This document provides an overview of electronic speckle pattern interferometry (ESPI), which is an optical technique used to measure surface deformations. It describes the basic experimental setup of ESPI including components like lasers, cameras, and computers. It also explains the working principle of ESPI, which involves recording speckle patterns before and after deformation and using phase shifting and image subtraction to calculate displacement fields. Finally, it discusses different ESPI configurations for measuring in-plane and out-of-plane deformations and provides the theoretical basis for intensity calculations in ESPI systems.

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Electronic Speckle Pattern Interferometry
Akash Marakani, Nivedita Madanala
Abstract
Interferometry is a measurement method using the phenomenon of interference of waves (usually light, radio or
sound waves). The measurements may include those of certain characteristics of the waves themselves and the
materials that the waves interact with. Interferometry is an investigative technique that uses light waves for the
study of changes in displacement. Electronic speckle pattern interferometry (ESPI) is a technique which uses laser
light, together with video detection, recording and processing to visualize static and dynamic displacements of
components with optically rough surfaces in the form of fringes. This paper reviews the working principle, benefits
and limitations of this technique. The paper also talks about the various applications and provides a brief summary
of the latest advancements in this field.
1. Introduction
Electronic Speckle Pattern Interferometry (ESPI), which
is also known as TV Holography is a non-contact optical
method which is generally used for studying surface
deformations. Surface deformation are a characteristic
property of three dimensional displacements, which
can be further translated into 3D strains and stresses,
and are a key parameter for design, manufacturing and
quality control. Due to the advancements in technology
various industrial sectors like automotive,
manufacturing has already adapted rapid optimization
design concepts. These concepts all require the support
of high sensitive measurement of 3D displacements.
The traditional technique for measurement of
displacement and strain is the resistance strain gage
method, however due to its low spatial resolution and
time consuming methodology, advanced optical
methods due to their non-contact, full field
characteristics, and high measurement sensitivity have
been widely accepted as displacement and strain
measurement tool in industry. Of these methods,
electronic speckle pattern interferometry is the most
sensitive and accurate method for full field 3D
displacement measurement.
Electronic Speckle Pattern Interferometry (ESPI) relies
on the interference between the reflected light from
the object to be tested and a reference beam from the
same laser source. It is an optical full-field
measurement method used to determine the
deformations on object that must be an optically rough.
The reflected beam and the reference beam from the
same laser light source are superimposed on a video
camera and interfere to form a speckle pattern. Speckle
pattern recorded before and after deformation of the
object yield a non-unique fringe pattern. Using a phase
shifting method, this non-uniqueness can be solved and
the fringe pattern is further evaluated using a computer
algorithm. This technique is very accurate and also has
the capability to detect fractures, micro-cracks and
surface flaws.
In this report, we provide a literature review on
Electronic Speckle Pattern Interferometry (ESPI), their
experimental setup, procedure and their applications.
This report also summarizes the areas of current
advancements in ESPI and how they have been
accepted as a displacement and strain measurement
tool in the industrial environment.
2. Experimental Setup
The Electronic Speckle Pattern Interferometry (ESPI) is
used for collecting data for surface strain analysis and is
based on a holographic interferometric (HI) technique
involving photographic recording of a light beam and
video recording. The video recording from the source is
filtered and displayed on the TV monitor. The main
advantage of ESPI is its real time capability, and the fact
that the recordings can be easily stored and processed
for later use. Further it uses an automatic fringe
analysis technique.
The setup consists of a laser diode (LD), microscope
objective (MO), directional coupler (DC), charge-
coupled diode camera (CCD), digital to analog

