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SPECIAL SECTION: INVITED REVIEW PAPER Laser Display and Lighting Conference (LDC’15),
Yokohama, Japan
Speckle reduction methods in laser-based picture projectors
M. Nadeem Akram
1
•Xuyuan Chen
1,2
Received: 13 August 2015 / Accepted: 21 November 2015
ÓThe Optical Society of Japan 2015
Abstract Laser sources have been promised for many
years to be better light sources as compared to traditional
lamps or light-emitting diodes (LEDs) for projectors,
which enable projectors having wide colour gamut for
vivid image, super brightness and high contrast for the best
picture quality, long lifetime for maintain free operation,
mercury free, and low power consumption for green
environment. A major technology obstacle in using lasers
for projection has been the speckle noise caused by to the
coherent nature of the lasers. For speckle reduction, current
state of the art solutions apply moving parts with large
physical space demand. Solutions beyond the state of the
art need to be developed such as integrated optical com-
ponents, hybrid MOEMS devices, and active phase mod-
ulators for compact speckle reduction. In this article, major
methods reported in the literature for the speckle reduction
in laser projectors are presented and explained. With the
advancement in semiconductor lasers with largely reduced
cost for the red, green and the blue primary colours, and the
developed methods for their speckle reduction, it is hoped
that the lasers will be widely utilized in different projector
applications in the near future.
Keywords Laser projection Laser speckle Speckle
reduction MOEMS Phase modulators
1 Introduction
The topic of speckle phenomenon in coherent wave for
imaging is very old [1]. In parallel, there has been con-
tinuous effort to use lasers in display applications [2]. A
comprehensive reference to the history of speckle and its
theoretical foundation are given in the famous book by
Goodman [3]. Briefly speaking, the speckle noise is formed
when coherent light is reflected or transmitted through a
region which imparts randomness onto the amplitude,
phase or both of the light. In addition, the light field has to
be detected by a detector which only records the intensity
of the electric field. Since intensity is squared magnitude of
the electric field, cross-terms appeared in the intensity field
create the speckle noise. A granular random dark-bright
pattern appears in the uniform image. Hence, the useful
information in the displayed picture hides under the
speckle noise.
2 Principle for suppressing laser speckle noise
The severity of speckle noise in a uniform picture is usually
characterised by the speckle contrast parameter Cdefined
as,
C¼rI
\I[ð1Þ
where r
I
is the standard deviation of the intensity of the
captured image, \I[is the mean intensity in the captured
image. For the fully developed speckle noise in a uniform
picture, the speckle contrast C=1 and if there is no
speckle noise in a uniform picture, C=0. Experiments
with human observers [4,5] have shown that if speckle
contrast Cis below 3 % in a uniform picture, an average
&Xuyuan Chen
xuyuan.chen@hbv.no
1
University College Buskerud and Vestfold, IMST,
3103 Horten, Norway
2
Key Laboratory of Instrumentation Science and Dynamic
Measurement, North University of China, Shanxi 030051,
China
123
Opt Rev
DOI 10.1007/s10043-015-0158-6
human observer will not notice the presence of speckle
noise and there will be no degradation to the perceived
picture quality.
Several methods have been proposed and demonstrated
for reducing the speckle noise [3–40]. The key point to be
noted is that speckle phenomenon itself cannot be elimi-
nated if a coherent light is used with rough screen. How-
ever, what can be done to eliminate the perceived speckle
on a CCD camera or human observer is to somehow benefit
from the spatial and temporal integration properties of the
human eye–brain or the CCD camera. It is well known that
the human eye-brain has a finite temporal response time of
30–60 ms [3,4]. There is also a finite spatial resolution of
the human eye lens–retina system which is around
595lm
2
[5]. Hence if many speckle patterns are gen-
erated which look different from each other, and are
superimposed during the finite spatial–temporal averaging
constants of the human eye–brain, then the perceived
speckle will be a spatio–temporal average of these patterns.
This is the key method used in many different ways to
reduce the perceived speckle in laser projectors. According
to Goodman, if Nindependent speckle patterns with equal
mean intensity are averaged, the speckle will be reduced by
1
ffiffiffiffi
N
p.
