Multispectral polarimetric imaging with coherent illumination:: towards higher image contrast

Combining active multispectral and polarimetric imaging significantly enhances detection capability of very low contrast targets through the control of both the polarization state and the wavelength of the illumination light. However, increasing the operation range of the imaging system relies on the use of coherent sources, such as lasers and optical parametric oscillators, to illuminate the scene, leading to a dramatic decrease of the image quality due mainly to speckle noise. In order to investigate the benefits and drawbacks brought by coherent illumination, a preliminary laboratory demonstrator of an active multispectral polarimetric imager has been designed to operate with both polarized natural light and coherent sources. The orthogonal state contrast images recorded at different wavelengths in both configurations (coherent and non-coherent) clearly demonstrate the benefits of using active illumination of the scene to discriminate between real and fake targets and also to reveal very low contrast objects. Noise characteristics of polarimetric images under coherent illumination are also investigated. In particular the study of noise statistics of recorded images shows that the actual distribution of noise is log-normal. As a result, the so-called "natural" representation of the polarimetric image offers important advantages in terms of image processing. Indeed, if the intensity image is perturbed with multiplicative noise, the noise in the image with natural representation has uniform variance and is quasi-gaussian. The potential increase of target detection performance brought by properly processing the active polarimetric image is illustrated on a very low contrast scene.