1

MECH 6491 Engineering Metrology

and Measurement Systems

Lecture 7 Continued

Instructor: N R Sivakumar

2

Holography

Introduction and Background

Theory and types of Holography

Holographic Interferometry

Theory

Applications

Speckle Methods

Speckle Introduction

Speckle intensity and size

Speckle Interferometry

Theory

Applications

Outline

Holography Introduction

Reflection

hologram

Transmission

hologram

Holography Introduction

5

Holography History

Invented in 1948 by Dennis Gabor

Leith and Upatnieks (1962) applied laser to holography

Holography is the synthesis of interference and diffraction

In recording a hologram, two waves interfere to form an

interference pattern on the recording medium.

When reconstructing the hologram, the reconstructing

wave is diffracted by the hologram.

6

Holography History

When looking at the reconstruction of a 3-D object, it

is like looking at the real object

By means of holography an original wave field can

be reconstructed at a later time at a different location

This technique has many applications; we

concentrate on holographic interferometry

A photograph tells more than a thousand words and

a hologram tells more than a thousand photographs

7

Holography Advantages

Conventional Photography:

2-d version of a 3-d scene

Photograph lacks depth perception or parallax

Film sensitive only to radiant energy

Phase relation (i.e. interference) are lost

8

Holography Advantages

Holographic Photography:

Freezes the intricate wavefront of light that carries all

the visual information of the scene

To view a hologram, the wavefront is reconstructed

View what we would have seen if present at the

original scene through the hologram window

Provides depth perception and parallax

9

Holography Advantages

Holographic Photography:

Converts phase into amplitude information (in-phase

= max amp, out-of-phase = min amp)

Interfere wavefront of light from a scene with

reference wave

The hologram is a complex interference pattern of

microscopically spaced fringes

“holos” – Greek for whole message

10

Holography Recording

Laser beam is split in 2

1 wave illuminates the object

The object scatters the light

onto the hologram plate

(object wave)

The other wave is reflected directly onto the hologram

plate. (reference wave) constitutes a uniform illumination

of the hologram plate

The hologram plate must be a light-sensitive medium,

e.g. a silver halide film plate with high resolution

11

Holography Recording

Let the object and

reference waves in the hologram

plane be described by the field

amplitudes uo and u.

These two waves will interfere

resulting in an intensity distribution

This intensity is allowed to blacken the hologram plate

Then it is removed and developed

This process is hologram recording

*

o

*

0

2

o

22

o uu u u u u u u I

12

Holography Recording

This hologram has a

transmittance t proportional to

intensity distribution

*

o

*

0

2

o

2uu u u u I t u

Replace the hologram back in the holder in

the same position

Block object wave and illuminate the hologram with the reference

wave (reconstruction wave) Ua which will be U multiplied by t

o

2*

0

22

o

2

a uu u uu u u uuut

13

Holography Reconstruction

The quantity IuI2 is constant –

uniform light and the last term thus

becomes (apart from a constant)

identical to the original object

wave uo.

We are able to reconstruct the

object wave, maintaining its

original phase and relative

amplitude distribution uo

by looking through the hologram, object can be seen in 3D

even though the physical object has been removed

Therefore this reconstructed wave is also called the virtual

wave

14

Direct wave: corresponds to zeroth order grating

diffraction pattern

Object wave: gives virtual image of the object

(reconstructs object wavefront) – first order diffraction

Conjugate wave: conjugate point, real image (not

useful since image is inside-out) – negative first order

diffraction

In general, we wish to view only the object wave – the

other waves just confuse the issue

Hologram Reconstruction uu u

2

o

2

a uu u *

0

2u o

2uu

15

Virtual image

Real image

-z z

Direct wave

Object

wave

Conjugate

wave

z=0

Reference wave

Hologram Reconstruction

16

Virtual imageReal image

Direct wave

Object

wave

Conjugate

wave

Reference wave

Use an off-axis system to record the hologram, ensuring separation of the

three waves on reconstruction

Hologram Reconstruction

17

Holography Reconstruction

Alternative method of recording

Fewer components hence more stable

Can you spot the difference …………..

