optical holography202.62.95.70:8080/jspui/bitstream/123456789/12012/1... · 2020. 5. 18. · module...
TRANSCRIPT
Module -5
Optical Holography
Basics of Holography
Holography is the technique of producing 3-dimentional image of an object on 2-
dimentional recording aid, by the phenomenon of interference. Holography is a Greek word,
Holos means complete and graphos means writing. A and B are two identical or coherent
beams incident on photosensitive surface at different angles. Due to interference effect,
interference fringes are recorded on developing the photographic plate.
Principle of Hologram construction:
Light wave reflected from an object are characterized by their intensity (square of
amplitude) and phase. When both intensity and phase attributes of the wave coming from
three dimensional object is recorded on a photographic plate, it is called construction of
hologram. When recorded photographic plate (hologram) is illuminated by a coherent light
source, the three dimensional image of the original object is formed. This formation of
image is known as reconstruction process.
Recording phase variation in a Hologram:
In recording hologram of an object a photographic plate is placed in front of an object at a
suitable distance. Consider a coherent light incident on the object. The light reflected from
two nearby points on the object travel slightly different distances in reaching the
photographic plate due to variation in depth on the object. Thus the two wave fronts arrive
at the photographic plate in a slightly different phase. Hence the light reflected from
different points on the object will have different phases and interfere with the reference
beam. The fringes recorded in the hologram carry information regarding the phase
difference.
In holography there are two phases:
1) Recording
2) Reconstruction of the image.
Recording has two methods
1) Wave front division technique.
2) Amplitude division technique
1) Recording of the image of an object by wave front division technique
Expanded coherent laser beam from the laser source is obtained. A portion of it is made to
incident on the mirror and other portion is made to incident on the object as shown in the
fig.
Photographic plate is placed at a suitable position so that it receives the light reflected from
both the mirror and the object. The light reflected from the mirror form a plane wave front.
It is called reference beam. The light reflected from each point on the object form a
spherical wave front. It is called object beam. Thus the interference effects of the two
beams are recorded on the photographic plate. As the spherical wave intersect the plane
wave in circular zones, the interference pattern consists of concentric circular rings having
constructive and destructive interference. It is called Gabor Zone plate.
Hologram consists of number of such zone plates. The centre of each is displaced from the
other. In the recorded pattern the neigh boring zones overlap each other and become
apparent, once the film is developed. It is called a hologram.
2) Recording the image of an object by amplitude division technique:
Expanded coherent laser beam from the laser source is obtained. It is made to incident on
the beam splitter. The beam splitter reflects the portion of the light which is incident on the
mirror. The transmitted light from the beam splitter is incident on the object. The reflected
plane wave front from the mirror and reflected spherical wave fronts from different points
on the object undergoes interference on the photographic plate kept at a suitable place.
The interference fringes are recorded on the photographic plate. The developed
photographic plate becomes the hologram of the object.
Reconstruction of the image from the hologram:
Original Laser beam is made to incident on the hologram in the same direction as the reference
beam was incident on it at the time of recording. The beam undergoes refraction in the hologram.
Secondary wavelets originating from each constituting zone plate interfere constructively and
generate real image on the transmission side and virtual image on the incident side.
Acoustical Holography:
Introduction
Acoustic holography makes it possible to determine the noise radiated by each of the
mechanical components of a complex system; it is the near field acoustic imagery. It delivers
a fine representation of the distribution of the sound sources on the surface of the
equipment or in any parallel plan near this surface. By measuring the pressure in the
immediate environment of the system, acoustic holography allows to calculate the field of
pressure in any point close to the sound sources or in the far field.
Principle
Acoustic holography is a method for estimating the sound field near a source by
measuring acoustic parameters away from the source by means of an array
of pressure and/or particle velocity transducers. The optical reconstruction of image
information contained in a sound field.first the diffraction pattern formed by an object
irradiated by ultrasonic rays, interferes with a military coherent reference wave. the
consequent spatial irradiation distribution is then recorded. the acoustical hologram is
illuminated by a light beam resulting in diffraction from the hologram that can be used to
form a 3-D visual image of the object.
