Imaging transparent objects as a unique technique is widely used in biology, medicine, industrial machine vision and other fields, among which special coatings, sample staining, phase imaging, structured light and multispectral imaging are some of the transparent object imaging techniques. However, the development of transparent object imaging technology faces challenges in many fields.
Digital holography, which utilizes the amplitude and phase data of light waves to reconstruct a 3D image, can provide significant imaging capabilities, even for transparent objects. The use of image data outside of the standard RGB range (e.g., multispectral imaging) can also add to the capabilities of digital holography for displaying previously unobserved structures and thus obtaining additional data on the observables. However, challenges remain in applying the benefits of multispectral techniques to holographic imaging based on how holographic images are generated.
Figure 1: The Imaging Source's black-and-white industrial camera, the DMK 72BUC02, as part of the Recording Interference Fringe System unit.
Recently, researchers published an article on an experimental protocol for digital holographic imaging, an experimental setup that includes an interferometer with acousto-optic tunable filters and The Imaging Source's DMK 72BUC02 monochrome industrial camera.
The researchers' goal is to increase the amount of information captured in digital holograms, which would be a significant breakthrough for viewing transparent objects.
Optical wavefront data acquired by an industrial camera
Generating holograms is closely related to coherent light, and in order to do so, the beams from a coherent light source (i.e., a laser) are split into object beams and reference beams. In the case of digital holography, the interference patterns generated by the object beams and reference beams are recorded by an industrial camera sensor and stored digitally. These optical wavefront data are then digitally reconstructed to produce quantized amplitude and also phase images, which are further processed to produce clear digital holograms.
The coherent light sources used to produce holograms are essentially monochromatic, and to produce multispectral holograms, image data from multiple coherent beams of different wavelengths are reconstructed and fused to form multispectral holograms.
In these systems, the set of operating wavelengths is usually limited, and one cannot select arbitrary wavelengths. At the same time, in many cases it is an interesting task to study a sample over a rather wide spectral range. Therefore, a pressing problem is the development of methods for recording 'multispectral holograms involving quasi-continuous spectral tuning'.
Experimental setup for acousto-optic filter tuning
To realize such an experimental system, the researchers installed a broadband light source and tunable acousto-optic (AO) filters at the entrance to the Mach-Zender interferometer "designed to form, in arbitrarily narrow spectral intervals, optically transparent object digital holograms".
Figure 2: A hologram of a biological sample captured by The Imaging Source's black-and-white industrial camera DMK 72BUC02, showing a typical interference pattern.
In the experiment, the object light wavefront and the reference light wavefront are spatially aligned by a beam splitter to create the interference pattern, and the image is then recorded by the DMK 72BUCO2 camera. A long-pass filter is mounted on the front of the camera to eliminate interference from the background light. The operating wavelength is adjusted by adjusting the "ultrasonic frequency added to the AO unit".
By spatially separating the background zero order from the +1 and -1 diffraction orders, the researchers realized an off-axis digital holography scheme capable of capturing Fourier holograms of transparent objects as well as test patterns and biological samples.
The researchers write, "...... arbitrary spectra make it possible to obtain multicolor holographic images that do not correspond to fixed wavelengths but allow for an arbitrary arrangement of spectral components."
Applications of digital holographic imaging
The non-contact imaging capability of digital holographic imaging makes it particularly suited for fine-grained applications such as the study of cells and structures (especially living specimens) in biomedical applications; non-destructive materials testing, such as internal defect detection in metals or composites; and refractive index fields in transparent media; "Qualitative and precise quantitative analysis of the properties of various objects during microcontour reconstruction, phase structure studies, stress state monitoring, particle trajectory investigations, microscopy, optical coherence tomography, etc."
The techniques described in this paper should "improve the informativeness of holograms" without the need for multiple coherent light sources, while also benefiting applications where "the amplitude-phase and spectral structure of a transparent object must be investigated simultaneously". (Text: The Imaging Source)