18

Transmission hologram: reference and object waves

traverse the film from the same side

Reflection hologram: reference and object waves traverse

the emulsion from opposite sides

Hologram

View in Transmission View in reflection

Reflection vs. Transmission

19

Hologram - Transmission

20

Hologram - Reflection

21

Hologram: Wavelength

With a different color, the virtual image will appear at a

different angle – (i.e. as a grating, the hologram disperses

light of different wavelengths at different angles)

Volume hologram: emulsion thickness >> fringe spacing

Can be used to reproduce images in their original

color when illuminated by white light.

Use multiple exposures of scene in three primary

colors (R,G,B)

22

Volume Hologram

Reconstruction wave must be

a duplication of the reference

wave

Reflection hologram can be

reconstructed in white light

giving images in their original

color

23

Hologram - Applications

Microscopy M = r/s

Increase magnification by viewing hologram with

longer wavelength

Produce hologram with x-ray laser, when viewed

with visible light M ~ 106

3-d images of microscopic objects – DNA, viruses

24

Hologram - Applications

Interferometry

Small changes in OPL can be measured by viewing

the direct image of the object and the holographic

image (interference pattern produce finges Δl)

E.g. stress points, wings of fruit fly in motion,

compression waves around a speeding bullet,

convection currents around a hot filament

25

Two waves reflected from two identical objects could

interfere

With the method of holography now at hand, we are

able to realize this by storing the wavefront scattered

from an object in a hologram.

We then can recreate this wavefront by hologram

reconstruction, where and when we choose.

Holographic Interferometry

26

For instance, we can let it interfere with the wave

scattered from the object in a deformed state.

This technique belongs to the field of holographic

interferometry

In the case of static deformations, the methods can be

grouped into two procedures, double-exposure and

real-time interferometry.

Holographic Interferometry

(Vest 1979; Erf 1974; Jones and Wykes 1989).

27

Double Exposure Interferometry

Two holograms of the object recorded in

same medium at different time instants

If conditions at the recordings different

→interference between the reconstructed

holographic images reveals deformations

simple to carry out

avoids the problem of

realignment

distortion minimized

compares only two time

instances

28

The observer sees any

deformation of the object

(in scale of λ) in real time

as interference between

the real object and the

holographic image of the

object at rest

Disadvantage is that the

hologram must be

replaced in its original

position with very high

accuracy

Real Time Interferometry

29

Holographic Interferograms

Deflection of a

rectangular plate

fastened with five

struts and subjected

to a uniform pressure

Detection of

debonded region of

a honeycomb

construction panel

A bullet in flight

observed through

a doubly-exposed

hologram

30

Make hologram of vibrating

object

Maximum vibration amplitude

should be limited to tens of

wavelengths

Illumination of hologram

yields image on which is

superimposed interference

fringes

Fringes are contour lines of

equal vibration amplitude

Holographic Vibration Analysis

31

Speckle Introduction

When looking at the laser light

scattered from a rough surface, one

sees a granular pattern

This so-called speckle pattern can

be regarded as a multiple wave

interference with random phases

Speckle is considered a mere

nuisance

But from the beginning of 1970

there were several reports from

experiments in which speckle was

exploited as a measuring tool.

32

Speckle Introduction

light is scattered from a

rough surface of height

variations greater than the .

In white light illumination,

this effect is scarcely

observable ???

Applying laser light,

however, gives the scattered

light a characteristic granular

appearance

33

Speckle Introduction

The probability density

function P, for the intensity in

a speckle pattern is given as

Where I is the mean intensity.

The intensity of a speckle

pattern thus obeys negative

exponential statistics

From this plot we see that the

most probable intensity value

is zero, that is, black.

34

Speckle Size

Objective speckle size

(without lens) is given by

Subjective speckle size (with

lens) is given by

Objective speckle size

Subjective speckle size

35

Laser speckle methods can be utilized in many ways; Speckle-

shearing enables direct measurements of displacement derivatives

related to strains

Speckle Interferometry

(Hung and Taylor 1973; Leendertz and Butters 1973).