The guiding principle of acoustic holography consists of measurements of pressures phased
acoustics on a regular level of collecting close to the sound-effects man. Since one cannot to
acquire all the microphones simultaneously, it is necessary to use fixed references of phase
on the body or close to the sound-effects man.
The basic equipment used in acoustic holography includes:
microphones
References
Acquisition system
A system for data analysis.
Near Field Acoustic Holography Methods
The complex field of sound measured by the antenna is broken up into infinity of
propagatives elementary plane and evanescentes waves. The evanescentes acoustic waves
describe the complex field of the sound existing close to the envelope and partly mirroing
the vibrations. The level and the direction of each acoustic wave are described by their
number of acoustic wave. The principal treatment of acoustic holography is to apply to each
acoustic element components (planes, cylinders, etc) an opposite operator of propagation,
in order to obtain it sound field on a surface parallel with the plan of measurement in near
field. Starting from the same data of measurement, it is possible to calculate the radiated
acoustic pressure in the far-field.
Plane acoustic holography allows, on complex equipment, to identify them 'sources'
responsible for the acoustic radiation perceived in the vicinity. Method reserve here
consists, starting from acoustic measurements, to find the vibratory components fields
sources while using, on the one hand a process of return towards the sources (or
propagation reverses), and in addition the existing relation between vibratory speed and
speed particulate in the fluid environment. These hot sources, or 'points', will be visualized
under form directly interpretable images.
This technique allows the 3D acoustic field reconstruction from the complex numerical part
of the pressure measured on an hologram surface which is located in the source near-field.
Microwave holography
Microwave imaging is a science which has been evolved from older detecting/locating
techniques (e.g., radar) in order to evaluate hidden or embedded objects in a structure (or
media) using electromagnetic (EM) waves in microwave regime (i.e , ~300 MHz-300 GHz).
Engineering and application oriented microwave imaging for non-destructive testing is
called microwave testing
Microwave imaging techniques can be classified as either quantitative or qualitative.
Quantitative imaging techniques (are also known as inverse scattering methods) give the
electrical (i.e., electrical and magnetic property distribution) and geometrical parameters
(i.e., shape, size and location) of an imaged object by solving a nonlinear inverse problem.
The nonlinear inverse problem is converted into a linear inverse problem (i.e., Ax=b where A
and b are known and x (or image) is unknown) by using Born or distorted Born
approximations. On the other hand, qualitative microwave imaging methods calculate a
qualitative profile (which is called as reflectivity function or qualitative image) to represent
the hidden object. These techniques use approximations to simplify the imaging problem
and then they use back-propagation (also called time reversal, phase compensation, or
back-migration) to reconstruct the unknown image profile. Synthetic aperture radar (SAR),
ground-penetrating radar (GPR), and frequency-wave number migration algorithm are some
of the most popular qualitative microwave imaging methods.
Principles
In general, a microwave imaging system is made up of hardware and software components.
The hardware collects data from the sample under test. A transmitting antenna sends EM
waves towards the sample under test (e.g., human body for medical imaging). If the sample
is made of only homogeneous material and is of infinite size, theoretically no EM wave will
be reflected. Introduction of any anomaly which has different properties (i.e.,
electrical/magnetic) in comparison with the surrounding homogeneous medium may reflect
Contents Principles A general view of a microwave imaging system. 3D image of re bars with
corrosion produced using microwave imaging, a portion of the EM wave. The bigger the
difference between the properties of the anomaly and the surrounding medium is, the
stronger the reflected wave will be. This reflection is collected by the same antenna in a
mono static system, or a different receiver antenna in bi static configurations.
Applications
Microwave imaging has been used in a variety of applications such as: non destructive
testing and evaluation (NDT&E, see below), medical imaging, concealed weapon detection
at security check points, structural health monitoring, and through-the-wall imaging.
Ageing of infrastructure is becoming a serious problem worldwide. For example, in
reinforced concrete structures, corrosion of their steel reinforcements is the main cause of
their deterioration. Recently, microwave imaging has shown great potential to be used for
structural health monitoring. Lower frequency microwaves can easily penetrate through
concrete and reach objects of interest such as reinforcement bars (rebars). If there is any
rust on the rebar, since rust reflects less EM waves in comparison with sound metal, the
microwave imaging method can distinguish between rebars with and without rust (or
corrosion). Microwave imaging also can be used to detect any embedded anomaly inside
concrete (e.g., crack or air void).