3 Available degrees of freedom for speckle
reduction
As explained in [29], the speckle contrast depends upon the
reduction factor Rfor a projection system,
C¼1
Rð2Þ
R¼RkRrRXRNð3Þ
where R
k
is the reduction factor due to the spectral diver-
sity of the laser source, R
r
is the reduction factor due to the
polarization diversity of the screen, R
X
is the reduction
factor due to the angular diversity of the projection system
and the observation system, and R
N
is the number of
independent laser sources with a certain minimum angular
separation between them. For a source with Gaussian
spectrum with 1/efull-width =dk, and a screen with rms
roughness r
h
, the speckle contrast C
k
is [29]:
Ck¼1
Rk¼1þ8p2dk
k
2rh
k
2
()
0:5
2
43
5
0:5
ð4Þ
For a projected image seen by the observer and a
moving diffuser is used to average out the speckle, the
speckle contrast C
X
is given by [3],
CX¼1
RX¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi
MþK1
MK
rð5Þ
where the plus sign is when the projection lens is overfilled,
and negative sign is when the projection lens is just filled or
under filled. Here Mis the temporal degree of freedom
depending upon how many independent speckle patterns
are introduced by the motion of the diffuser, and Kis the
spatial degree of freedom equal to the squared ratio of the
numerical aperture of the projector lens to that of the
human observer with respect to the screen.
The polarization diversity factor Rr¼ffiffiffi
2
pfor a screen
which fully depolarizes the incoming electromagnetic
wave after scattering [3]. It will be less than 2 if the screen
has preferential reflection in a certain polarization state.
If Nindependent lasers are used in the projector, and if
the angular separation between them is larger than the
angular substance of the observer eye on the screen [3],
then the speckle contrast C
N
is:
CN¼1
RN¼1
ffiffiffiffi
N
pð6Þ
The combined speckle reduction factor Rin a specific
projector can be assessed from these fundamental
equations.
4 Speckle measurements
Laser speckle can be measured in free space, which is
called objective speckle, and in an image plane, which is
called subjective speckle. In the condition that the CCD
sensors of the same pixel size are used in both the free
space and the image plane measurements, and the mea-
surement configuration as well as the parameters of the
apparatus were chosen to result in the same objective and
subjective speckle size on the CDD sensor, the speckle
contrast measured in the image plane match well with it in
the free space [41]. The measured speckle contrast depends
on the ratio of the speckle size over the pixel size of the
CCD sensors, the offset of the optical axis to the normal
direction of the screen, the environmental straight light, as
well as the settings of the CCD sensor such as the exposure
time. In the piled publications on suppression of the
speckle, no standard configuration of speckle measurement
was applied. Therefore, it is less meaningful to compare the
speckle contrast published in different articles in which the
different measurement configurations were employed. In a
projector, the speckle on screen percept by a human
observer becomes practically important. Therefore, con-
figuration for measuring the subjective speckle must be
applied due to the similarity to the situation of human
Opt Rev
123
observers. To evaluate the effectiveness of any speckle
reduction technique in a projector, the projection optics
must be configured in the measurement, and the parameters
of the CDD camera must model the eye optics. The articles
[5,41–44] are excellent to refer to for setting up the
speckle measurement in laser projection systems correctly.
As the summary of this paragraph, a standard measurement
setup and procedure for the speckle in projection system
must be developed and adapted. The speckle contrast data
referred below cannot be compared between different
articles but can be an indication of the effectiveness of the
speckle reduction method presented in a specific article.
5 Possible methods of speckle suppression
In Fig. 1, a summary of possible methods for speckle
suppression is presented. The instantaneous and sequential
decorrelation are two principles for creating the speckle
reduction.
5.1 Speckle reduction using time-sequential creation
of many independent speckle patterns
There are many methods to create time varying indepen-
dent speckle patterns. One important point to be noted is
that even though the integration time of the human eye is
30–60 ms, the integration time of the projector should also
be considered [3]. Full-frame DLP, LCD or LCoS projec-
tors project a complete picture on the screen at one instant
of time. In such projectors, the time for a pixel illuminated
on the screen is 16.68 ms for the frame rate of 60 pictures/
s. If we want to reduce speckle in each individual frame of
the picture especially for video, then the effective inte-
gration time available for speckle reduction is 16.68 ms.
