
  reaching the retina acts as a physical stimulus
  to produced electrical impulses that are
  conducted via the optic nerve to the occipital
  cortex of the brain, which provides the sensation
  of vision.

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The IR spectrum

• The IR spectrum extends from 760 to 106 nm.
• It is divided into three portions
• IR-A extends from 760-1400nm.
• IR-B extends from 1400-3000nm.
• IR-C extends from 3000 nm 1 mm.

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Classification of radiation effects

• Drapers law states that for radiation to have an
  effect on a substance through which it travels,
  it must be absorbed by the substance.
• Radiation has no effect (beneficial or
  deleterious) on a substance through which is
  completely transmitted or by which it is
  completely reflected.
• Radiation in the region of the visible spectrum
  causes the sensation of vision because it is
  absorbed by the phpotopigments of the retina.
• Ionizing radiation
• Non-ionizing radiation

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Ionizing radiation

• Most ionizing radiation pass through the eye, but
  small amount is absorbed.
• The damage depends on the exposure time,
  concentration, and the type of radiation.
• Ionizing radiation may have direct or indirect
  effect on ocular tissue.
• A direct effect may produce cellular anomalies or
  death.
• Indirect effect can result in damage to the blood
  vessels and thus restrict the blood supply to the
  tissue.

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Ionizing radiation

• Ionizing radiation can affect nearly all ocular
  tissue. Of the ocular tissue, the conjunctiva,
  cornea, and lens are the most vulnerable.
• At low level, the conjunctiva vessels become
  engorged and the cornea loses its normal luster.
• Heavier doses result in exfoliation of the
  epithelium cells, cornea ulcer, and keratitis.
• The most common effect of ionizing radiation is
  the formation of cataract.
• High level of ionizing radiation can result in
  retinal damage and degeneration extremely high
  levels can result is sudden blindness.

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Nonionizing radiation

• When radiation is absorbed by an ocular tissue,
  various effects are produced by the transfer of
  radiant energy to the molecules and atoms of the
  absorbing tissue.
• The absorbed energy can affect the visual
  apparatus in the following ways
• The thermal effect
• The photochemical effect
• Photoluminescence (fluorescence)

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Nonionizing radiation

• The thermal effect
• Heating effect
• Solar retinopathy, cause by looking directly at a
  solar eclipse.
• The photochemical effect
• In the visible spectrum, produces a chemical
  reaction in the retina initiating the sensation
  of vision.
• Harmful photochemical effects can occur with
  other ocular tissues, such as photokeratitis
  produced by excessive absorption of UV radiation
  by the cornea.
• Photoluminescence (fluorescence)
• The lens is capable of visible flurescence when
  illuminated by UV light.

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Concentration of radiant energy by the eye

• As radiant energy passes through the eye, it is
  attenuated in a number of ways
• Absorption by the ocular media
• Scattering within the eye
• Reflection by the various optical interfaces
• Loss caused by the aberrations of the eyes
  optical system

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Concentration of radiant energy by the eye

• The concentration of radiant energy within the
  eye also depends on the size of the pupil and the
  angular extent of the source.
• For a point source of high intensity, refraction
  by the eyes optical system concentrates the
  energy of the retina (A) and cause tissue damage,
  but has little effect on the cornea and the lens.
• Solar retinopathy occur after exposure to a
  solar eclipse.

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Concentration of radiant energy by the eye

• A small source of low-intensity radiation is
  usually harmless to the retina, an extended
  source of the same intensity may provide a
  dangerous concentration of radiant energy in the
  lens. (B)

A
B
Concentration of energy in the eye. A, point
source B. extended source.
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Absorption of radiation by the ocular tissue

• The tear layer absorbs only a small amount of
  radiation.
• absorbs below 290 nm and IR radiation above about
  3000 nm.
• transmits radiation from approximately 290 to
  3000 nm.
• The cornea absorbs UV radiation.
• absorbs below 290 nm and IR radiation above about
  3000 nm.
• transmits for UV in the range 290 to 315 nm and
  for IR in the range of 1000 to 3000 nm.
• High transmission in the range extending from 315
  to 1000 nm, which includes the long UV
  wavelengths, all the visible spectrum, and the
  shorter IR wavelengths.
• The transmission of the cornea particularly for
  the shorter wavelengths decreases markedly wit
  age.

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Absorption of radiation by the ocular tissue

• The aqueous humor absorbs very little radiation,
  with the result that any radiation that is
  transmitted by the cornea is also transmitted by
  the aqueous humor, and passes to the iris and the
  lens.
• In the iris, the uveal pigment absorbs radiation
  and converts in the heat.
• This conversion can be accompanied by a marked
  contraction of the pupil, probably because of the
  release of histamine.

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Absorption of radiation by the ocular tissue

• The lens, like the cornea, has variable
  absorption properties, depending on age.
• The child absorbs UV radiation below about 310 nm
  and IR radiation beyond 2500 nm, and thus
  transmits UV radiation between 310 380 nm.
• Old adult absorbs almost all radiation below
  about 375 nm and therefore transmits very little
  UV radiation.
• There is no change in the absorption of IR
  eadiation with increasing age.

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Absorption of radiation by the ocular tissue

• The vitreous mainly absorbs radiation below 290
  nm and above 1600 nm and therefore transmits to
  the retina radiation in the range from 290 to
  1600 nm.
• As the lens absorbs more UV radiation with
  increasing age, the amount of UV radiation
  available to the vitreous gradually decreases.

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Absorption of radiation by the ocular tissue

• The radiation received by the retina is the
  radiation transmitted by vitreous.
• Although UV radiation received by the retina
  decreases in amount with age, IR radiation does
  not decrease in amount -94 of IR radiation of
  770 nm reaches the retina, then falls to 90 at
  900 nm to a very low level beyond 1500 nm.

