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A heterodyne lock‐in imaging scheme is introduced to overcome NIR camera speed limitations and poor signal‐to‐noise ratios, thereby generating high‐frequency carrierographic imaging up to 10 kHz. A theoretical model was established to investigate the heterodyne signal generation mechanism as well as to extract the major carrier transport parameters by best‐fitting the entire modulation frequency dependence. Good agreement was found between the theoretically predicted and experimentally measured heterodyne amplitude dependence on excitation intensity. A contrast inversion feature in the high‐frequency heterodyne amplitude images was observed. High‐frequency information presents a method for resolving local near‐surface carrier transport parameters from the lumped effective lifetime, thereby providing ac‐diffusion‐length‐controlled depth‐selective/resolved imaging. Heterodyne lock‐in carrierography (LIC) amplitude images of a crystalline silicon wafer at different frequencies.

Silicone rubber composite insulators are increasingly used in high-voltage power transmission and distribution grids. However, the aging of composite insulators is a serious safety issue of power systems. Understanding the aging mechanism of composite insulators is therefore of great importance. In this paper, a 3D photothermal radiometry (PTR) is employed to determine the thermal and absorption properties of silicone rubber composite insulators aged in natural environment. A 3D PTR model was developed to extract precisely the thermal diffusivity and absorption coefficients of the composite insulators. In this PTR model both surface and bulk absorptions were accounted for by taking into consideration the fact that a surface absorption of aged composite insulators would be created and changed by surface crazing, chalking, and contamination during aging process. By multi-parameter fitting the measured modulation frequency dependences of PTR amplitude and phase to this 3D PTR model, the thermal diffusivity, surface absorption, as well as bulk absorption coefficient of the composite insulators were extracted simultaneously. The comparisons between the thermal diffusivities, absorption characteristics of fresh and aged composite insulators indicated that aging caused significant decrease of thermal diffusivity and substantial increase of absorption of composite insulators, and the thermal diffusivity and absorption changes were aging level dependent. These experimental results demonstrated that both thermal and absorption properties may be useful parameters for evaluating the aging level and understanding the aging mechanism of silicone rubber composite insulators.

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