Enhanced angular domain optical imaging by background scattered light subtraction from a deviated laser source

Imaging structures within a turbid medium using Angular Domain Imaging (ADI) employs angular filter array aligned to a laser source to separate ballistic and quasi-ballistic photons from the highly scattered light by means of angular filtration. The angular filter consists of a high aspect ratio linear array of silicon micromachined tunnels, 51 micron wide by 10 mm long with a 0.29 degree acceptance angle. At heavy scattering ratios of > 1E7:1 image detectability declines due to the non-uniform scattered background light fraction still within the acceptance angle. This scattered signal can be separated out by introducing a wedge prism to deviate the laser source where it enters the medium by an angle slightly larger than the acceptance angle. This creates a second image consisting of pure scattered photons with the filtration characteristics of the angular filter, and a pixel by pixel correspondence to the fully scattered illumination emitted from the medium. Experiments used an 808 nm laser diode, collimated to an 8 x 1 mm line of light, entering a 5 cm thick medium with a scattering ratio of > 1E6:1, with a wedge prism creating a 0.44 degree deviation. Digitally subtracting the deviated scattered signal from the original image significantly reduced the scattered background and enhanced image contrast. We can obtain images with scattering ratios at least 40 times more than our previous scattering limits. The contrast level can be increased from 0.04 of the total dynamic range to over 0.50, depending on test phantom object location, which results in higher definition and visibility of our micro-scale test structures in the turbid medium.