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Transmission of radiation by the ocular media
Ultraviolet (nm) Visible (nm) Infrared (nm)
Tear layer 290 -380 380-760 760-3000
Cornea 290-380 380-760 760-3000
Aqueous 290-380 380-760 760-3000
Lens (child) 310-380 380-760 760-2500
Lens (older adult) 375-380 380-760 760-2500
Vitreous 290-380 380-760 760-1600
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Effects of ultraviolet radiation

• UV radiation can have harmful effects on the
  conjuctiva and cornea by causing photophthalmia
  and development of pterygia, piguecelase, and
  band-shaped keratopathy it can effect the lens
  and cause cataracts, and it can affect the retina
  and cause macular degeneration.

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Effects of ultraviolet radiation

• Photophthalmia
• The primary effect resulting from absorption of
  UV radiation of 300nm and below is photochemical
  damage to the cornea epithelium.
• This is known as photophthalmia, photokeratitis,
  or photoconjuctivitis.
• The corneal epithelium absorbs most UV radiation,
  corneal damage is confined to this layer.
• Effect occur from 30 mins to 24 hrs, the length
  of time depending on the intensity of exposure.

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Effects of ultraviolet radiation

• Repeat exposures, with intermission equivalent to
  a single long exposure as long as the
  intermissions are sufficiently short(24 hr or
  less), keep physiologic healing from occurring.
• In acute photokeratitis, the pt experiences the
  sensation of a foreign body, photophobia,
  lacrimation, blepharospasm, redness, and edema.
• It occurs with long exposure to UV reflected from
  large areas of snow, calls snowblindness.
• Photokeratitis is self-limiting, the acure
  symptoms disappear within 24 to 48 hrs.
• Permanent damage is rare, and occurs only with
  extremely high-intensity exposure.

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photokeratitis
FromPacific University
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Effects of ultraviolet radiation

• Pterygia, pingueculae, and band-shaped
  keratopathy
• Repeat, long-continued exposure to UV radiation
  is widely thought to be a causative factor in the
  development of pterygia, pinguecula, and nodular
  band-shaped keratopathies.

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Effects of ultraviolet radiation

• Pterygia
• are growths of vascular and connective tissue
  into the epithelium of the bulbar conjunctiva and
  the cornea.
• A significantly high incidence of pterygia is
  found among outdoor workers who are exposure to
  UV, wind, and dust.

From Florida lion foundation for the blind, Inc.
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Effects of ultraviolet radiation

• Pingueculae
• It is small, yellowish elevated concretions of
  bulbar conjunctiva.
• They have long been associated with continued
  exposure to solar radiation.
• Microtrauma from windborne particles may also
  play a role in pingueculae.

From www.mrcophth.com
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Effects of ultraviolet radiation

• Band-shaped keratopathy
• It has white or cream-colored opacities between
  the epithelium and Bowmans layer, which are
  distributed symmetrically in the interpalpebral
  portion of the two corneas.
• The terms spheroid degradation and climatic
  droplet keratopathy are also used for this
  condition.
• The association of UV with band-shaped
  keratopathy is more firmly established than it is
  with pterygia or pingueculea.

Frommrcophth.com
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Effects of ultraviolet radiation

• Cataract
• One of the cumulative effects of the radiation is
  the formation of lens pigments that cause an
  increasing yellow coloration of the lens nucleus.
• The pigments are mainly produced in the nucleus
  and lead to decrease in the light transmission of
  the lens as one grows older.
• The cumulative effects of exposure to UV over a
  period of many years may be responsible for
  producing lens opacities, in particular, the
  brown or brunescent cataract of the nucleus.

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Effects of ultraviolet radiation

• The avascular lens, with its inefficient
  metabolic system, is vulnerable, apparently
  because its repair mechanisms are not as well
  developed as those of the cornea or the retina.
• UV-B (290-320 nm) has been implicated as the
  causative factor, on the basis of biochemical,
  photochemical, and physiologic studies.
• Both the UV-absorbing pigments in the lens and
  fluorescence of the lens increase with age, and
  fluorescent substances in the lens may be
  responsible for other changes, such as darkening
  of the lens, which leads to the brunescent
  (brown) form of senile cataract.

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Effects of ultraviolet radiation

• Environmental, nutritional, and genetic factors
  are also known to play a role in the etiology of
  cataract, but epidemiologic and experimental data
  suggest that UV is an important factor.
• A hat with brim and closefitting sunglasses with
  UV-B absorbing lenses should be worn at time of
  maximal exposure to sunlight.

From??????????
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Effects of ultraviolet radiation

• Retina
• In the normal eye, the UV by the filtering action
  of the cornea and lens.
• Under ambient solar radiation, the small amount
  of UV reaching the retina is not likely to cause
  any serious retinal damage.
• It is possible that repeated exposure for period
  of years may lead to some degree of damage
  because of slow, cumulative effect.

From Tom H Williamson.
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Effects of ultraviolet radiation

• When the lens has been removed because of
  cataract, the aphakic eye is subjected to UV in
  the range of 320 380 nm, which had previously
  been filtered out by the lens.
• The absorption of UV by pigment epithelium of the
  retina and by the choroid enhances the potential
  for photochemical and thermal damage.
• The study (Ham et al.) found that in the absence
  of the lens there was sufficient UV-A in the
  environment to damage the retina.
• Cystoid macular edema is well-known complication
  of cataract surgery and may be caused by the
  increased amount if UV-A and visible reaching the
  retina of the aphakic eye.