Comparison of optical and millimeter wave imaging on speckle

In this paper, the origin of speckle phenomenon is explained from the wave point of view. The finite power collection ability of the system is considered to be the 1st order factor which can be solved by infinite large lens theoretically, while the path fading and interference of the wave during propagation can not be avoided by a larger lens. Hadamard speckle reduction approach is effective to deal with speckles generated by electrically large roughness (several wavelengths). In active millimeter wave imaging, the power issues due to electrically small cell size may be the bottleneck to high resolution. Therefore, Hadamard technique can be effective in practical active millimeter wave imaging systems conditionally, and it usually ends up underperforming to theoretical best.