This document provides an overview of using digital holography to characterize the hygroscopic properties of wood. It discusses the basic principles of digital holography, the experimental setup used, and the methodology. The methodology involves recording holograms of wood samples as they dry, numerically reconstructing the holograms, and using phase detection algorithms to calculate strain and determine the hygroscopic shrinkage coefficient from changes in the wood's moisture content. Next steps include working on algorithms to unwrap phases from holograms and correlate phase changes with deformation measurements.

1. Characterization of hygroscopic
properties of wood using
digital holography
Puneet Singh Thakur
M.E. (Digital Instrumentation)
Roll No. 15DI3109
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CONTENTS
 Introduction
 Basic principle
 Components
 Experimental setup & digital recording of hologram
 Reconstruction process(using image processing )
 Problem domain
 Objectives
 Methodology
 Strategy & description
 References

3. WHAT IS DIGITAL HOLOGRAPHY?
The word “holography” simply means “entire recording”.
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PHOTOGRAPHY DIGITAL HOLOGRAPHY
CCD

4. BASIC PRINCIPLE
Field Distribution for object wave 𝜑 𝑜 𝑥, 𝑦 = 𝐴 𝑜(𝑥, 𝑦)𝑒−𝑖𝜃 𝑜(𝑥,𝑦)
Field Distribution for reference wave 𝜑 𝑟 𝑥, 𝑦 = 𝐴 𝑟(𝑥, 𝑦)𝑒−𝑖𝜃 𝑟(𝑥,𝑦)
The object and reference wave interfere on the recording medium which records the resultant intensity
distribution and is given by
I(x, y) = 𝐴 𝑜
2+𝐴 𝑟
2+2𝐴 𝑜 𝐴 𝑟 cos(∅ 𝑜 − ∅ 𝑟)
The recorded intensity pattern has also the phase of the object ∅ 𝑜.
Phase is nothing but the indicator of surface profile of the given object. As phase difference is directly related
to the path difference, hence it can provide depth information of the given surface.
Phase Difference=
2×Π
𝜆
(Path Difference)
PHASE CALCULATION FOR DIGITAL HOLOGRAPHY
Δø=
Δø1 − Δø2 𝑖𝑓Δø1 ≥ Δø2
Δø2 − Δø1 + 2Π 𝑖𝑓Δø1 ≤ Δø2
Where
Δø1= Undeformed phase
Δø2 = Deformed phase
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5. COMPONENTS
Spatial Filter
Arrangement
Beam Splitter
Mirrors
Laser Source
(wavelength
632.8 nm,
15mw)
CCD Camera
(1392×1024 pixel
with each 4.65
micrometer)
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6. DIGITAL HOLOGRAPHY METHOD
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BEAM
SPLITTER
SF1 L1 M3
CCD
SF2
LASER
M1
M2
WOOD
SAMPLE
Back
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7. Wood has some unusual characteristic that must be understood and considered for its
optimal utilization.
The dimensional changes i.e., the shrinkage and swelling of wood, are
responsible for the major source of structural problem in parts made of wood.
This coefficient is a measure of the shrinkage caused by a unit decrease in the moisture
content of the wood sample.
PROBLEM
DOMAIN
Here we try to figure out the Hygroscopic Shrinkage Coefficient.
The dimensional changes (shrinkage and swelling) of wood are related to the
moisture content in the environment and the temperature. But there are more
changes when there are changes in the moisture content in the environment
rather than changes in temperature.

8.  Wood shrinks when it dries, and the shrinkage is uneven over the cross section due to the different shrinkage
properties in the radial, tangential and longitudinal directions.
 Radial shrinkage is the amount the wood moves perpendicular to the growth rings.
 Tangential shrinkage the amount the wood moves along the growth rings.
 Longitudinal shrinkage is the lengthwise change in the wood.
Longitudinal shrinkage is very small as compare to the tangential and radial shrinkage.
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RADIAL
SHRINKAGE
TANGENTIAL
SHRINKAGE
GROWTH
RINGS

9. Devising strategies for testing the hygroscopic properties of wood.
Using the developed set up, sample holograms are recorded.
Development of appropriate techniques for estimating phase from the recorded
hologram (like Fourier Transform Method & Windowed Fourier Transform).
Effort will be made to minimize the noise, phase ambiguity and other factors in
estimation of phase.
Design and development of digital holography setup for the inspection of the sample.
OBJECTIVE