For a line-scan projector, a column of pixels is scanned
across the screen to create an illusion of a 2D picture in the
human eye–brain. For the 60 pictures/s frame rate and a
picture of 1920 91080 pixels, the time during which a
column is illuminated is 8.68 ls. Hence the effective
integration time available for speckle reduction is 8.68 ls
in each frame. For pico-projectors which project a single
spot on the screen and scans it in two-dimensions to create
a 2D picture, the available time a single pixel is illuminated
is even smaller. For the 60 pictures/s frame rate and
1920 91080 pixels picture resolution, this time is 8.03 ns.
Hence in such situation, the available time for speckle
reduction is 8.03 ns in each frame. Alternatively, since eye
with 30/60 ms integration time can integrate two/four
successive frames in 60 frames/s video, the effective
integration time for speckle reduction can be doubled/
quadruples, respectively. In both line-scan and raster scan
projectors, extra speckle reduction can be achieved
depending on the screen property and the dimension of the
laser line/spot along the scanning direction. Because the
laser line/spot progressively scans through the spot of eye
resolution on the screen, different areas within the eye
resolution spot at the different moments scatter the incident
light as schematically shown in Fig. 2, and create different
speckle patterns which can be partially independent. The
Fig. 1 Overview of the speckle
reduction possibilities
Opt Rev
123
correlation between those speckle patterns depends mainly
on the spatial correlation length of the rough screen. The
more the number of the correlation length within the eye
resolution spot on the screen, the less the independence of
those speckle patterns. Theoretical analysis of the speckle
in the scan (line or raster) projectors can be found in Refs.
[15,16,45].
5.1.1 Polymer dynamic diffraction gratings
A dynamic grating is made using an electroactive polymer
which can be electrostatically actuated (Fig. 3)[6,7]. Time
varying speckle patterns through the angle and spatial
diversity are generated in this method by creating different
diffraction orders through the grating as light diffracts from
it. When these independent speckle patterns are added on
the CCD camera during the integration time of the camera,
speckle reduction happens. In [6], four different grating
patterns were averaged to give a speckle contrast of 0.37
from its original value of 0.73. However, the maximum
number of independent speckle patterns that can be gen-
erated with such a device is limited due to limited number
of internal reflections that can be designed without any loss
of light.
5.1.2 MEMS deformable mirror
In this method [8], a thin reflective flexible membrane is
vibrated by a number of independent piezo actuators
underneath it. The piezo actuators are driven at kHz rate,
and ripples appear on the surface of the mirror, which is
otherwise flat. Hence, the reflected light at different angles
and diffraction orders is generated and dynamically chan-
ged. When such a light is used to illuminate a random
surface, independent speckle patterns are produced for
speckle reduction (Fig. 4). One particular advantage of
such a device is that the surface of the mirror remains
relatively smooth when it is actuated, thus avoiding scat-
tering losses. The extent to which speckle can be reduced
greatly depends upon how the projector illumination
Fig. 2 Schematic presentation of the scatter areas at different moments during the scanning process
Fig. 3 Side-view and top-view of polymer diffraction gratings with
three electrode patterns
Fig. 4 Speckle pictures without (left) and with (right) actuation of
the flexible mirror (adopted with permission)
Opt Rev
123
system is designed, and the degree of spatial diversity
factor K.
5.1.3 MEMS in-plane vibrating device
Silicon based microelectromechanical system (MEMS)
device with movable part is applied for speckle reduction
[9] (Fig. 5). The central mass of the device has random
rough surface etched onto it. The central mass is hung from
springs to the anchors and is electrostatically actuated by
the dual comb fingers on the two sides of the central mass.
Light is reflected by the central random surface, and when
the MEMS device is vibrated in plane, independent speckle
patterns are generated and speckle averaging results during
the integration time of the observer or the camera. A
speckle contrast reduction of 43.8 % is reported from its
initial value for the free space propagation, and 26.8 % for
the imaging geometry.
5.1.4 MEMS oscillating mirror
In this technology [10], a MEMS mirror made for rotating
on two axes is used to drive the laser beam for illuminating
a random diffuser (Fig. 6). Different angles of the laser
beam falling on the random surface and passing thorough
the illumination path create independent speckle patterns,
which result in very good speckle reduction when averaged
by the CCD camera (Fig. 7). Such MEMS mirror can be
placed in the illumination path of the projector to achieve
speckle reduction base on angle diversity. A very high
speckle reduction is reported with speckle contrast
C=3.3 % for the free space geometry and C=4.4 % for
the imaging geometry indicating that the MEMS mirror can
provide very high degree of temporal diversity M.
