FIGURE 1.

The principle of Brillouin microscopy. A low-power, narrowband NIR laser light is focused into the corneal tissue, and the Doppler Brillouin frequency shift of scattered light from the focus is analyzed by a confocal spectrometer. Spontaneous Brillouin scattering originates from thermodynamically induced pressure (acoustic) waves. The magnitude of Brillouin frequency is proportional to the acoustic propagation speed of tissue at the focus and provides a direct measurement of local longitudinal modulus of the tissue. NIR, near-infrared.