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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 components, hybrid MOEMS devices, and active phase modulators 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.
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
Xuyuan Chen
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
where r
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
University College Buskerud and Vestfold, IMST,
3103 Horten, Norway
Key Laboratory of Instrumentation Science and Dynamic
Measurement, North University of China, Shanxi 030051,
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 [340]. 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
[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
3 Available degrees of freedom for speckle
As explained in [29], the speckle contrast depends upon the
reduction factor Rfor a projection system,
where R
is the reduction factor due to the spectral diver-
sity of the laser source, R
is the reduction factor due to the
polarization diversity of the screen, R
is the reduction
factor due to the angular diversity of the projection system
and the observation system, and R
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
, the speckle contrast C
is [29]:
For a projected image seen by the observer and a
moving diffuser is used to average out the speckle, the
speckle contrast C
is given by [3],
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 Rr¼ffiffi
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
The combined speckle reduction factor Rin a specific
projector can be assessed from these fundamental
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
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
[5,4144] 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
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
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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.
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)
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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
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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 [1720], 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)
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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
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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)
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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)
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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 [3638], 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
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)
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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)
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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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... Celles-ci fonctionnent toujours par moyennage des interférences sur un petit temps d'intégration et requiert l'ajout d'un simple composant optique. Elles viennent majoritairement du domaine de la projection d'image [77] et se basent sur l'usage d'un élément tournant ou vibrant [78,79] tel qu'un diffuseur tournant ou une membrane. Georgiades et al. [80] montrent par exemple comment une membrane vibrante peut atténuer les interférences sur l'ordre de 3 ms (Figure 1.23). ...
La microscopie de fluorescence et la microscopie de localisation de molécules uniques (SMLM) permettent d’imager spécifiquement les entités subcellulaires et sont donc des outils indispensables en Biologie. Cependant ces techniques d’imagerie sont intimement liées aux propriétés de l’illumination, qui possède des limites en termes d’uniformité et de contrôle de l’éclairement.Nous proposons une nouvelle méthode d’illumination nommée ASTER (Adaptable Scanning for Tunable Excitation Regions), capable de délivrer une illumination modulable et uniforme, et compatible avec les méthodes de sectionnement optique classiques. Nous l’appliquons en premier lieu à la microscopie de fluorescence où nous montrons sa compatibilité avec l’imagerie d’échantillons vivants. Ensuite, dans le contexte de l’imagerie SMLM nous démontrons l’obtention de résolutions uniformes, mais également l’impact sur d’autres paramètres et possibilités d’acquisitions. Ainsi ASTER permet de réduire le fond ambiant, d’imager des champs larges de 200x200µm² ou de réaliser une image SMLM sur l’ordre de quelques minutes.Nous présentons ensuite le bénéfice du couplage d’ASTER avec l’imagerie 3D, et une méthode d’imagerie multi-couleur SMLM. Cette méthode quantifiée analytiquement permet d’obtenir des cross talk de l’ordre de 2%, nous montrons sont application à l’imagerie à deux et trois couleurs, ainsi qu’à l’imagerie 3D. Différentes pistes d’amélioration d’ASTER et de l’imagerie multi couleur sont ensuite proposées.
... Laser-based projectors are more efficient than discharge lamp-based projectors, produce greater color gamut and brightness, have a longer lifetime, and are mercury-free; however, they are more expensive. Furthermore, the image can be affected by the speckle noise due to the coherent nature of the lasers (Akram and Chen, 2016), which is observed as a grainy pattern superposed on the projection (Pauwels and Verschaffelt, 2017). ...
We assessed how rod excitation (R) affects luminance (L + M + S) and chromatic [L/(L + M)] reaction times (RTs). A four-primary display based on the overlapped images of two spectrally modified monitors, which allowed specific or combined [L + M + S + R, L/(L + M) + R] photoreceptor stimulation, was used to present a C-target stimulus differing from the background only by the selected stimulation. For the luminance pathway, rod input increased RTs, suggesting a suppressive rod–cone interaction. The responses of the chromatic pathway were faster when rods were involved, suggesting a major role of rods in mesopic color perception.
... In the earlier stages, the methodology of the speckle measurement and evaluation had not been established. Many publications focused on speckle-reduction methods and devices [5][6][7][8] and researchers proposed their own methodologies for speckle measurement, which caused some difficulties in fair comparisons among different speckle-related studies on the laser projectors [9]. Therefore, the international standardization of the methodology was expected. ...
