2.3 Review of Other Optical Research Techniques
2.3.3 Interferometry Method
Interferometry unlike shadow photography is a qualitative method suited to
measuring changes in the refractive index of the disturbance gradient density.
Interferometry is a quantitative method suitable for investigating the density gradient.
These methods are very sensitive to visualizing the patterns of mixing processes in
high speed disturbance fields.
Figure 2. 23 A Michelson Interferometer System [35]
Michelson and Morley 1887 [35] used optical Intererometry to measure
different phases by a half-silvered mirror. The two parts of the beam are reflected by
two mirrors and recombined into one beam by the half-silvered mirror. The resulting
interference fringes are photographed.
Many researchers make use of Interferometry method to study wave
propagation properties. These methods are employed to measure refractive index,
phase shifting and frequency shifting. A Diffraction grating is a common
interferometer who combines diffraction and Interferogram. A light beam is split by a
grating into several beams traveling at different diffraction angles. Then it generates
fringes surface on a plate by intereferogrametry in darkness and brightness or
chromatic colour in an alternating pattern. According to the Huygens-Fresnel [36]
principle, each beam from the grating slits can be considered as a new point source.
The diffraction equation is:
(2-2) Where is the slit spacing of grating is; is the incident angle of incoming beams;
is the maximum angle of diffraction beam where m is an integer; is the wavelength of light beam; m= 0, .
Interferometry methods are commonly used to measure the phase shift in the
wave front propagation or density gradient. The Mach-Zehnder interferometer is a
typical system used to measure the phase shift of a wave front. In a simple
two beams are reflected by mirrors to pass another half-silvered mirror and into the
detector. If one beam crosses a disturbance field there is a phase shift. The phase
shifted beam is compared with the other reference beam and the disturbance measured.
Figure 2.24 is a mathematics module of Mach-Zehnder interferometry system.
Figure 2. 24 A Sample of Mach-Zehnder Interferometry [37]
Figure 2. 25 High-Speed Interferograms Recorded by Differential Interferometry-
Mach 0.4 [38]
density. Due its high sensitivity, Interferometry is well suited to measuring weak
vortex gas gradients in polarized light. Figure 2.25 shows a recording of gas density by
Interferometry methods [38]. The Figure illustrates the reconstruction images of gas
density and indicates the size of gas vortexes in different directions.
2.3.4 Holography Methods
Holography was invented by Dennis Gabor who developed the theory in 1947
[39, 40]. This technique is primarily used in electron microscopy also known as
electron holography. Optical holography methods were first used to record 3D objects
by Yuri Denisyuk in 1962 [41]. The usefulness of holography in many fields including
research has ensured its rapid development.
Figure 2. 26 Formation of a Hologram
Holography is an interference method of recording light waves diffracted by a
subject illuminated with coherent light [42]. The diffraction waves are interfered with
diffraction wave information including amplitude and phase shifting are photographed
to model the subject. Hence, illuminating the photograph by white light or laser light, a
hologram of wave front is vividly visualized. The system is shown in Figure 2.26. A
photography only records object intensity information in a spatial distribution.
Holography records both amplitude and phase information of light intensity of an
unfocused subject.
Figure 2. 27 A Transmission Holography System [38]
Two types of holography can be identified: transmission holography and
reflection holography. In transmission holography, a light beam is split into two. One
beam is passed over the disturbance object and the other beam is a reference beam.
The beams are interfering on a hologram medium and the holographic image is
transmitted to the observer side by interferometric lighting. A sharp virtual image of
the object is obtained (Fig 2.27). In a reflection system, a hologram image is recorded
in the same way as the transmission system. The difference is that the light beam is
reflected by the object and the hologram is received on the observer’s side of the
holography medium (Fig 2.28)
Reflection holography exhibits greater interference than transmission
holography because the higher sensitivity of reflection holography to variations. The
drawback of reflection holography is that parts of the beam are reflected by the object
affecting the results. For example, the holographic image of a diamond shows sparkles.
Many types of holography have been investigated [43]:
Embossed holography commonly used in identification cards records the object
information on a photo resist material.
Integral holography records the target object as a series of holographic images
using transmission or reflection holography. The images are combined to give a
stereoscopic image. The sensitivity to small changes of disturbance parameters
makes holographic Interferometry or real time holographic Interferometry
Multichannel holograms use viewers to obtain holographic images from
various angles.
Computer-generated holography uses mathematic methods to model objects.
This technique is developing rapidly and popularly used in clinical medicine
and movie applications.
The analysis of wave front holograms is based on temporal and spatial modulations
[42]. The complex amplitude of the wave front is relative to the Fourier transform; it is
present in temporal –frequency domains and spatial- frequency domains. It assumes
that in a spatial domain the point source light amplitude is a(x,y); the a(x,y) represents
light propagation in the spatial coordinates x,y of an observation plane. The complex amplitude distribution corresponds to the frequencies and . Hence, the amplitude in
a spatial domain (x,y) can be represented as A( , in frequency domain. The equation
for point source holograms can be expressed as:
(2-3) Where, a(x,y) is the inverse Fourier transform of A( , :
(2-4)
The Shadowgraphy, Schlieren, Interferometry and holography methods are
present good results of flame density measurements. However, there are some distinct
measure the invisible density. The Interferometry and Holography methods are too
complex to detect in cylinder flame.