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Instrumentation and interpretation of the frequency domain thermoreflectance(FDTR) technique will be presented.FDTR is an alternative to time domain thermoreflectance that can be used to measure thermal conductivity and thermal interface resistance,but has more recently been used to measure spectral contributions of phonons to thermal conductivity.FDTR uses a modulated CW laser(a.k.a.the pump)to periodically heat a sample,and an unmodulated CW laser(a.k.a.the probe) to sense the temperature response based on the samples thermoreflectance.The amplitude and phase of the thermal response,relative to the heating,are used to determine the thermal properties of the underlying sample.Periodic modulation at high frequency f(0.1-200 MHz)confines the thermal penetration depth Lp=(α/πf)1/2(where α is thermal diffusivity)so that it is possible to probe thin films and interfaces without being overwhelmed by the substrate properties.Recent experiments done by both TDTR and FDTR [1-3] show that when Lp or the laser spot size are commensurate to the mean free paths of energy carriers,non-diffusive transport results in an apparent reduction in thermal conductivity.I will discuss analytical solutions to the Boltzmann Transport Equation that explain these results and create a pathway to study spectral contributions of phonons to thermal conductivity in non-diffusive thermoreflectance experiments.