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convertor (D/A), piezoelectric phase shifter (PZT) as
shown in the figure below.
Fig. 1 Experimental setup of ESPI
An illustrative speckle pattern subtraction wherein two
patterns are recorded at the surface of the specimen
before and after the application of load has been
demonstrated in the figure below.
Fig 2. Before (A) and after (B) a fluid pressure gradient
was applied to the surface
Fig 3. (A) is subtracted from (B) to obtain the speckle
pattern to obtain the displacement of the fringes
2.1 Components
2.1.1 Laser Diode (LD)
A laser diode (LD) is an electrical diode in which the
laser beam medium is formed by a p-n junction of a
semiconductor diode which is similar to that found in a
light emitting diode. The modulation of light intensity is
done to obtain different sensitivity and phase shifting
can be achieved by tuning the laser beam wavelength.
Ideally, Helium-Neon (HeNe) lasers are used for looking
at relatively small objects, while Argon (Ar) lasers are
used for large objects and for two-wavelength surface
geometry measurements. In rare cases, Co2 can also be
used.
2.1.2 Microscope Objective (MO)
The laser beam which is coherent in nature is passed
through the microscopic objective to get a single mode
optical light to avoid feedback. The beam is further
divided in two beams of equal intensity by a directional
coupler.
2.1.3 Phase Shifter
A piezoelectric phase shifter is generally glued to a
mirror and it also has a voltage input from the
computer interface. The voltage then distorts the phase
shifter and the mirrors moves which produces a small
variation of the optical phase. For example, a
displacement of λ/8 produces a phase shit of π/2.
2.1.4 Charge-Coupled Device (CCD) Camera
The reference beam and the reflected beam before
incident on the CCD camera are made to pass through a
mirror with a pin hole. The pin hole is necessary to
ensure the two beams are in line and to cleanse the
reference wave. The CCD camera is used to resolve the
beams before projecting in onto the TV camera.
2.1.5 Computer – TV Interface
The video signal from the camera is digitized by the PC.
The computer is provided with a video digitizer board
giving it a wide spectrum of image processing
capabilities. It is also used to read out fringe
coordinates. The video signal from the computer is
passed through a filter rectifier before displaying onto
the TV monitor. An illustrative figure comprising of the
computer interface has been shown below.
Fig 4. ESPI computer interface

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3. Working Principle of ESPI
The coherent laser beam from the laser source is first
split into a reference beam and an object beam using a
beam splitter. The object beam illuminates the object
and is then scattered, producing an object image on the
recording device. The object image and the reference
beam are superimposed on the charge coupled device
(CCD) camera, and the interference patterns obtained
on the device are called speckles or an interferogram.
The intensity, phase and amplitude of the scattered
beam are dependent on the structure of the area of the
object from which it reflects. The speckle pattern is an
inherent attribute of the object.
The reference beam and the object beam are
recombined using either a pinhole mirror aperture or
an optical component like a glass wedge. However,
using a mirror gives the best results as the beam is
subjected to less intensity fluctuations when compared
to passing the beam through an optical component like
a lens.
When the object under consideration is subject to
loading, the surface of the object undergoes
deformation. Since the interference pattern is an
inherent property of the object surface, it too changes
according to the deformation, resulting in a new
speckle pattern. Comparing this to the original
undeformed interference pattern, we can qualitatively
obtain the displacement the surface undergoes in the
form of contour lines or by calculating the order of
fringes. However, the presence of speckles gives us a
low contrast, noisy image. A more quantitative analysis
of the patterns can be carried out by a phase shifting
procedure to remove the non-uniqueness in the
pattern. A deformation in the object will change the
distance between the object and the image, and
therefore the interference pattern of the deformed
surface will undergo a phase change. The interferogram
for the deformed object is subtracted pixel by pixel
from the original interferogram. The result obtained is
sent through a rectifier to give a contour map made of
bright and dark fringes called correlation fringes that
give us the displacement of the object.
When there is a phase difference between the
reference and reflected beam we obtain grey or white
fringes and dark fringes when there isn’t a phase
difference.
The ESPI has two basic configurations:
1. In-plane measurement system
2. Out of plane measurement system
3.1 Different types of ESPI Configuration
Depending on the direction of deformation of the
specimen we have two different types of ESPI
experimental setup configuration.
3.1.1 Out-of-plane configuration
In this setup the laser beam is initially split using a
beam splitter into two beams, namely object beam and
reference beam as seen in the fig (5). The object beam
is used to illuminate the surface of the object and
scattered back to the camera. The reference beam and
the reflected beam are combined together using a
beam splitter and directed back towards the camera. As
the object displaces in the direction parallel to the
direction of viewing, the distance travelled by the
object beam changes, due to which there is a phase
change. The final image recorded by the camera is the
speckle pattern or interference pattern formed by
these two beams. If φ is considered as the phase
difference between reference beam and object beam
before any displacement and (φ + Δ) is the phase
change of the object beam after displacement where, Δ
is the change due to deformation. The two speckle
patterns are subtracted to get the fringed pattern that
gives the information about the displacement.
Fig 5. Schematic representation of the setup and
speckle pattern of out of plane configuration

