Optical Imaging through Non-Transparent Small Aquatic Creatures with Angular Domain Imaging

摘要

When imaging through small aquatic creatures, scattered photons produce problems in image quality and resolution. Angular Domain Imaging (ADI) reduces scattered photons and improves the image quality and resolution. ADI is an imaging technique which utilizes the angular spectrum of photons to filter multiple-scattered photons and accept only photons with small angular deviation from their original trajectory. Advantages of the ADI technique are that it is insensitive to wavelength and the sources are not required to be high optical quality, coherent, or pulsed, as with OCT or time domain. Our target is to image a small species called Branchiostoma lanceolatum, a lancet that is 5-8cm long and 5mm thick, by using ADI to remove the scattering in order to image internal structures. A laser illuminates the lancelet in a water-filled container and a spatiofrequency filter removes the scattered photons before the imager. Experimentally, a coherent Nd:Yag second harmonic (533nm) laser creates images but also optical interference occuring within the internal structures of the lancelet. Conversely, an incoherent broad-band white light source eliminates the structural interference effect; however, the wavelength variation of the scattering coefficient combined with the limitation of the image sensor's dynamic range limit the ability to distinguish the internal structures in many areas. Thus, an IR diode laser (780nm) is used to lower the scattering coefficient as compared to conventional visible light source and to diminish the interference effects due to its shorter coherence length.