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.