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3.1.2 In-plane configuration
For the in-plane measurement, two symmetric laser
beams from the same source are directed towards the
object from opposite directions. This is produced with
the help of beam splitter and mirror setup. The
interference pattern is formed by subtracting the two
beams reflected back to the camera. The interference is
obtained only if there is a deformation or displacement
in the direction perpendicular to the direction of
viewing. There is phase change as one beam phase
increases and the other beam phase decreases due to
displacement and is recorded as Δ.
Fig 6. Schematic representation of the setup and
speckle pattern of in plane configuration
3.2 Theoretical Calculation
Consider the in-plane ESPI system. The two coherent
laser beams of wavelength λ, and equal and opposite
angles θ fall on the object and get scattered. The
intensity of the scattered light along the normal to the
surface is given by
𝐼 = 𝐼# + 𝐼% + 2 𝐼# 𝐼% 𝑐𝑜𝑠φ (1)
where 𝐼#and 𝐼% are the intensities of the two beams
and φ is the phase difference between them.
On deformation, if a point on the surface of the object
moves by a displacement d, the phase differences
between the light beam changes by Δφ,
∆𝜑 =
4𝜋
𝜆
𝑑 𝑠𝑖𝑛𝜃
(2)
Therefore, using Eq. (1) we can write the new intensity
as,
𝐼5
= 𝐼# + 𝐼% + 2 𝐼# 𝐼%cos (φ + ∆𝜑) (3)
The difference in the intensities gives
𝐼 − 𝐼5
= 4 𝐼# 𝐼% sin φ +
∆𝜑
2
sin
∆𝜑
2
,
(4)
which denotes a fringe pattern with intensity maxima
at ∆𝜑 = (2𝑝 + 1) 𝜋, and minima at ∆𝜑 = 2𝑝𝜋, where
p is an integer.
The bright and dark fringes are obtained
according to Eq. (2) and Eq. (4), if d is spatially
dependent. Then the in plane displacement of the
object is obtained as
𝑑 =
𝑛𝜆
2 sin 𝜃
(5)
where n is the number of fringes at displacement d.
Similarly, in case of out-of-plane configuration by
carrying out a similar analysis we find the spatial
dependent,
where,
λ = wavelength of laser light
d = out of plane displacement of the object due
to the applied stress
α = angle between the direction of object
normal and camera viewing angle
β = angle between the direction of object
normal and the object beam
4. Applications
Electronic speckle pattern interferometry (ESPI) has
wide range of application in diverse fields because of its
ability to measure deformation or displacement with
variable sensitivity for in-plane and out-of-plane
directions, 3D object shape, surface roughness and
vibrations etc. Below are the areas where ESPI is widely
used in industries.
𝑑 =
𝑛𝜆
𝑐𝑜𝑠𝛼 + 𝑐𝑜𝑠𝛽
(6)

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4.1 Deformation measurements of solder joints on
electronic device boards
Electronic speckle pattern interferometry is preferred
for the investigation of solder joint deformation, since
it is a non-contact measurement method with accuracy
in the range of the order of the wavelength of the
source light, and has a full field image of the
deformation. It uses digital information, which suits
industrial application.
4.2 3D shape measurement with light-in-flight
electronic speckle pattern interferometry
There are various techniques that can be used for
three-dimensional shape measurements of small
components. The common problems with traditional
methods are that they are not efficient enough, and are
time consuming. ESPI is much more effective and
provides measurements much faster.
4.3 Continuous deformation measurement
Measurement of continuous deformation is difficult,
since collection of useful information is hard as the
state of the object changes dynamically. However, with
the help of high speed cameras, multi-camera systems,
and piezoelectric translators, ESPI is a very favorable
method for dynamic system measurements.
4.4 Cutting tool monitoring
The efficiency of the cutting process directly defines the
cost and productivity of machining operations. This is
because the cutting time increases as material strength,
complexity of work pieces’ increases and more
stringent machining tolerances are desired. The ideal
conditions vary significantly for the tool-machine-work
piece combination. Because of these reasons,
traditional methods like stylus probes cannot be used
to check the cutting tool condition. In such cases ESPI
performs much better and is a favorable measurement
tool.
4.5 Hole drilling method
The ‘locked in’ stresses which are also known as
residual stresses of a material which are formed during
common manufacturing processes can also be
calculated by using a ESPI optical phenomenon as they
avoid the lengthy procedure of attaching strain gages
as shown Fig 9.
Fig 7. Speckle pattern obtained for thermal distortions
in Printed circuit board (PCB)
Fig 8. Speckle pattern for flaw recognition for a surface
with cracks
Fig 9. Speckle pattern for a prism formed by examining
a drilled hole

