Biomedical Engineering Reference
In-Depth Information
12.1 Introduction
Healthy vision is arguably one of the most important concerns of human
health. Therefore it makes good sense to apply non-invasive Raman spec-
troscopy techniques also to investigations of ocular tissue components. The
visual process is enabled by the light-sensitive retina, which is positioned as
a hemispherical tissue layer system in the back of the eye (Fig. 12.1). The
retina is light sensitive, depending on age, over approximately the 400-750 nm
wavelength range, with highest sensitivity at green wavelengths. To reach the
photoreceptor layer of the retina, the light has to traverse an optical path
length of
2 cm through all anterior ocular tissue structures. Apertured by
the iris, the light has to propagate through the cornea, the aqueous humor,
the lens, the vitreous humor, and also through part of the anterior retinal
layer structure, the nerve fiber layer. While nature has designed all these oc-
ular tissues for high transmission in the visible wavelength region, any tissue
degeneration effects can easily attenuate the propagating light via induced
absorption and scattering processes. To view an object with high visual acu-
ity, humans have to move their eyes such that the object light falls into the
small central region of the retina, the macula, where the color-sensitive cone
photoreceptor cells are located. Outside the macula, the retina is mainly used
for peripheral vision, enabled by the rod photoreceptor cells. All interior oc-
ular tissue structures are protected by the surrounding thick, white sclera, a
tough tissue layer consisting of non-transparent collagen and elastin, which is
partially visible on the outside of the eye.
Due to the transparent nature of all ocular tissue components anterior
to the retina, Raman spectroscopy holds potential as a non-invasive optical
probe of the internal tissue components. It offers the possibility to identify
molecular tissue constituents with high chemical specificity, to quantify their
∼
Fig. 12.1.
Schematics of the human eye with main anatomical components. Raman
probing of any ocular tissue component in the living eye requires light delivery and
collection through the preceding ocular media
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