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We propose and demonstrate a color-speckle assessment method based on a threedimensional Jzazbz color space, which is appropriate for both three-primary and multi-primary systems. In the proposed scheme, new physical quantities are defined to describe the color-speckle characteristics, which provides a general and intuitive color-speckle evaluation for different laser projectors. Experimental verification is also performed using three-primary and six-primary laser projectors. The simulation and measurement results are consistent.
... Laser-based projectors are more efficient than discharge lamp-based projectors, produce greater color gamut and brightness, have a longer lifetime, and are mercury-free; however, they are more expensive. Furthermore, the image can be affected by the speckle noise due to the coherent nature of the lasers (Akram and Chen, 2016), which is observed as a grainy pattern superposed on the projection (Pauwels and Verschaffelt, 2017). ...
This chapter describes the most common setups that scientists use for generating light stimulation, from lab-made approaches to commercially available technologies. The studied optical stimulation systems are divided into nonimage-forming and image-forming arrangements. Two classical systems widely used are among the first: the Maxwellian view system and the Ganzfeld stimulator. Between the image-forming arrangements, the focus is on approaches that consider off-the-shelf devices and the recent appearance of multi-primary displays, which allow the inclusion of more primaries and the generation of stimulation for independent and combined photoreceptor and postreceptoral excitations. Some of the several limitations that can have important implications in research practice are also examined, such as those related to color gamut, sampling frequency, light range, and spatial resolution. Since experimentation on how optical radiation is processed by the human neural system requires the reliability of the parameters and variables under study to be assured, the characterization and consequent calibration of experimental devices are essential. Therefore the chapter discusses a set of characterization and calibration principles that researchers should consider when carrying out experiments with the described optical stimulators. Outstanding characteristics are stimulator response curve, primaries' spectral power distribution, additivity, modulation transfer function, and temporal stability. Finally, some possible sources of artifacts that researchers should consider when these stimulators are used are presented. Throughout this last section, data based on different optical stimulator measurements is provided.
... Very high-power burst-mode lasers [35,36] are an alternative source of narrowband (<1 nm) illumination. If coupled with a method for reducing coherent speckle [37,38], these devices can potentially be used for BOST imaging with high contrast. Increased illumination would enable the use of a smaller camera aperture, which would reduce the effects of blur and facilitate improved deflection sensing. ...
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The understanding and predictive modeling of explosive blasts require advanced experimental diagnostics that can provide information on local state variables with high spatiotemporal resolution. Current datasets are predominantly based on idealized spherically symmetric explosive charges and point-probe measurements, although practical charges typically involve multidimensional spatial structures and complex shock-flow interactions. This work introduces MHz-rate background-oriented schlieren tomography (BOST) to resolve transient, three-dimensional density fields, as found in an explosive blast, without symmetry assumptions. A numerical evaluation is used to quantify the sources of error and optimize the reconstruction parameters for shock fields. Average errors are ∼3% in the synthetic environment, where the accuracy is limited by the deflection sensing algorithm. The approach was experimentally demonstrated on two different commercial blast charges (Mach ~1.2 and ~1.7) with both spherical and multi-shock structures. Overpressure measurements were conducted using shock front tracking to provide a baseline for assessing the reconstructed densities. The experimental reconstructions of the primary blast fronts were within 9% of the expected peak values. The MHz time resolution and quantitative reconstruction without symmetry assumptions were accomplished using a single high-speed camera and light source, enabling the visualization of multi-shock structures with a relatively simple arrangement. Future developments in illumination, imaging, and analysis to improve the accuracy in extreme environments are discussed.
... 1 Following Huygens' principle, 2 each point of the surface acts as a spherical wave scatter emitting light with random phase distribution among them. The coherent superposition of these spherical wavefronts creates a random intensity pattern of constructive and destructive interference, dark and shiny spots, to produce the speckle pattern. 1 The speckle noise is, therefore, a random pattern of dark and shiny spots that appears on the images in any coherent (e.g., laser-based) imaging system, 1 including laser-based photography, 3 digital holography (DH), 4 and digital holographic microscopy (DHM). 5 While laser-based projectors provide a wide color gamut for vivid, super bright, and high contrast images, their major limitation is the speckle noise caused by the lasers' coherent nature. ...