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5. Advantages and Disadvantages
The ESPI system has several benefits when compared to
other traditional methods for determining surface
deformation,
• It is a non-contact measurement method with
wavelength order accuracy.
• This method provides a full field measurement.
• It is well suited for computer aided measurements
as information is acquired and evaluated digitally.
• The sensitivity is much higher than that of
holographic plates and thus allows one to use
shorter exposure times than those in classical
holography.
• Almost a real-time operation. The correlation
fringes can be displayed on a monitor without the
recourse to any form of photographic processing,
or plate relocation.
• The resolution of the recording medium used, need
not to be high compared with that required for
traditional holography.
• With phase modulation, the sensitivity can be
increased by 20 times.
Limitations of an ESPI system,
• The measurement range of ESPI is small and limited
by the speckle correlation.
• For large objects, high power lasers are required to
increase the average speckle pattern size.
• Equipment and installation cost of the setup is high.
6. Advancements
ESPI relies on technologies which have a huge scope of
improvement and those of which are driven by other,
much larger, forces. Components such as computers,
CCD cameras, laser diodes and image processing boards
are all high volume devices which are developed by
major industries which have high R&D budgets. In this
way, ESPI has the potential to advance much more
quickly than others. With continued advancements in
image processing techniques, it is probable that ESPI
will take a big leap towards producing holographic
quality data which could eliminate the need for
holographic interferometry.
The future scope of ESPI appears to be even brighter
than the already documented success. The laser diodes,
phase shifters, PC’s, CCD camera that will be developed
in the forth coming years seems to be at least an order
of magnitude more powerful than those available
today. Future advancements in optical and image
processing techniques will help to develop a better
fringe contrast, reduce the noise and increase the
resolution of the speckle pattern. Finally, a complete
displacement map of the specimen under deformation
can be obtained by implements phase shift methods.
The phase shift can be achieved by modulating the
wavelength of the laser diode.
7. Conclusion
This paper explains how surface deformations can be
evaluated using electronic speckle pattern
interferometric (ESPI) optical method. Two different
configuration of ESPI have been explained in detail
followed by the theoretical calculation for determining
spatial dependent (d).
The speckle pattern is obtained by the contouring
method which is based on the holographic two-beam
illumination technique. The recording, digitizing, storing
and processing of video signals, as well as the data
collection and all calculation are done by using a
personal computer (PC) which follows an algorithm
derived from the video digitized board.
We can also conclude that ESPI has a short exposure
time (< 1/25 sec), high repeatability rate (1/25 sec) and
less sensitive to noise when compared to other
holographic techniques.
ESPI has wide range of applications because of which it
is the measurement tool used in most industries.
Further, this paper also summarizes the advantages,
disadvantages and applications of ESPI and outlines the
advancements and some potential future
improvements to ESPI.

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8. References
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Interferometry, 1989, Cambridge University
Press
2. Winther Svein, 3D Strain Measurements using
ESPI, 1987, Norwegian Institute of Technology
3. Moore J. Andrew & Tyrer R. John, 2D Strain
measurement with ESPI, 1995, Loughborough
University of Technology
4. Moore, A. J. & Tyrer, J. R., An electronic speckle
pattern interferometer for complete in-plane
displacement measurement. Meas. Sci.
Technol., 1 (1990), 1024-1030.
5. Sharp Brad, Electronic Speckle Pattern
Interferometry, 1989, Newport Corporation –
CA, USA
6. Angel F Doval, “A systematic approach to TV
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(2000)
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Wolfgang, Simulation and Experiment in laser
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ESPI, Vol. 46 [1], 2008
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measurement, Chinese journal of ME, Vol. 27
[1], 2014
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Press, 1978
11. Moore R. Thomas, A simple design for an
electronic speckle pattern interferometer,
Rollins College, 2004
12. Raghavendra Jallapuram, Con Healy, Emilia
Mihaylova, and Vincent Toal, In-Plane sensitive
electronic speckle pattern interferometer using
a diffractive holographic optical element,
Dublin Institute of Technology, 2010