The principle of speckle-

shearing (shearography) is

to bring the rays scattered

from one point of the object

into interference with rays

from neighboring point

36

This can be obtained in a speckle-shearing interferometric camera

where one half of the camera lens is covered by a thin glass wedge

In that way, the two images focused by each half of the lens are

laterally sheared

If the wedge is oriented to shear in the x, the rays from a point P(x,

y) on the object will interfere in the image plane with those from a

neighbouring point P(x+x, y)

The shearing x is proportional to the wedge angle

When the object is deformed there is a relative displacement

between the two points that produces a relative optical phase

change

Speckle Interferometry

37

For small shear angles x the equation can be

approximated to (k= 2/)

For out of plane measurement normal angle (=0) is

enough and the equation becomes

For both in plane and out of plane measurement that is

both u and w, we need to use different angle

Speckle Interferometry

38

Electronic Speckle Interferometry

39

Electronic Speckle Interferometry

(a) Out-of-plane displacement fringes (w)

and slope fringes (w/x) for a aluminium

plate loaded at the centre. x is 6 mm,

and w = 2.5 µm and

(b) Out-of-plane displacement fringe

pattern (w) and slope pattern (w/y) for

the same object. The shear y is 7 mm.

40

Speckle and Holography

Electronic shearography (ES) used for non-destructive

testing of a ceramic material.

(a) A vertical crack is clearly visualized by ES as a

fringe in the centre of the sample and

(b) The crack is not detected using TV holography

41

ESPI for NDT

GOOD part BAD part

Digital Shearography

Setup

Able to detect surface/subsurface

defects effectively and efficiently

To develop a non-destructive In-line

subsurface defects detection system

42

READY FOR IC FABRICATION PROCESS

RECYCLE

BIN

Defect?yes no

New process

Unpolished Silicon Wafers

Defect?no

RECYCLE

BIN

yesSilicon wafers

Patterned Wafer

Unpolished Silicon Wafers

Polishing(whole batch)

Polishing(good wafers only)

Conventional process

Estimated cost

savings more than

S$1million/year

for ISP

ESPI for NDT Application

43

MECH 6491 Engineering Metrology

and Measurement Systems

Lecture 8

Instructor: N R Sivakumar

44

Light Sources

Incoherent Light Sources

Coherent Light Sources

Detectors

PhotoElectric Detectors

CCD Camera

Outline

45

Most light sources are incoherent (candle light to Sun)