These applications of microwave imaging are part of non-destructive (NDT) testing in civil
engineering. More on microwave imaging in NDT is described in the following.
Microwave testing
Microwave testing uses the scientific basics of microwave imaging for the inspection of
technical parts with harmless microwaves. Microwave testing is one of the methods of non-
destructive testing (NDT). It is restricted to tests of dielectric, i. e. non-conducting material.
It can be used to inspect components also in a built-in state, e. g. built-in non-visible gaskets
in plastic valves.
The microwave frequencies extend from 300 MHz to 300 GHz corresponding to wavelengths
between 1 m and 1 mm. The section from 30 GHz to 300 GHz with wavelengths between 10
mm and 1 mm is also called millimeter waves. Microwaves are in the order of the size of the
components to be tested. In different dielectric media they propagate differently fast and at
surfaces between them they are reflected. Another part propagates beyond the surface.
The larger the difference in the wave impedance, the larger is the reflected part. In order to
find material defects, a test probe, attached or in a small distance, is moved over the surface
of the device under test. This can be done manually or automatically. The test probe
transmits and receives microwaves. Changes of the dielectric properties at surfaces (e. g.
shrinkage cavities, pores, foreign material inclusion, or cracks) within the interior of the
device under test reflect the incident microwave and send a part of it back to the test probe,
which acts as a transmitter and as a receiver.
Applications
Microwave testing is a useful NDT method for dielectric materials. Among them are plastics,
glass-fiber reinforced plastics (GFRP), plastic foams, wood, wood-plastic composites (WPC),
and most types of ceramics. Special applications of microwave testing are non-destructive
moisture measurements wall thickness measurements of paint thickness on carbon
composites (CFRP) condition monitoring, e. g. presence of gaskets in assembled valves,
rubber based piping in heat exchangers[8] measurement of material parameters, e.g.
permittivity and residual stress disbond detection in strengthened concrete bridge members
retrofitted with carbon fiber reinforced (CFRP) composite laminates corrosion and precursor
pitting detection in painted aluminium and steel substrates flaw detection in spray-on foam
insulation and the acreage heat tiles of the Space Shuttle. Microwave testing is used in many
industrial sectors: aerospace, e. g. non-destructive paint thickness measurements on CFRP
automobile, e. g. NDT of organo sheet components and of GFRP leaf springs civil
engineering, e. g. radar applications energy supply, e .g. test of rotor blades of wind power
plants, riser pipe security, e .g. body scanner on airports.
Applications of Optical Holography
Security and authentication is most common area for holography and diffractive
optics applications: The increase in demand on security is dictated by steady growing
rating of counterfeiting and piracy. Analysis carried out by Organization for Economic
Co-operation and Development (OECD) showed that amount of counterfeit and
pirated products was about USD 200 billion in the world market. Various types of
holograms and Optically Variable Devices (OID) and Optically Variable Image Devices
(OVID) are widely used to improve security of documents for ant counterfeiting and
for brand authentication.
Transmission rainbow holograms were first commercially used by United States bank
note company (USBC) IN 1980 and rainbow holograms were first application of
holography for bank credit card security.
Dot-matrix hologram is the most common type of hologram used for authorization
of products and against document forge.
In 1965 the digital methods for optical filtering were invented. Developing this
method lead to invention of computer generated hologram employing binary mask
to record 2-D AND 3-D objects.
Limitations
Conventional holography of the real object using some wave interference between
two laser beams with a high degree of coherence between the in a dark room.
The system must be kept very stable since ever a very slight movement can destroy
the interference fringes, in which both intensity and phase information of the 3-D
object are contained. The device will be heavy and will have alignment and
packaging issues.
High cost makes them impractical for many applications. There are packaging and
alignment issues. It is difficult to be implemented in smaller displays.
Most holographic products such as the holographic stickers had already been
decorated with reliefs which would let the machine copying become relatively easy
and cheap so, it would be hard for the people to put a coherent distinguishing
between the holographic products and the source products.