Infrared phase imaging using complex scattering media

Abstract

Infrared imaging finds numerous applications in airborne measurements, structural monitoring, medical diagnostics, and spectroscopy. Long wave infrared (LWIR) radiation (λ= 8-14 m) enables self-illuminated thermal imaging, and can uniquely identify different chemical species. The investigation of thermal emission control using plasmonic antenna devices,¹ and the study of tunable free-form planar optics² has motivated our development of a novel high-resolution thermal imaging technique. Speckle imaging has been successfully used to image optical intensity or phase through complex inhomogeneous scattering media - particularly at visible wavelengths,³ and recently in the infrared.⁴ Single-shot high-resolution images of the scattered light capture sufficient information to reconstruct images through opaque media and around corners with diffraction-limited resolution. In this context, we have developed a high-resolution broadband speckle imaging setup in the LWIR for phase reconstruction, using a thin scattering medium in front of an uncooled microbolometric camera. Our method utilizes the large angular memory effect of a thin scattering medium. Local phase gradients within the incoming beam produce distorted speckle images after scattering by the scatterer's surface. Local translation shifts between the speckle patterns are estimated by a fast diffeomorphic image registration algorithm to obtain a phase gradient map. Integrating this gradient map in 2-D finally yields the wavefront profile. We demonstrate infrared phase image reconstruction using our broadband LWIR speckle imaging methodology, which promises future applications in imaging through visually opaque objects like semiconductor circuits, solid-state nanoelectronics, and infrared optical components, for defect monitoring.