... 1 Following Huygens' principle, 2 each point of the surface acts as a spherical wave scatter emitting light with random phase distribution among them. The coherent superposition of these spherical wavefronts creates a random intensity pattern of constructive and destructive interference, dark and shiny spots, to produce the speckle pattern. 1 The speckle noise is, therefore, a random pattern of dark and shiny spots that appears on the images in any coherent (e.g., laser-based) imaging system, 1 including laser-based photography, 3 digital holography (DH), 4 and digital holographic microscopy (DHM). 5 While laser-based projectors provide a wide color gamut for vivid, super bright, and high contrast images, their major limitation is the speckle noise caused by the lasers' coherent nature. ...
Full-text available
Images recorded by coherent imaging systems, including laser-based photography, digital holography (DH), and digital holographic microscopy (DHM), are severely distorted by speckle noise. This work presents a single-shot image processing method to reduce the speckle noise, coined hybrid median-mean filter (HM2F). The HM2F is based on the average of conventional median-filtered images with different kernel size. The synergic combination of the median filter and mean approach provides a denoised image with reduced speckle contrast while the spatial resolution is kept up to 97% from the original value. The HM2F method is compared with the conventional median filter approach (CMF), the 3D Block Matching (BM3D) filter, the non-local means (NLM) filter, the 2D windowed Fourier transform filter (WFT2F), and the Wiener filter using different speckle-distorted images to benchmark its performance. Based on the experimental results and the simplicity of the technique, HM2F is proposed as an effective denoising tool for reducing the speckle noise in laser-based photography, DH, and DHM.
Many correlations exist between spectral reflectance or transmission with various phenotypic responses from plants. Of interest to us are metabolic characteristics, namely, how the various polarimetric components of plants may correlate to underlying environmental, metabolic, and genotypic differences among different varieties within a given species, as conducted during large field experimental trials. In this paper, we overview a portable Mueller matrix imaging spectropolarimeter, optimized for field use, by combining a temporal and spatial modulation scheme. Key aspects of the design include minimizing the measurement time while maximizing the signal-to-noise ratio by mitigating systematic error. This was achieved while maintaining an imaging capability across multiple measurement wavelengths, spanning the blue to near-infrared spectral region (405–730 nm). To this end, we present our optimization procedure, simulations, and calibration methods. Validation results, which were taken in redundant and non-redundant measurement configurations, indicated that the polarimeter provides average absolute errors of (5.3±2.2)×10 ⁻³ and (7.1±3.1)×10 ⁻³ , respectively. Finally, we provide preliminary field data (depolarization, retardance, and diattenuation) to establish baselines of barren and non-barren Zea maize hybrids (G90 variety), as captured from various leaf and canopy positions during our summer 2022 field experiments. Results indicate that subtle variations in retardance and diattenuation versus leaf canopy position may be present before they are clearly visible in the spectral transmission.
Laser sources have been considered to be better light sources compared to traditional lamps or light‐emitting diodes (LEDs) used in projectors, which enables the projector to create images with higher color saturation, brightness, and energy efficiency. However, the speckle noise caused by the coherent nature of the laser is a major technology obstacle for laser projection, which severely affects the image quality. To suppress laser speckle, a TiO2‐Sapphire Composite (TSC) with rough and high reflectivity surface and high thermal conductivity substrate is designed, which reduces the laser speckle by creating spatial diversity. Moreover, combined with a micro motor, the design and fabrication of the TSC rotating wheel are realized, which further reduces the laser speckle by creating time diversity. The experimental results show that the laser speckle contrast can be reduced from 9.0% to 2.2% when placing the TSC rotating wheel in the light path, which is below the speckle perception limit of the human eye (<4%). This new type of laser speckle suppression device is simple to use, low in cost, high in energy utilization, high in thermal conductivity, easy to mass production, and has great application potential in high‐power laser projection. This article is protected by copyright. All rights reserved
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A new type of 4000-5000 lm-class white lamp system using high-power blue laser diodes and phosphors has been developed for high-luminance lighting applications. Using fibre-optics, the lamps can be arranged separately from a light engine containing the blue laser diodes. One of the new lamp systems achieved much higher luminance of 140 Mcd/m2 than the conventional white LEDs. Their lighting quality and safety categorization have been discussed based on speckle contrast as an index of coherence. Speckle contrast values less than 2% as low as the LEDs have been obtained, implying that these lamps are considered to be incoherent as well as the conventional white LEDs. Multiple scattering processes in the lamp structure including the phosphor layer are found to be indispensable for reducing speckle contrast completely, adding to multi-longitudinal mode operation of the blue laser diodes.