They all radiate light due to spontaneous emission

Here we will consider some sources often used in

scientific applications

These are incandescent sources, low-pressure gas

discharge lamps and high-pressure gas discharge-arc

lamps

They are commonly rated according to their electric

power consumption

Spontaneous Emission

46

/12 hchEE

Spontaneous Emission

Energy-level diagram for a molecule is shown

The atom by some process is raised

to an excited state E3

Then it drops to E2, E1 and E0 in

successive steps

Energy difference between E2 and E1 is released as

electromagnetic radiation of frequency given by

This might be the situation in an ordinary light source

where the transition occurs spontaneously – hence called

spontaneous emission

where h = 6.6256 x 10-34Js

is the Planck constant

47

Tungsten halogen lamps - common incandescent source

Quartz tungsten halogen lamps (QTH) produce a bright,

stable, visible and infrared output and is the most

It emits radiation due to the thermal excitation of source

atoms or molecules

Tungsten evaporates the filament and deposits inside the bulb

This blackens the bulb wall and thins the tungsten filament, gradually

reducing the light output

halogen gas removes the deposited tungsten, and returns it to the hot

filament, leaving the inside of the envelope clean, and greatly

increases lamp life. This process is called the halogen cycle

Incoherent Light Sources

48

Low Pressure Gas Discharge lamps - Electric current passes through

a gas

Gas atoms or molecules become ionized to conduct the current

At low current density and pressures, electrons bound to the gas

atoms become excited to well-defined higher-energy levels

Radiation is emitted as the electron falls to a lower energy level

characteristic of the particular type of gas. The spectral distribution is

then a number of narrow fixed spectral lines with little background

radiation

Incoherent Light Sources

49

Low Pressure Gas Discharge lamps - brightest conventional sources

of optical radiation

High-current-density arc discharges through high-pressure gas

Thermal conditions in the arc are such that gas atoms are highly

excited resulting in a volume of plasma

The hot plasma emits like an incandescent source, while ionized

atoms emit substantially broadened lines

The most common sources of this type are the Xenon (Xe) and

mercury (Hg) short arc lamps

Incoherent Light Sources

50

Spontaneous Emission

Excited atoms normally emit light spontaneously

Photons are uncorrelated and independent

Incoherent light

51

Stimulated Emission

Excited atoms can

be stimulated into

duplicating passing

light

Photons are

correlated and

identical

Coherent light

52

Spontaneous Emission

Stimulated Emission

Stimulated Absorption

Population Inversion

Optical Pumping

As postulated by Einstein, also another

type of transition is possible

If a photon of frequency given by Equation passes the atom it might

trigger the transition from E2 to E1 thereby releasing a new photon

of the same frequency by so-called stimulated emission

Coherent Light Sources

/12 hchEE

53

Under normal conditions, the

number of atoms in a state tends

to decrease as its energy

increases

This means that there will be a

larger population in the lower state

of a transition than in the higher

state

Therefore photons passing the

atom are far more likely to be

absorbed than to stimulate

emission

Coherent Light Sources

54

Under these conditions,

spontaneous emission dominates

However, if the excitation of the

atoms is sufficiently strong, the

population of the upper level might

become higher than that of the

lower level

This is called population inversion

Then by passing of a photon of

frequency given by equation it will

be more likely to stimulate

emission from the excited state

Coherent Light Sources

than to be absorbed by the lower state

55

This is the condition that must be

obtained in a laser

This results is laser gain or

amplification, a net increase in the

number of photons with the

transition energy

They produce narrow beams of

intense light

They often have pure colors

They are dangerous to eyes

Reflected laser light has a funny

speckled look

Coherent Light Sources

56

LOSER - “Light Oscillation by STIMULATED

Emission of Radiation”

LASER - “Light Amplification by STIMULATED

Emission of Radiation”

LASER

57

Laser Amplification

Stimulated emission can amplify light

Laser medium contains excited atom-like systems

Photons must have appropriate wavelength, polarization, and

orientation to be duplicated

Duplication is perfect; photons are clones

58

Laser Oscillation

Laser medium in a resonator produces oscillator

A spontaneous photon is duplicated over and over

Duplicated photons leak from semitransparent mirror

Photons from oscillator are identical

59

Laser Oscillation

60

Properties of Laser Light

Coherence – identical photons

Monochromaticity; Controllable

wavelength/frequency – nice colors

Beam divergence; Controllable spatial structure –

narrow beams

Brightness

Energy storage and retrieval – intense pulses

Giant interference effects

Apart from these issues, laser light is just light

61

Wavelength

I

λ0 λ2λ1

Spectral Width = λ2 - λ1

Monochromaticity

62

Coherence

1

c

Lcc

63

Laser Modes

Although the laser light

has a well-defined

wavelength (or

frequency), it has

nevertheless a certain

frequency spread.

By spectral analysis of the light, it turns out that it consists of one

or more distinct frequencies called resonator modes, separated by

a frequency

where c = the speed of light and L is the distance between the

laser mirrors, i.e. the resonator length

64

FLASH

LAMP

LASER

= 250

= 0.020

Beam Divergence

NGLEUNIT SOLID AUNIT AREA

POWERBRIGHTNESS

/

65

t t

E E

Pulse duration

Peak Power

Continuous

Wave

Pulsed

Laser

Average power

CW and Pulsed Lasers

66

Types of Lasers

Gas (HeNe, CO2, Argon, Krypton)

Powered by electricity

Solid state (Ruby, Nd:YAG, Ti:Sapphire, Diode)

Powered by electricity or light

Liquid (Dye, Jello)

Powered by light

Chemical (HF)

67

Inside a discharge tube is a gas

mixture of helium and neon. 5 to

12 times more helium than neon.

Coherent Light Sources

By applying voltage to the electrodes, the resulting electric field will

accelerate free electrons

These collide with helium atoms raising them to a higher energy

level. By collision between helium and neon atoms, the latter are

raised to a higher energy level

This constitute the pumping process.

The neon atoms, which constitute the active medium, return to a

lower energy level and the energy difference is released as

electromagnetic radiation

68

He Ne Laser

69

Detectors

Chemical

detectors

photographic

film,

photopolymers

They do not

give a signal

output in the

usual sense Electronic detectors

thermal detectors

photon detectors

70

Detectors

In thermal detectors,

the absorption of light

raises the

temperature of the

device

This in turn results in changes in some temperature-

dependent parameter (e.g. electrical conductivity)

Most thermal detectors are rather inefficient and quite

slow (hence, not useful in optical metrology)

