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Speckle reduction in THz imaging systems with multiple phase patterns Host Publication: Finds and Results from the Swedish Cyprus Expedition: A Gender Perspective at the Medelhavsmuseet Authors: I. Jäger, J. Stiens, G. Koers, G. Poesen and R. Vounckx Publisher: SPIE Publication Date: Apr. 2006 Number of Pages: 12
Abstract: Free-space active millimeter wave imaging where THz radiation is employed to illuminate the object and the signals reflected from the object are collected by the receiver, has a number of advantages for security applications in W band . Among them is imaging of concealed objects. Because all existing THz sources are coherent speckle is one of the ultimate limiting factors of the free space imaging techniques. Speckle arises when coherent light scattered from a rough surface is detected by an intensity detector with a finite aperture, hiding the image information. This problem is of special importance for THz imaging, because surface roughness is closer to the object dimension as in optical imaging. The reduction of speckles is highly desirable and we propose here two solutions, introducing uncorrelated phase patterns in the object plane. The first solution employs a Hadamard diffuser for multiple phase patterns , the second using of a polychromatic radiation. We use computer simulations to demonstrate that small random phase perturbations manifest themselves as random illumination modulation for speckle reduction.
Because speckle patterns are best described in statistical terms, we will follow Goodman, and use speckle contrast as a measure of speckle. It is defined as the ratio of the standard deviation to the mean of the speckle intensity, and its value is between 0 and 1. For fully developed speckle patterns from a monochromatic light source speckle contrast is unity. Standard deviation of nearly zero shows that illumination is approaching a perfectly incoherent system. Averaging over many pixels reduces speckle.
A Hadamard transform is a way which would completely eliminate speckle. It is a square matrix of order 2x2 or 4x4 or 8x8 and so on, over (ǃ, 1), where each row and each column are perfectly uncorrelated. All of them, considering as vectors - are orthogonal. A Hadamard diffuser is a phase selective surface, giving maximum speckle contrast reduction with the minimum number of distinct phase patterns. The Hadamard phase patterns can be fabricated as a dielectric (Teflon, n=1.75) mask by means of micro-drilling. The non-drilled cells correspond to relative phase, and the drilled cells . For f=100 GHz, the drilled depth is 3.5 mm. We examined how effective is the Hadamard transform if the phases are not exactly 180 deg. Simulations show an improvement of the original speckle contrast under Hadamard transform. The improvement from 1 to 0 starts almost linear with the 90 deg phase. This phase 90deg corresponds to the wavelength 1.75 mm or to the frequency 171 GHz, being used in transmission (and even better in reflective way). It means that a very broadband imaging system can be designed with Hadamard phase patterns. For the wavelengths, not exactly suitable for the surface depth, the transform works better in a combination with other speckle reduction techniques.
We demonstrated theoretically, following again the Goodman procedure, how phase perturbations in the object plane, due to the partially coherent illumination, reduce speckle. As mentioned in [2], three cases of polychromatic radiation are possible: (a) multiple sources, (b) a broadband tunable THz-source a pulsed source. The speckle pattern produced by polychromatic radiation is composed of a sum of the speckle produced independently by all monochromatic components and depends both on temporal coherence of the radiation and on surface roughness [5]. We examined the sensitivity of speckle contrast reduction to the imaging parameters: number of wavelengths, central wavelength, wavelength difference and distance from the center of the object to the center of the receiver. Simulations show that the speckle contrast is getting larger with the growing number of wavelengths. In F-band (90끔 GHz) with 60 pixels it saturates from 60% for 2 wavelengths to 99% for 8. The configuration giving a reasonable contrast (60%) is achieved already with 2 wavelengths. This shows that dichromatic illumination is sufficient for speckle reduction in security applications (but may be not enough for elimination of Gibbs ringing). For dichromatic illumination we fixed two imaging parameters of changing the third and compared speckle reduction. After this procedure we have chosen all imaging parameters for security applications. Speckle size for such a setup is maximum 3 mm.
In this paper we quantified the effect of phase perturbations on the speckle reduction, The effect will be proved experimentally and published elsewhere. External Link.
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