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    Re: Telescope danger to sight. Was: Venus transit ...
    From: Jim Thompson
    Date: 2004 May 14, 07:58 -0300

    Here are some of the medical literature results that I found in my limited
    search using Pubmed.  (By the way, a search on "sextant" produced only 29
    references: 28 regarding prostate cancer, and 1 on dental work.  None had
    anything to do with the sextants we use in navigation.  So the world's
    medical literature appears to be silent on the issue of sextants and human
    How light reaches the eye and its components. Sliney DH. Int J Toxicol. 2002
    Nov-Dec;21(6):501-9.  United States Army Center for Health Promotion and
    Preventive Medicine, Aberdeen Proving Ground, Maryland 21010-5403, USA.
    "The human eye is exquisitely sensitive to light (i.e., visible radiant
    energy), and when dark-adapted, the retina can detect a few photons of
    blue-green light. It is therefore not at all surprising that ocular tissues
    are also more vulnerable to ultraviolet (UV) and light damage than the skin.
    For this reason, humans have evolved with certain anatomical, physiological,
    and behavioral traits that protect this critical organ from the UV damage
    that would otherwise be certain from the intense bath of overhead solar
    ultraviolet radiation (UVR) when we are outdoors during daylight. For
    example, the UV exposure threshold dose for photokeratitis ("welders' flash"
    or "snow blindness")--if measured as falling on a horizontal ground
    surface--would be reached in less than 10 minutes around midday in the
    summer sun. There are three critical ocular structures that could be
    affected by UV exposure: the cornea, the lens, and the retina. The cornea
    transmits radiant energy only at 295 nm and above. The crystalline lens
    absorbs almost all incident energy to wavelengths of nearly 400 nm. In
    youth, a very small amount of UV-A reaches the retina, but the lens becomes
    more absorbing with age. Thus there are intraocular filters that effectively
    filter different parts of the UV spectrum and allow only of the order of 1%
    or less to actually reach the retina. Nevertheless, this small fraction of
    energy--if phototoxic--could still be of concern. Finally, oblique rays
    entering the eye from the temporal side, can actually reach the equatorial
    (germinative) area of the lens."
    Phototoxicity to the retina: mechanisms of damage.  Glickman RD. Int J
    Toxicol. 2002 Nov-Dec;21(6):473-90. Department of Ophthalmology, University
    of Texas Health Science Center, San Antonio, Texas 78229-3900, USA.
    "Light damage to the retina occurs through three general mechanisms
    involving thermal, mechanical, or photochemical effects. The particular
    mechanism activated depends on the wavelength and exposure duration of the
    injuring light. The transitions between the various light damage mechanism
    may overlap to some extent. Energy confinement is a key concept in
    understanding or predicting the type of damage mechanism produced by a given
    light exposure. As light energy (either from a laser or an incoherent
    source) is deposited in the retina, its penetration through, and its
    absorption in, various tissue compartments is determined by its wavelength.
    Strongly absorbing tissue components will tend to "concentrate" the light
    energy. The effect of absorbed light energy largely depends on the rate of
    energy deposition, which is correlated with the exposure duration. If the
    rate of energy deposition is too low to produce an appreciable temperature
    increase in the tissue, then any resulting tissue damage necessarily occurs
    because of chemical (oxidative) reactions induced by absorption of energetic
    photons (photochemical damage). If the rate of energy deposition is faster
    than the rate of thermal diffusion (thermal confinement), then the
    temperature of the exposed tissue rises. If a critical temperature is
    reached (typically about 10 degrees C above basal), then thermal damage
    occurs. If the light energy is deposited faster than mechanical relaxation
    can occur (stress confinement), then a thermoelastic pressure wave is
    produced, and tissue is disrupted by shear forces or by cavitation-nonlinear
    effects. Very recent evidence suggests that ultrashort laser pulses can
    produce tissue damage through nonlinear and photochemical mechanisms; the
    latter because of two-photon excitation of cellular chromophores. In
    addition to tissue damage caused directly by light absorption, light
    toxicity can be produced by the presence of photosensitizing agents. Drugs
    excited to reactive states by ultraviolet (UV) or visible light produce
    damage by type I (free radical) and type II (oxygen dependent) mechanisms.
    Some commonly used drugs, such as certain antibiotics, nonsteroidal
    anti-inflammatory drugs (NSAIDs), and psychotherapeutic agents, as well as
    some popular herbal medicines, can produce ocular phototoxicity. Specific
    cellular effects and damage end points characteristic of light damage
    mechanisms are described."
    Eclipse retinopathy.Michaelides M, Rajendram R, Marshall J, Keightley S.
    Eye. 2001 Apr;15(Pt 2):148-51.
    Department of Ophthalmology, Queen Mary's Hospital, Sidcup, Kent, UK.
    "PURPOSE: Solar retinopathy is a well-recognised clinical entity of macular
    damage caused by viewing the sun, induced by a photochemical process. The
    term 'eclipse retinopathy' is frequently employed when the condition is
    sustained as a result of viewing a solar eclipse. Considerable public
    excitement had been raised in anticipation of the full solar eclipse on 11
    August 1999. Whilst experience has shown that visual morbidity is likely to
    be temporary, current evidence is anecdotal and restricted to isolated case
    reports and series. This study was conducted to establish the true visual
    morbidity associated with a solar eclipse, and whether it was temporary or
    permanent. METHODS: A 3 month active case ascertainment study was carried
    out from July to September 1999 to record cases presenting to
    ophthalmologists with visual symptoms arising from solar viewing. Further
    information about the cases was sought using a short questionnaire. A
    follow-up questionnaire requesting outcome data at 6 months was also
    employed. RESULTS: There were 70 reported cases of visual loss. The average
    age was 29.5 +/- 12.9 years. Half the cases presented to an ophthalmologist
    within 2 days of the eclipse. An abnormal macular appearance was reported in
    84% of patients at presentation. There have been no reported cases of
    continued visual loss or symptoms at 6 months. CONCLUSIONS: This is the
    largest nationwide study of the visual effects of a solar eclipse ever
    undertaken. There were no recorded cases of permanent visual loss, which
    corroborates the previous evidence that visual morbidity is likely to be
    temporary. It would appear probable that public health education was most
    effective in reducing visual morbidity and hence keeping the consequent
    burden on the NHS to a minimum."
    "The increase in solar ultraviolet radiation can have various direct and
    indirect effects on human health, like the incidence of ocular damage. Data
    of eye damage in residents of three suburban regions in Greece and in two
    groups of monks/nuns and fishermen are examined here. The statistics
    performed on these data provides new information about the plausible
    association between increased levels of solar ultraviolet radiation,
    air-pollution at ground level, and the development of ocular defects."
    "The retinal damage arising soon after exposure to sunlight showed many
    different aspects in the OCT images of the 4 cases examined. All retinal
    layers seemed to be altered, but these alterations disappeared after 1
    month, and the OCT findings remained the same after 1 year."
    "After gazing at a solar eclipse the ocular surface and tear film changed.
    While the aqueous layer remained pathological in many patients, the lipid
    layer and the mucous layer recovered spontaneously."
    "Following the present study, the existence of an excess risk of ocular
    melanoma in welders may now be considered as established. Exposure to
    ultraviolet light is a likely causal agent, but a possible role of other
    exposures in the welding processes should not be overlooked."
    Case reports of solar retinopathy in Nigeria:
    Chronic sun-gazing tomography findings:
    Unusual case of solar retinopathy:
    Jim Thompson
    Outgoing mail scanned by Norton Antivirus

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