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Figure 1-5, Measurement of the anterior chamber depth using optical low-coherence interferometry. The graphs display the magnitude oi the reflected optical intensity as a function of distance. The measurement is analogous to ultrasound A-mode ranging, except that light is used rather than sound, A large reflection is observed from the anterior surface of the cornea while smaller reflections originate from the posterior corneal surface (cornea-aqueous boundary), and the lens or iris. Note the presence of scattered light which originates from within the cornea. The data plots show the structural features which occur along the axis of the optical beam. If the transverse position of the optical beam is changed, then different features are measured (the depth of the anterior chamber versus the depth to the iris).

echo delay or distance (axial or longitudinal range). Echoes may be observed from the anterior and posterior surfaces of the cornea as well as from the anterior capsule of the lens. The intensity of reflections is a measure of the discontinuity of the optical properties of the tissue. The reflection of the optical beam from the anterior surface of the cornea is relatively large; however, reflections from internal structures occurring at boundaries between different tissue types {i.e., such as between the cornea and aqueous, or different layers of the retina) are relatively small. In addition, the scattering of light from different tissues {i.e., such as within the cornea and lens, or, in the posterior eye, within different layers of the retina) can also be mea-

j sured.

The axial range measurement shown in Figure 5 permits a direct measurement of corneal thickness as well as anterior chamber depth. The thickness of the tissue is calculated by measuring the optical echo delay and multiplying it by the speed of light in the tissue. The speed of light in the tissue is given by the speed of light in vacuum times the index of refraction of the tissue. Thus, measurement of physical thickness from OCT relies on knowing or assuming a value for the index of refraction of the tissue. The strength or magnitude of the reflected light is extremely small, on the order of approximately 10~5 to 10"9 (-50 dB to -90 dB) of the incident light power Thus, very high detection sensitivity to very weak reflected echoes is required for ranging or measurement of structures within the eye.

Once an axial range measurement has been made, the relative positions of different structures may be measured by changing the transverse position or aiming of the optical beam within the eye. Figure loB shows a second axial range measurement with the optical beam aimed on the iris rather than the anterior capsule of the lens. Because the light beam can be fo-cussed to a small spot size, the transverse position of the beam can be known with high precision. Thus, information on both the axial or longitudinal as well as the transverse microstructure of tissue can be measured. This is the basis for performing optical tomography or cross sectional imaging.

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