Fig. 1. （a）Schematic diagram of the inspection principle using the electrical parameters of epitaxial wafers instead of the electrical parameters of individual LED chips；（b）Cross-section diagram of LED wafer along X-Y axis^［13］；（c）Microscope image of Micro-LED array^［17］；（d）Luminance pseudo-color image and luminance 3D distribution of a single Micro-LED chip^［17］；（e）Schematic diagram of LED inspection system based on eddy-current heating^［20］；（f）Thermal image of two samples under eddy-current heating^［20］.

Fig. 2. （a）EL measurement system with double spectrometer^［24］；（b）Schematic diagram of EL measurement system^［24］；（c）Schematic diagram of inspection based on the transparent conductive film；（d）Microscope image of LED electrode after measurement^［25］；（e）Microscope image of LED electrode after measurement with the transparent conductive film^［25］.

Fig. 3. （a）Schematic of working mechanism of Micro-LED under forward electric field；（b）Schematic of working mechanism of Micro-LED under reverse electric field；（c）Schematic diagram of Micro-LED inspection based on non-electrical contact mode；（d）Schematic diagram of Micro-LED array and the luminescence image.

Fig. 4. （a）Schematic diagram of PL inspection；（b）Luminescence image of PL inspection^［27］；（c）Luminescence image of CL inspection^［27］；（d）Equivalent circuit of LED under light excitation^［30］；（e）Principle diagram of EL and PL.

Fig. 5. （a）Schematic diagram of AOI system^［38］；（b）Inspection image of LED array^［38］；（c）Microscope image of a single LED^［39］；（d）Microscope image of LED with defects ^［41］；（e）Schematic diagram of visual inspection process^［45］；（f）Inspection images of LED with different kinds of defects^［45］.

Fig. 6. Comparison of various LED inspection technologies