PHYSICS

(J..), polarization (P), duration, frequency of light pulses, etc. Similar
to movement parameters, light parameters may be divided
into intensity which is characterized by absolute magnitude (I),
gradient in space (dl/dx), and time (dI/dt).
23.3.6. Photosensitization
Photosensitization is increased sensitivity (hypersensitivity) of
living organisms to light due to photochemical reactions induced
loy different chemical substances (photosensitizers). This phenomenon
has been a problem of economic importance in livestock in
various parts of the world for hundreds of years. For example,
Arabs in Tunis painted white horses with henna as protection
against hypericism - a disease resulting from infection by several
species of Hypericium that is facilitated by excessive sunlight. How
light exerted its effect was not known until O. Raab (Munich,
1900) observed that the killing of paramecia by a low concentration
of acridines was greatly hastened by exposure to bright
light. We now know that the general mechanism of photosensitization
is related to the formation of highly reactive species
(e.g., free radicals or singlet oxygen) that can lead to damage
and in some instances death.
The critical factor in animals that modulates photosensitization
is the sensitivity of lightly pigmented skin to sunlight
(e.g., in sheep the most susceptible sites are the ears, eyelids,
face, lips and coronets; in cows the teats, udder and escutcheon).
The first signs of photosensitization are often restlessness, exhibited
as shaking of the head and ears, rubbing or scratching of
the affected parts, and seeking of relief in the shade. In more
severe cases, erythema is soon apparent and oedema develops
rapidly. If the photosensitization is particularly severe, the animal
may become comatose and die within a few hours.
Diseases such as hypericism and fagopyrism, in which the photosensitizers
are plant pigments, are the oldest known examples of
photosensitization in domestic animals - sheep, cattle, goats,
pigs and horses. Hypericism results from ingestion of Hyperiicum
perforatum and fagopyrism is induced by buckwheat, Fagopyrum
esculentum. Photosensitization of animals eating Tribulus terrestris
and Sisymbrium altissimum has also been recognized for some time.
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23.4. EFFECT OF NEAR-INFRARED RADIATION
ON LIVING ORGANISMS
Visible and near-infrared radiation (400 to 2500 nm) is
important in medical applications; the longer the wavelengths in
the visible part of the spectrum, the deeper the penetration of
the radiation into the skin (fig. 23.2). There are many chromo-
Ultraviolet Visisble Infrared
,
region , region region
200 400 500 :800 1000 1200 nm
" '
,
• 0 ~. ·n '
Dermis~ c
(JO
Epidermis
..........~.. ~
" Ii
... .
• Subcutaneous • 4 • •• • •• l • • • • • •
• • • tisse·· • • • • • • • • • • • • • • • • •
Fig. 23.2. Penetration of light into light-colored human skin
phores in skin and blood: (e.g., proteins, amino acids, DNA,
RNA, hemoglobin, bilirubin, melanin). Two primary spectroscopic
phenomena partici pate in the interaction of infrared radiation
with human tissue - light scattering in the region of 1.52 ­
1.85 urn and molecular absorption (e.g., by water, fat, and protein)
in the 2.0 - 2.5 urn region. Tissue to be treated must be
thick enough to provide absorption of NIR radiation by glucose
but thin enough to allow sufficient NIR energy to pass through.
Water, hemoglobin (the pigment that renders blood red), and
melanin (the pigment that gives skin its color) present in human
surface tissue, exert a significant affect on the absorption of
optical radiation. The spectral range within the "therapeutic
window" (fig. 23.3) is between 600 and 1300 nm due to absorption
by water and pigments.
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