Study of active millimeter-wave image speckle reduction by Hadamard phase pattern illumination

Abstract

Active millimeter-wave images typically exhibit characteristic speckle noise, due to the coherence of artificial millimeter-wave sources. We study the Hadamard speckle contrast reduction (SCR) technique, which has been successfully used in laser projection systems, in the context of millimeter-wave imaging. We show the impact of Hadamard pattern order and size and of image and pattern resolution on speckle reduction efficiency. Practical limitations of Hadamard pattern implementations and their effect on speckle reduction efficiency are also discussed.

Discussion on Validity of Hadamard Speckle Contrast Reduction in Coherent Imaging Systems

“…So whenever the pi phase hop happens, it would always exist. Considering the small adopted f -numbers of 1/2 and 1/3, which are already much better than practical millimeter wave imaging systems which have an f -number around one [5], we can conclude that practical field distributions for the projected Hadamard phase patterns can not be absolutely flat. As shown in Figure 3 in Figure 3(d).…”

“…These examples validate that the performance of the Hadamard phase projection sub-system is influenced by both the electrical size of a cell and the f -number of the lens. Rather than simple geometrical descriptions of the Hadamard diffuser in term of "overfill" or "non-overfill" the lens, the simulations above based on wave optics show that the phase projection process is not as clean as in previous discussions [5,6]. Generally, phase patterns can be projected well, but amplitude modulations occur due to the Gibbs ringing effect, interference of the waves accompanied with aberrations during diffraction [17], and power loss.…”

“…Unlike [6,11], in which only the final synthesized image can be attained, we show all the partial images. In case of millimeter wave imaging systems, we emphasize the non-paraxial features during wave propagation by calculating diffractions, which can work beyond the limitations of Fourier optics which is applied in [5]. To quantize the discussions of speckle reduction, we follow the optical concept of speckle contrast [1], which is the ratio of the standard deviation and the mean value of all the pixels in terms of intensities.…”

“…By coupling the incident power on sensors, detected signals fluctuate and speckles appear on the attained images. Speckles generally blur the image, making information extraction quite difficult even by image postprocessing techniques [5]. In some serious cases, images are totally blurred and can not be recognized at all [1].…”

“…The working principle is explained based on the concept of point spread function (PSF) and geometrical optics. In [5], the Hadamard SCR system is simulated by Fourier optics at 100 GHz with corresponding SCR performance of 75% for Hadamard 4 and 62% for Hadamard 16 compared to the theoretical values of 2 and 4 respectively. In [7], the experiment setup at 100 GHz shows 80% SCR compared to the theoretical value by applying Hadamard 4.…”