September 2015
Spotlight Summary by Kedar Khare
Inline digital holographic movie based on a double-sideband filter
Digital holography has become an important modality for fast and accurate quantitative phase imaging applications relevant to biomedical diagnostics, optical metrology, displays, etc. There are two important classes of digital holographic imaging systems being currently used: Gabor’s original in-line holography configuration (1948) and off-axis holography as first proposed by Leith and Upatnieks (1965). The image reconstruction in-line configuration typically suffers from the presence of the conjugate or twin-image term. The off-axis method, on the other hand, enables removal of this twin image by Fourier-domain filtering. The removal of the twin-image term in in-line configuration generally requires a multi-frame phase-shifting approach, making it somewhat unsuitable for imaging dynamic phenomena such as particle flow in a liquid.
An important but less used alternative holographic configuration - the single sideband (SSB) holography – as first proposed by Bryngdahl and Lohmann (1968) has the capability of removing the twin image as well. In this configuration, one half of the Fourier plane of spatial frequencies corresponding to the complex valued object to be imaged are blocked in the recording stage while the other half plane is blocked in the hologram reconstruction process. The technique results in phase images that are somewhat distorted since one half of the frequency information is blocked. The distortion is removable, however, if two separate SSB holograms that block complementary regions of the Fourier plane are recorded. In the present Optics Letters article, Ramírez et al. use a liquid crystal based phase screen, polarization elements and two CCD sensors to achieve the two complementary SSB hologram recordings simultaneously. The complex field in the hologram plane can be computed by means of the holograms recorded on the two CCD sensors. Once the complex field in the hologram plane is recovered, standard diffraction calculations can lead to computational refocus to any desired plane. The authors demonstrate the dynamic imaging capability of their setup by showing a holographic movie of microscopic objects located at different planes in depth. This technique has several interesting applications for tracking of dynamic microscopic objects (e.g. micro-organisms or live cells) at a frame rate determined by the CCD sensor alone.
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An important but less used alternative holographic configuration - the single sideband (SSB) holography – as first proposed by Bryngdahl and Lohmann (1968) has the capability of removing the twin image as well. In this configuration, one half of the Fourier plane of spatial frequencies corresponding to the complex valued object to be imaged are blocked in the recording stage while the other half plane is blocked in the hologram reconstruction process. The technique results in phase images that are somewhat distorted since one half of the frequency information is blocked. The distortion is removable, however, if two separate SSB holograms that block complementary regions of the Fourier plane are recorded. In the present Optics Letters article, Ramírez et al. use a liquid crystal based phase screen, polarization elements and two CCD sensors to achieve the two complementary SSB hologram recordings simultaneously. The complex field in the hologram plane can be computed by means of the holograms recorded on the two CCD sensors. Once the complex field in the hologram plane is recovered, standard diffraction calculations can lead to computational refocus to any desired plane. The authors demonstrate the dynamic imaging capability of their setup by showing a holographic movie of microscopic objects located at different planes in depth. This technique has several interesting applications for tracking of dynamic microscopic objects (e.g. micro-organisms or live cells) at a frame rate determined by the CCD sensor alone.
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Article Information
Inline digital holographic movie based on a double-sideband filter
Claudio Ramirez, Angel Lizana, Claudio Iemmi, and Juan Campos
Opt. Lett. 40(17) 4142-4145 (2015) View: HTML | PDF