COL 9(suppl.), S10702(2011) CHINESE OPTICS LETTERS June 30, 2011
Research on the Moir´e fringes formed by circular and
linear grating
Xiaoyu Chen (陈陈陈晓晓晓钰钰钰), Jinbo Su (苏苏苏锦锦锦博博博), Xiangqun Cao (曹曹曹向向向群群群),
Bin Lin (林林林 斌斌斌), and Bo Yuan (袁袁袁 波波波)
∗
State Key Laboratory of Modern Optical Instrumentation, CNERC for Optical Instrumentation, Zhejiang University,
Hangzhou 310027, China
∗
Corresp onding author: yuanb o@zju.edu.cn
Received December 30, 2010; accepted January 25, 2011; posted online June 29, 2011
The features of Moir´e fringe generated by overlapping a circular and a linear grating are studied. Given
that the pitch of circular grating is a and that of linear grating is P , the shapes of the Moir´e fringe that
they form are hyp erbola, parabola, and ellipse when a > P , a = P , and a < P , respectively. As the pitches
of these two gratings become close to each other, the magnification of the Moir´e fringe is over 100, which
is useful for the measurement of small displacement. This letter also discusses how the fill factor influences
Moir´e fringe visibility.
OCIS code: 050.2770.
doi: 10.3788/COL201109.S10702.
Given that the Moir´e fringe has high rate of am-
plification, metrology gratings that can form a Moir´e
fringe have been widely applied in many fields, such
as precision instruments, sup er finishing, CNC machine,
and so on
[1]
. In particular, the system composed of two
parallel gratings is often used for displacement measure-
ment. Moir´e fringe technology has been widely applied to
machining, laboratory, and photoelectric instruments
[2]
.
In recent years, grating interferometers used in the field
of urban construction have become low-cost, miniatur-
ized, and portable, making them easy to use for out-
door measurements. Constructing a sensor monitoring
network on the building surface can effectively help con-
struction engineers learn about the change of the stress
inside the building and displacement in the plane by an-
alyzing Moir´e interference patterns
[3]
. In addition, when
fringe image as shadows of the grid is analyzed by wavelet
transform, micro-sized products can be measured using
this three-dimensional (3D) measurement technique
[4]
.
Nowadays, the most widely applied kind of metrologi-
cal grating is formed by two circular or two linear grat-
ings. The shape of this kind of Moir´e fringe is simple
and fixed, and is typically used in measuring tiny rela-
tive displacement. In this letter, a new grating composite
system is proposed (Fig. 1 (a)). This system consists of
a circular grating and a linear grating. The shapes of the
Moir´e fringe are hyperbola, parabola, or ellipse depend-
ing on the different pitches of two gratings. This kind
of Moir´e fringe contains the amplification effect and has
much more shape facility than the previous ones. Thus,
it can be used in the new field.
In the past, using metrological gratings to acquire
and analyze data depends mainly on photoelectric de-
tectors, integrated circuit, and MCU. However, b ecause
the Moir´e fringe consisting of circular and linear gratings
is more manifold and complicated, it is very difficult to
analyze it in the traditional way. Fortunately, with the
development of charge coupled device (CCD) and digital
imaging processing technology, this new grating system
can be better studied.
By exploring the relationship between grating parame-
ters containing pitch and fill factor, as well as the shape,
visibility and size of the Moir´e fringe, the choice of grating
parameters can be analyzed and summarized to achieve
an ideal observation result. The ordinal equation method
is used to study the general performance of the grating.
At the same time, along with the change of fill factor
and pitch of grating, the Moir´e fringe possesses different
properties that provide significant theoretical foundation
for the application of this kind of Moir´e fringe.
We establish a rectangular coordinate system whose
origin is the center of the circular grating (Fig. 1 (b)).
Taking the initial phases of the two gratings as zero, we
will have grating equations of circular grating and linear
grating given by
x
2
+ y
2
= (ma)
2
, (1)
x = nP, (2)
where m and n are grating ordinals, a is the pitch of cir-
cular grating, and P is the pitch of linear grating. Using
the relationship m − n = ±N, where N is Moir´e fringe
ordinal, the Moir´e cluster equation can be obtained as
follows
[5]
:
(P
2
− a
2
)x
2
± 2aP N x + P
2
y
2
= a
2
P
2
N
2
. (3)
From Eq. (3), the hyperbola, parabola, and ellipse are
represented by a > P , a = P , and a < P , respectively,
which are shown as
"
p
(a
2
− P
2
)x ∓
aP N
p
(a
2
− P
2
)
#
2
− P
2
y
2
=
a
2
P
2
N
2
(a
2
− P
2
)
− a
2
P
2
N
2
, (4)
y
2
= ±2P N x + P
2
N
2
, (5)
1671-7694/2011/S10702(4) S10702-1
c
° 2011 Chinese Optics Letters