Fig. 5 Top-view of MEMS diffuser device
Fig. 6 MEMS dual-axis micromirror
Fig. 7 Speckle pictures without (a) and with (b) rotating mirror
Opt Rev
123
5.1.5 Vibrating Hadamard binary phase matrix
According to the statistical property of Hadamard matrix, a
special set of binary phase matrices based on Hadamard
matrix was proposed to create independent speckle patterns
[11]. Different set of matrices were etched on to glass, and
placed at an intermediate image plane in the laser projector.
When the glass plate is sequentially cycled through the set of
Hadamard matrices, effective speckle suppression results.
One particular advantage of this method is the better speckle
reduction can be achieved in fewer steps as compared to a
moving random diffuser. A speckle contrast factor C=9%
was reported when the binary diffuser is vibrated.
5.1.6 Binary micromirror array
A novel electrode configuration was developed to drive a
two-dimensional micromirror array of pixels for creating a
series of independent phase patterns, which can be fabri-
cated using Silicon MEMS technology [11]. Each pixel can
be individually moved up and down at two binary levels
giving a phase of 0 or pradians to the reflected light. The
electrical driving scheme will enable an orthogonal phase
patterns. The MEMS micromirror array is placed at the
intermediate image plane of the laser projector. When
successive matrices of orthogonal phase patterns are
imposed on the micromirror array, independent speckle
patterns are produced. One disadvantage of such a device is
that the total number of independent pixels needed for
efficient speckle reduction is too large. Moreover, it has to
be placed at the intermediate image plane of a laser pro-
jector, which is not always accessible.
5.1.7 Sinusoidal vibrating random diffuser
In this method [13], a random diffuser with continuous
height profile (Fig. 8) was placed at an intermediate image
plane and vibrated in a pure sinusoidal motion using a
tuning fork. As shown theoretically in [13], if the diffuser
has a pure sinusoidal motion, the temporal degree of
freedom Mbecomes infinity and the speckle contrast
depends only upon the spatial degree of freedom K, which
is in the range 500–18000 for laser projectors [29]. Hence,
a very low speckle contrast C=3.4 % can be achieved
(Fig. 9). However, if the diffuser has non-sinusoidal
motion or is not placed exactly at an intermediate image
plane, less speckle reduction would result.
5.1.8 Two-layer Hadamard binary phase matrix
In this novel technology [14], the two-dimension binary
Hadamard phase matrix is replaced by a two 1D binary
phase matrices placed adjacent to each other (Fig. 10). The
net effect of light passing through the pair of binary phase
matrices is the same as if light passed through a single 2D
binary phase matrix. The advantage of using two 1D binary
phase codes is that it is easier to fabricate, and the control
electronics for electrically actuated binary phase code is
simpler as compared to a single 2D binary phase matrix. For
the proof of concept, the binary phase codes were etched in
glass [14], and experimental results prove the effectiveness
of speckle reduction (Fig. 11) to the level of 8.96 %.
5.1.9 Binary phase code
In scanning projectors, a specially designed binary phase
code can be used to reduce speckle [15,16]. The binary
phase code has to have the property that its autocorrelation
is a narrow delta-like function. The binary phase code is
etched on to a glass plate and placed at an intermediate
image plane. Due to the scanning action on the laser beam,
the speckle is time-averaged in the human eye (Fig. 12).
An example of such code is Barker 13 code =[11111
-1-111-11-1 1]. Other possible codes are MPS 28,
MPS 51, MPS 69. One important factor is that the laser
spot on the screen should be smaller than the eye-resolution
spot on the screen. Using the longest MPS 69 code, a
speckle contrast factor of 6 % can be achieved theoreti-
cally, but the experimental value reported was 8.7 %.
5.1.10 Rotating diffuser
In this method [17–20], an optically rough diffuser with a
certain scattering angle is placed in the illumination path and
rotated. Due to different areas of the diffuser coming under
light path, different phase patterns are imposed on the light
beam, the speckle can be brought down even if the laser has
very narrow spectrum. The higher the roughness of the
Fig. 8 Random diffuser (adopted with permission)
Opt Rev
123
diffuser or the faster the rotation speed, the larger the tem-
poral degree of freedom M. However, the maximum speckle
reduction that can be achieved is limited by the spatial
diversity K. Speckle contrast below 4 % can be achieved by
choosing appropriate scattering angle of the diffuser.