Fire detection and alarms

71

Detectors

Photon detectors work on the photoeffect

Absorption of photons by some materials results directly in

an electronic transition to a higher energy level

Since the energy of a single photon is E = h = hc/,

photon detectors have a maximum for operation

For detectors operating in the infrared, photon energy

thermal energy of the atoms in the detector

Detectors operating above a of 3 µm must be cooled

below 77K

72

Detectors

The photoeffect takes two forms: external and internal

The former process involves photoelectric emission, in

which the photo-generated electrons escape from the

material (the photocathode) as free electrons with a

maximum kinetic energy given by Einstein's photoelectric

equation

where the work function W is the energy difference

between the vacuum and the Fermi levels of the material

73

Detectors

Photoemissive devices usually take the form of vacuum

tubes called phototubes

Electrons emitted from the photocathode travel to an

electrode (the anode) which is kept at a higher electric

potential

As a result, an electric current proportional to the photon

flux incident on the cathode is created in the circuit

In a photomultiplier, the electrons are accelerated towards

a series of electrodes maintained at successively higher

potentials

74

Detectors

From the electrodes a cascade of electrons are emitted by

secondary emission, resulting in an amplification

A microchannel plate consists of an array of channels (ID

~ 10 µm) in a slab of insulating material (0.5 mm thick)

Each channel acts like a miniature photomultiplier tube

Emerging from the channels, the electrons can generate

light (optical image) by striking a phosphor screen.

In the internal photo-effect, the photo-excited carriers

(electrons and holes) remain within the material

75

Detectors

Photoconductors rely on

the light-induced increase

in the conductivity - almost

all semiconductors

The absorption of photon results in the generation of a

free electron, and a hole is generated

An external voltage applied causes the electrons and

holes to move, resulting in a detectable electric current

The detector operates by registering the current

proportional to the photon flux

76

Detectors

Photodiode is a p-n

junction structure where

photons absorbed

generate electrons and

holes which are subject to electric field within that layer

The two carriers drift in opposite directions and an electric

current is induced in the external circuit

Here the circuit current is directly proportional to the

incident light irradiance

77

CCD Cameras

CCDs are a series of Metal

oxide semicon (MOS)

capacitors

A semiconductor substrate is covered with a thin layer of

insulating silicon oxide - insulates the Si from electrode.

When a positive voltage is applied between the electrode

and the Si, holes in p-type Si will be repelled, creating a

region free of mobile carriers directly underneath the

electrode

78

CCD Cameras

This region is known as

depletion region and has

a thickness of few microns

The electrodes are transparent for >400 nm

If incident photon has an energy larger than the

bandgap in Si, a charge packet is formed consisting of

photon-electrons which were created in the vicinity of a

specific electrode

79

At the heart of every digital camera is a Charge Coupled

Device (CCD) typically about a square centimeter in size.

CCD Cameras

80

The CCD is comprised

of many individual

signal capture units,

each of which

corresponds to a

single pixel in the final

digital image.

CCD Cameras

81

Light - incoming photons falls onto

the CCD chip surface

This generates free electrons in

the silicon of the CCD in proportion

to the number of photons striking it

CCD Cameras

These electrons collect in little packets created by the

silicon geometry and surrounding electrical circuitry laid

out in a 2D grid on the chip

Typical CCD chips have from 1 to 5 million charge packets

82

At the heart of the CCD is these metal oxide

semiconductors (MOS) which allow the charge of electrons

to build up in wells in the silicon base.

CCD Cameras

83

a time by varying the voltage of adjacent rows thereby

creating a potential well which couples two rows and

causes the charge to move over

CCD Cameras

The CCD operates on

the principle of charge

coupling.

The packets of

charged electrons can

be moved one row at

84

Buckets on conveyor belts depict how each bucket

contains a different amount of light (shown as rainwater)

and how these buckets are shifted in an orderly fashion

CCD Cameras

85

In this way the quantity of water (or electrons representing

light) in each bucket (or packet) are counted. In a typical

CCD this happens very fast: about 30 times per second for

every one of the million or so "buckets" on the CCD.

CCD Cameras

86

To increase the efficiency of reading the output of the CCD

array there are several different designs. One type transfers

the entire frame into an empty storage array, while others

alternate empty rows with collecting rows.

87

CCDs can be used to collect an image in one of three ways,

either one pixel at a time, one row at a time, or as an entire

area at once.