 In this case the echo shape when measured through the optical phase variation is proportional to a spatial integral of the strain distribution (see below).Surface displacement detection has been demonstrated with ultrafast optical beam deflection and with interferometry.For a homogeneous isotropic sample in vacuum with normal optical incidence, the optical amplitude reflectance ("r") modulation can be expressed as

where formula_2 ("n" the refractive index and "κ" the extinction coefficient) is the complex refractive index for the probe light in the sample, "k" is the wave number of the probe light in vacuum, "η"("z", "t") is the spatiotemporal longitudinal strain variation, formula_3 is the photoelastic constant, "z" is the depth in the sample, "t" is the time and "u" is the surface displacement of the sample (in the +"z" direction)