5.1.11 Rotating light pipe
Similar to the rotating diffuser, a homogenizing light pipe
of rectangular shape in the illuminating optics path of a
projector is rotated to produce the angle diversity [21]. A
speckle contrast of \7.5 % can be achieved with this
method. However, as the light pipe rotates, the rectangular
spot on the display chip (DMD, LCoS, LCD) also rotates,
hence causing light loss since the illuminating area will
have to be overdesigned as compared to the true size of the
display chip.
5.1.12 Rotating microlens array
A microlens array (Fig. 13) is designed to create the
angular diversity when it is rotated in the illumination path
[22]. The advantage of a microlens array is that no higher
angle scattering rays are produced, which would result if a
rough diffuser is used in the optical path. Improved speckle
reduction and picture homogeneity were demonstrated by
using such a microlens array.
5.1.13 Vibrating microlens array beam shaper or diffuser
In articles [23,24], a microlens array beam shaper is
designed to convert the Gaussian intensity laser beams into
a top-hat profile (Fig. 14) to efficiently illuminate the dis-
play chip, which has rectangular dimensions. In addition, if
the beam shaper is vibrated axially or laterally using a
piezo motor, angle diversity happens and resulted speckle
is effectively minimized (Fig. 15). A low speckle contrast
of 5.5 % was reported for the green colour and even lower
for red and blue colours.
5.1.14 Vibrating multimode optical fibre
In articles [25,26], the laser light is coupled into a multi-
mode fibre with large numerical aperture. The light power
couples into different modes depending upon the coupling
condition. Each mode of the fibre has its unique mode pat-
tern and propagation velocity. When the optical fibre is
vibrated, the light power is dynamically shifted between
different modes of the fibre, and hence the electric field at the
output end of the fibre is randomly changing its intensity and
phase profile. When such a light is used to illuminate a laser
Fig. 9 Speckle pictures with stationary diffuser (a) with moving
diffuser (b) (adopted with permission)
Fig. 10 A pair of 1D binary phase elements
Opt Rev
123
projector, speckle will be reduced due to the angle diversity,
reduced spatial coherence and temporal coherence. The
authors reported a speckle contrast down to 5 %.
5.1.15 Wavefront randomization by ferroelectric liquid–
crystal cell
Article [27] demonstrates the technology using an electro-
optic ferroelectric liquid crystal (FLC) cell through which
light passes. By applying short voltage pulses to the FLC, a
spatially inhomogeneous structure with random distribu-
tion of refractive-index gradient is generated in the FLC
layer, which results chaotic space and time phase modu-
lation needed for speckle suppression. A speckle reduction
efficiency of 50 % was achieved.
5.2 Speckle reduction using instantaneous creation
of many independent speckle patterns
This is the second major category of speckle reduction
methods. In this category, different independent speckle
patterns are created instantly, and averaged by the CCD
camera or the human eye instantly. Therefore, the finite
integration time of the CCD camera or the human eye is
Fig. 11 Speckle pictures without (a) and with (b) moving a 2D single plate, and with moving pair of 1D plates (c)
Fig. 12 Speckle reduction in line-scan projector with different binary
phase codes
Fig. 13 Schematic of rotating microlens array in a LCD projector
(adopted with permission)
Opt Rev
123
not important. Such methods can be used in combination
with sequential method as well, where a number of inde-
pendent speckle patterns are created instantaneously and
sequentially, to get a better effective speckle reduction.
5.2.1 Array of independent lasers
When Nindependent lasers are used in the projector, the
speckle reduction can be achieved [28,29]. If each laser
will illuminate the screen homogenously and the effective
angular separation between the lasers is larger than the
angular substance of the observer eye on the screen,
independent and uncorrelated speckle patterns are pro-
duced on the screen by each individual source, resulting in
speckle reduction. The higher the number of independent
laser and broader the spectral bandwidth of the laser array,
the better the speckle reduction will be.
