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New Techniques in Digital Holography

  • Erscheinungsdatum: 23.02.2015
  • Verlag: Wiley-ISTE
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New Techniques in Digital Holography

A state of the art presentation of important advances in the field of digital holography, detailing advances related to fundamentals of digital holography, in-line holography applied to fluid mechanics, digital color holography, digital holographic microscopy, infrared holography, special techniques in full field vibrometry and inverse problems in digital holography

Produktinformationen

    Format: ePUB
    Kopierschutz: AdobeDRM
    Seitenzahl: 100
    Erscheinungsdatum: 23.02.2015
    Sprache: Englisch
    ISBN: 9781119091929
    Verlag: Wiley-ISTE
    Größe: 7476 kBytes
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New Techniques in Digital Holography

2
Digital In-line Holography Applied to Fluid Flows

Due to the simplicity of its optical configuration, in-line holography offers an attractive way to study fluid flows [COË 01, PAN 01, SHE 03, FOU 04, GAR 06]. The use of a single beam is particularly interesting in industrial or laboratory flow studies where optical access is difficult. Then, this configuration opens the way for onboard measurement systems [OWE 00, FUG 04]. Generally, it can be said that for a given situation where classical transmission imaging is feasible, holography can also be applied. This chapter is organized as follows: two typical applications are described; the first application is concerned with near-wall velocity measurements and the second application is concerned with bubbles sizing in a cavitation tunnel. A secondary advantage of the in-line configuration is that the coherence of the light source is not as critical as it is for off-axis configuration. Then, theoretical models of holograms recorded with partial coherent sources (temporal or spatial) are described and the reconstruction of these holograms by fractional Fourier transform is presented. Finally, it must be said that the tracers generally encountered in flows (for example, bubbles or droplets) are not opaque. This chapter extends the application of digital in-line holography (DIH) to the determination of size and three-dimensional (3D) location of phase objects.

2.1. Examples of measurements in flows

The following examples describe direct applications of DIH to particle characteristics measurements in flows. The first example presents the measurement of 3D velocity vector field in a turbulent boundary layer. In the second example, a similar setup is used to measure the size of micrometric bubbles in a cavitation tunnel. For both examples, a holographic magnification is required not only for enlarging particle images but also to increase the spatial resolution of reconstructed images. Through these examples, we will see hereafter how to position the different elements (laser source and camera) from the sample volume for recording hologram under optimal conditions.

2.1.1. Increasing NA with a divergent wave

The use of micrometric tracers needs to use an optical magnification. When the two-dimensional (2D) sensor (charge-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) camera) cannot be placed near the sample to be studied, an optical system is used in order to report particle images near the camera by using a microscope objective lens [SHE 08, KAT 06, STA 13, MAL 08]. However, the introduction of optical elements may lead to inconveniences, such as astigmatism. A magnification factor can also be easily obtained by illuminating the sample volume with a diverging beam. It must be said that a diverging illumination does not directly produce an improvement of the spatial resolution. Nevertheless, the interference fringes are spread over a larger area so that the image sensor can be brought closer to the objects to be recorded without violating the Shannon's criterion [LEB 11, KEL 11]. Consequently, the numerical aperture (NA) of the recording system can be better adapted to micrometric objects and a resulting significant change of the scaling is observed in the reconstructed images [XU 03, GAR 06]. The optical configuration used in the two following examples is given in Figure 2.1. A single laser beam coming from a point source S illuminates the objects and a 2D detector records the hologram.

Figure 2.1. Description of coordinates system

The point source and the object are located, respectively, at distances zs and ze from the camera. According to [LEB 11], a magnification factor K, defined by:

[2.1]

can be efficiently introduced to describe the intensity distributi

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