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Absorptive lenses and lens coatings

Absorptive lenses and lens coatings. The optical spectrum. From: Leroy Davis . The optical spectrum. We are regularly exposed to some UV radiation, the visible spectrum, and the IR portion of the electron-magnetic spectrum.

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Absorptive lenses and lens coatings

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  1. Absorptive lenses and lens coatings

  2. The optical spectrum From: Leroy Davis

  3. 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.

  4. 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.

  5. 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.

  6. 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.

  7. Classification of radiation effects • Draper’s 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

  8. 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.

  9. 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.

  10. 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)

  11. 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.

  12. 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 eye’s optical system

  13. 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 eye’s 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.

  14. 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.

  15. 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.

  16. 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.

  17. 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.

  18. 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.

  19. 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.

  20. Transmission of radiation by the ocular media

  21. 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.

  22. 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.

  23. 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.

  24. photokeratitis From:Pacific University

  25. 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.

  26. 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.

  27. 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

  28. Effects of ultraviolet radiation • Band-shaped keratopathy • It has white or cream-colored opacities between the epithelium and Bowman’s 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. From:mrcophth.com

  29. 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.

  30. 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.

  31. 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:高須眼科ホームへ戻る

  32. 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.

  33. 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.

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