5.2.2 Long multimode fibre illuminated with broad
spectrum laser
In such method [29], a stationary multimode fibre is used to
create the temporal diversity M. Due to phase delay
between different modes in a multimode fibre, the light at
the output end of a long multimode fibre behaves temporal
incoherent. As the spectrum width of the light and the
numerical aperture of the multimode fibre are increased, a
shorter length of the fibre is needed to achieve sufficient
speckle reduction as shown in Table 1. By combining this
method with the spectral diversity and using a bundle of
multimode fibres, a very low speckle contrast down to 1 %
can be achieved.
5.2.3 Broadband laser
For green laser, special frequency doubled laser source with
broadband spectrum have been proposed (Fig. 16)[32].
When such a laser is used in the projector, the speckle
reduction happens due to the spectral diversity (Fig. 17).
Similarly, if the temperature distribution of a laser diode array
is made to be non-uniform [33], the total spectrum of the array
broadens even further, resulting in much less speckle.
5.2.4 Partial coherent laser beams
If a single laser beam with low coherence length is split
into many paths while optical path difference among dif-
ferent paths is greater than the coherence length, the split
Fig. 14 Schematic of three chip
LCoS projector with vibrating
microlens array beam shaper
(adopted with permission)
Fig. 15 Speckle picture
without and with vibration of
the microlens array beam shaper
(adopted with permission)
Opt Rev
123
Nlaser paths behave as independent laser sources [34], and
provide Ndegree of freedom for speckle reduction. The
laser beam can be split into many paths using optical fibre
loop or beam splitter with different thickness.
5.2.5 Random laser
In a novel proposal [35], the structure of the laser cavity
itself is modified to result in low spatial and temporal
coherence photon stream. The lasing action is based on
disordered active region without any laser cavity. Lasing
action happens due to multiple scattering of trapped light.
5.2.6 Volume scattering
In such methods [36–38], spatial and temporal de-coher-
ence is achieved by passing light through a volume having
multiple scattering microspheres or particles. Due to the
multiple scattering and refraction, the light photons get
decorrelated. The losses due to the volume scattering can
be reduced by optimizing the particle density, size and
refractive index. If such a light is used to illuminate a
projector, minimal speckle contrast will result depending
upon the spectral width or coherence time of the source and
the standard deviation of the scattering path length distri-
bution of the scattered photons [4].
Table 1 Length of the fibre (in meters) of specific numerical aperture
(NA) needed for speckle contrast C=1 % at the end of the fibre
when the laser source of spectral width (BW) is used (adopted with
permission)
BW\NA 0.1 nm 1 nm 10 nm
0.1 3200 320 32
0.2 800 80 8
0.4 200 20 2
0.6 90 9 0.9
0.8 50 5 0.5
Fig. 16 Spectrum of the green laser having broadband emission
(adopted with permission)
Fig. 17 Spectrum width and
speckle reduction factor for
different laser sources (adopted
with permission)
Opt Rev
123
5.3 Compound methods for speckle reduction
In line-scan projector as shown in Fig. 18 [39], speckle
reduction is achieved by a multitude of methods. A com-
bined effect is utilized to reduce speckle, such as polar-
ization diversity because of the depolarizing property of the
screen, angular diversity because of the fast moving dif-
fuser, line scanning which results in extra angular diversity,
and splitting of the laser beam into multiple paths with
sufficient optical path differences resulting in independent
non-interfering laser beams (Fig. 19).
In [40], effective speckle reduction is achieved by
combination of spectral, angular, polarization diversities
and by using nine individual laser diodes. A rotating dif-
fuser is used to provide the angular diversity.
6 Conclusion
The speckle noise in laser projectors can be reduced by a
variety of methods. These methods exploit the available
degrees of freedom for speckle reduction, that is: polar-
ization, angular, spatial, temporal, and spectral. The tem-
poral and spatial averaging properties of the human
observer eye–brain play a crucial role to determine the
extent to which speckle can be reduced. The choice of
Fig. 18 Schematic of the line-scan laser projector (adopted with permission)
Opt Rev
123
speckle reduction methods depends greatly on the archi-
tecture of the laser projector as well. Eventually, a partic-
ular speckle reduction solution will depend upon the choice
of the laser sources and the optical engine design used in
the projector.
Acknowledgments We acknowledge LasePro BIA Project# 210598
funded by Research Council of Norway for this research work.
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