100 CHINESE OPTICS LETTERS / Vol. 6, No. 2 / February 10, 2008
An effective method for reducing speckle noise in
digital holography
Xin Kang (
xxx
###
)
Department of Applied Mechanics, Nanjing University of Science and Technology, Nanjing 210094
Received March 13, 2007
An effective method for reducing the speckle noise in digital holography is proposed in this paper. Different
from the metho ds based on classical filtering technique, it utilizes the multiple holograms which are
generated by rotating the illuminating light continu ously. The intensity images reconstructed by a series of
holograms generated by rotating the illuminating light possess different speckle patterns. Hence by properly
averaging the reconstructed intensity fields, the speckle noises can be reduced greatly. Experimental results
show t hat the proposed method is simple and effective to reduce speckle noise in digital holography.
OCIS codes: 090.0090, 030.0030, 070.0070, 120.0120.
Recent development in charge-coupled devices (CCDs)
and digital computers have made it possible to rec ord
a hologram using a CCD camera and r e c onstruct the
object wavefront numerically. This has led to the de-
velopment of digital holography. Compared with con-
ventional holography, digital holography has the advan-
tage that the amplitude as well as the phase information
of the object wavefront can be obtained quantitatively
and analyzed digitally without the need fo r wet chemical
processing. However, digital holography possesses draw-
backs which are similar to those of c onventional holog-
raphy such as the pr e sence o f speckle noise due to the
roughness of object surface. Moreover, speckle noise in
digital holography has more overwhelming effects than
that of conventional holography.
Great effort has been focused on removing the
sp e ckle noise in digital holography
[1−9]
and spe ckle
interferometry
[10−15]
. Garcia-Sucer quia et al.
[1]
pre-
sented an effective method to reduce the speckle noise
in reconstructed image. This method is based on resiz-
ing (minishing) the reconstructed image together with
median filtering. Morimoto et al.
[2]
used the divided
holograms to decrease the effect of speckle noise in de-
formation measurement by phase-shifting digital holog-
raphy. They divided the original full-size hologram into
16 small holograms. Each s mall hologram is pasted at
the or iginal position on the full-size hologram with null
data at the other pixels. By searching the max imum av-
erage amplitude among the 16 reconstr ucted wavefronts
befo re and after deformation at each pixel, the phase
value at this pixel corresp onding to the deformation is
determined. Recently, an effective method based on mul-
tiple holograms
[3,4]
is used to reduce the speckle noise in
reconstruction of digital holography. Baumbach et al.
[3]
generated multiple holograms by a set of differ e nt lateral
positions of a CCD camera which is used to record the
holograms. They utilized the shift theo rem of Fourier
transform to reduce the speckle noise which can be in-
terpreted as generation of a large synthetic aperture con-
sisting of many small apertures given by a single CCD.
Kebbel et al.
[4]
obtained multiple holograms by moving
continuously a diffusing screen perpendicular to the op-
tical axis of a CCD sensor. By properly averaging sev-
eral phase images with differ e nt speckle patterns, they
improved drastically the accuracy of the phase measure-
ment.
In this paper, a method based on multiple holograms
to reduce the s peckle noise in digital holography is pre-
sented. However, the multiple holograms are gener-
ated by rotating an illuminating beam through different
angles. This is different from the above-mentioned
methods
[3,4]
. A series of off-axis Fresnel holograms of
a same object are captured. By properly averaging the
reconstructed intensity images, the speckle noise can be
suppressed drastically.
It is known that the effect of spe ckle noise in lase r in-
terferometry can be measured using the contrast value of
a sp e ckle pattern. The higher the contrast value is, the
more significant the effect of speckle no ise will be. The
contrast of a speckle pattern is defined as
ν =
σ
hIi
, (1)
where hIi =
P
i
P
j
I(i, j)/(n × m) is the mean
value of intensity I of a speckle pattern, n ×
m is the size of the speckle pattern, and σ =
P
i
P
j
(I(i, j) − hIi)
2
/(n × m − 1)
!
1/2
is the standard
deviation of intensity of speckle pattern. For a standard
interferometric speckle pattern, its intensity presents a
distribution of negative exponent. Hence the contrast
value for a standard speckle pa ttern is always eq ual to 1.
Assuming that I
1
, I
2
, · · · , I
p
represent a series of
sp e ckle patterns (p is the number of speckle patterns)
and follow the same intensity distribution but are statis-
tically independent o f each other, we have
D
ˆ
I
E
= hI
i
i , (2)
where
D
ˆ
I
E
is the mean value of
ˆ
I and
ˆ
I =
1
p
p
P
i=1
I
i
, while
hI
i
i is the mean value of any speckle pa ttern I
i
. In addi-
tion, we have
1671-7694/2008/020100-04
c
2008 Chinese Optics Letters