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— A novel principle and simple technique of suppressing the speckle noise in images displayed by a laser projection system is proposed. Wave coherence in a laser beam and speckles are destroyed in real time when the beam passes through a single FLC cell where spatially inhomogeneous phase light modulation takes place due to special FLC material and an electrical pulse regime.
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In quantitative measurement of the speckle observed in laser projection displays, it is essential how accurately one simulates the human eye. Based on the eye model given by Westheimer, we succeeded in simulating the optical transfer function of the eye model using that for the circular aperture. The equivalent circular aperture diameter is dependent on the eye model used in the simulation and its viewing conditions, but particularly for the eye models given by Westheimer and Williams, they substantially agreed, yielding approximately 1.2 mm at green under a 3 mm pupil diameter viewing condition.
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A broadband green light source was demonstrated using a tandem-poled lithium niobate (TPLN) crystal. The measured wavelength and temperature bandwidth were 6.5 nm and 100°C, respectively, spectral bandwidth was 36 times broader than the periodically poled case. Although the conversion efficiency was smaller than in the periodic case, the TPLN device had a good figure of merit owing to the extremely large bandwidth for wavelength and temperature. The developed broadband green light source exhibited speckle noise approximately one-seventh of that in the conventional approach for a laser projection display.
High brightness fiber-coupled RGB laser sources offer an expanded color gamut, lifetimes in excess of 20,000 hours, scalable power (just add more lasers), reduced cost of ownership (no lamps to replace), as well as smaller system size and efficiency. We present the basic design and performance data for a laser-based projection system using Novalux Necsel™ lasers.
Through theoretical analysis the reasons of generating laser speckle, we use an optical scheme for reducing composite speckle in the three LCOS laser projection system. The project of rotating diffuser has a simple optical structure and can be easily implemented. Finally, experimental verification of rotating diffuser plate can effectively suppress laser speckle contrast.
Reducing the coherence of laser light without energy loss is critical for speckle suppression in laser projectors. In this paper, it is shown that more than 90 % of incident light is forward-scattered with complete de-coherence. We modeled a scattering volume with particles in ZEMAX as a scattering medium and studied the forward-scattered light energy and de-coherence dependence on the following three parameters: particle concentration, normalized radius of particles \(\upalpha \) and refractive index ratio m. By varying all three parameters, a number of parameter combinations to achieve complete de-coherence are obtained, and which are utilized for speckle suppression. The forward-scattered light energy loss is up to 60–70 % of incident light at these combinations. But, at smaller particle concentration, the loss drops quickly. When setting the particle concentration at \(1\times 10^{8}\,\hbox {cm}^{-3}\) , the forward-scattered light energy can be more than 90 % of incident light by adjusting \(\upalpha \) and m according to Mie scattering theory. The structure parameter L of the scattering volume is also analyzed to collect forward-scattered light more than 90 % of incident light. These results can benefit the parameters’ choice of scattering particles and the design of a scattering volume used for speckle suppression.
Laser illumination for projection introduces undesirable speckle artifacts that must be measured accurately and suppressed. A useful and accurate standardized method to measure and quantify the effect has been elusive. This paper describes work that investigates the most significant measurement setup parameters affecting correlation between manufacturer measurements and observed speckle image quality.
Based on the theoretical analysis of the generation of laser speckles, a specific scheme is proposed to reduce composite speckles in the tricolor liquid‐crystal‐on‐silicon laser projector.The rotated diffuser is relatively simple in its optical structure which can be easily implemented. Furthermore, the experiment with regard to the detected images at the receiver is also performed to verify the feasibility of our method. It is shown by simulated results that the rotated diffuser plate can effectively suppress the contrast ratio of laser speckles. © 2012 Wiley Periodicals, Inc. Microwave Opt Technol Lett 55:138–141, 2013; View this article online at DOI 10.1002/mop.27221
We proposed a design for a LASER pico-projector with a low speckle contrast value and high contrast ratio that maintains system efficiency. The method for speckle contrast reduction utilizes two diffusers and a Voice Coil Motor (VCM) oscillator. The two different diffusers for a high contrast ratio and high system efficiency can be divided into two categories. In Category 1, the speckle contrast value can be decreased to 2.80% by using a circular symmetric diffuser. At the same time, the full-on/full-off (FO:FO) contrast ratio can be maintained within 1200:1-1300:1, but the system efficiency decreases 1.50%. In Category 2, the speckle contrast value can be reduced to 6.50% by using an elliptically scattering diffuser. At the same time, the FO:FO contrast ratio can be maintained within 1300:1-1400:1, and the system efficiency decreases by only 1.00%.