10. Basically proposed set up (Slide No. 6) will be used to capture the
Holograms.
Hologram will be reconstructed numerically. The image processing carried
out in MATLAB.
The phase map generated by the MATLAB for the different hologram will
Give us the information about the drying process, hence we can extract
Hygroscopic properties of wood.
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01
02
03
METHODOLOGY
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11. BS
M1
BS: BEAM SPLITTER
SF: SPATIAL FILTER
M1,M2: MIRROR
Experimental set-up for Digital Holography
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Graphical representation of reconstruction process
Reference wave
Real image
Virtual image
First order
zero order

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Diffraction field
calculation
Reference light
introduction
Obtain field in
object plane
Raw
Hologram
Reconstructed
Hologram
Phase
Calculation
Algorithm to reconstruction Process of Hologram
(Requires Digital Image Processing)

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Reconstruction Process of Hologram
(Requires Digital Image Processing)
The complete simulation can be separated into three steps.
Step 1: Diffraction field calculation (Fresnel diffraction).
Step 2: Introduce a reference light to the CCD plane and calculate the resulting interference fringes. This
step together with step 1 is to simulate holographic recording.
Step 3: Find the field in the object plane from the field producing from the hologram (backward propagation).
This step is to simulate holographic reconstruction.

15. START
Read the Reconstructed image
Take a FFT
Central lobe selection
Subsidiary lobe selection
Replace central lobe by subsidiary lobe
Take IFFT
Phase detection
Wrapped phase information
Stop
Flowchart to calculate Phase from the Hologram :
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The reconstructed fringe pattern can be described by

16. 𝑢 𝑥 =
𝜆
4 cos(
𝛼
2
)
∆∅
𝜀 𝑥𝑥 =
𝑢 𝑥 𝑥+Δ𝑥 ,𝑦 −𝑢 𝑥(𝑥, 𝑦)
𝑥
𝜀 𝑥𝑥 =
𝑢 𝑥 𝑥+Δ𝑥 ,𝑦 −𝑢 𝑥(𝑥, 𝑦)
𝑥.Δ𝑡
 The deformation (𝑢 𝑥) induce due to the change in the moisture content of the wood is given below:
Where 𝛼 is the angle between the illumination and observation direction, ∆∅ Phase difference
between consecutive wood sample with different moisture content, 𝜆 is the wavelength of the LASER
source used.
 To calculate the induced strain (𝜀 𝑥𝑥) , the displacement will be differentiated, which given by:
 The strain rate is given by
Strategy to calculate the Hygroscopic Shrinkage Coefficient of wood :
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 For the calculation of the hygroscopic shrinkage coefficient, a curve will be fitted to the strain data.
The slope of the fitted curve is the hygroscopic shrinkage coefficient of the wood.

17. Recorded Hologram
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18. Working on the Algorithm of FTM (Fourier Transform Method) to get the phase and
deformation information from the Hologram.
Also working on the phase unwrapping algorithm so that we can get the unwrap phase map
of the recorded Hologram. That will we correlated with the deformation, which will give
us the strain generated in wood sample due to different moisture content.
Till now we have generated the sample Hologram of the wood sample and trying to
reconstruct it numerically.
Next Steps
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Communications 214, 115-121 (2002).
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2959-2966 (2004).
5. M. Hossain, G. Sheoran, D. S. Mehta, and C. Shakher, “Contouring of diffused objects by using digital holography,”
Optics and Lasers in Engineering 45, 684–689 (2007).
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Optics and Lasers in Engineering 45, 684–689 (2007).
7. M. K. Kim, “Principles and techniques of digital holographic microscopy,” SPIE Reviews 1, 018005-1- 50 (2010).
8. P. Tankam, and P. Picart, “Use of digital holography for crack investigation in electronic components,” Optics and
Lasers in Engineering 49, 1335–1342 (2011).
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(2014).
10. V. Kumar and C. Shakher ,“Study of heat dissipation process from heat sink using lens less Fourier transform digital
holographic interferometry,” Applied Optics 54, 1257-65 (2015).
REFERENCES

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