High-speed 3D imaging based on structured illumination
and electrically tunable lens
Dongping Wang (王东平)
1,2
, Yunlong Meng (孟云龙)
1,2
, Dihan Chen (陈頔瀚)
1,2
,
Yeung Yam (任 揚)
1,
**, and Shih-Chi Chen (陳世祈)
1,2,
*
1
Department of Mechanical and Automation Engineering, The Chinese University of Hong Kong, Shatin,
Hong Kong SAR, China
2
Shenzhen Research Institute, The Chinese University of Hong Kong, Shenzhen 518057, China
*Corresponding author: scchen@mae.cuhk.edu.hk; **corresponding author: yyam@mae.cuhk.edu.hk
Received April 18, 2017; accepted June 16, 2017; posted online July 19, 2017
In this Letter, we present a high-speed volumetric imaging system based on structured illumination and an
electrically tunable lens (ETL), where the ETL performs fast axial scanning at hundreds of Hz. In the system,
a digital micro-mirror device (DMD) is utilized to rapidly generate structured images at the focal plane in syn-
chronization with the axial scanning unit. The scanning characteristics of the ETL are investigated theoretically
and experimentally. Imaging experiments on pollen samples are performed to verify the optical cross-sectioning
and fast axial scanning capabilities. The results show that our system can perform fast axial scanning and three-
dimensional (3D) imaging when paired with a high-speed camera, presenting an economic solution for advanced
biological imaging applications.
OCIS codes: 180.6900, 180.2520, 220.1080.
doi: 10.3788/COL201715.090004.
In microscopy, optical cross-sectioning means the extrac-
tion of in-focus signals in the form of thin slides by the
rejection of the out-of-focus signals or noises within a
thick specimen
[1]
. Conventionally, this can be achieved
by multi-photon microscopy
[2,3]
, confocal microscopy
[4]
,
or structured illumination microscopy (SIM)
[5,6]
. However,
multi-photon microscopy suffers from the high cost of
laser sources; confocal microscopy has the issue of high
phototoxicity. As point-scanning systems are typically
slow compared with wide-field systems, SIM becomes a
suitable and economic solution for a variety of high-speed
biological imaging applications.
In SIM, three modulated images are obtained sequen-
tially with a phase interval of 2π∕3 to reconstruct an optical
cross-sectional image. Optical cross-sections are realized by
the fact that high-frequency patterns attenuate rapidly
when away from the focal region, and only the in-focus
signals are modulated by the structured pattern. The im-
aging speed of SIM is limited by the speed of switching of
structured patterns, which is conventionally achieved by
mechanically scanning an optical grating
[7,8]
.Notably,this
also results in motion/image artifacts and low optical sec-
tioning efficiency
[9]
. More accurate phase shifts can be
achieved by using a liquid-crystal-based spatial light modu-
lator (LC-SLM) that has a speed of hundreds of Hz
[10,11]
.
However, the LC-SLM is polarization-dependent and only
modulates phases, which confines its application to a single
excitation wavelength
[12]
. Compared with LC-SLMs, a dig-
ital micro-mirror device (DMD) is a cost-effective solution
with substantially improved speed (4.2–32.5 kHz) and a
broader spectral range
[13–15]
.
Although the DMD-based SIM can rapidly generate
two-dimensional (2D) optical cross-sections, a fast axial
scanning method is still needed in order to realize
high-speed volumetric imaging. Conventionally, volume
imaging is achieved by axially scanning the specimen or
objective lens to acquire images at different depths. How-
ever, the mechanical scanning process is slow and may
introduce unwanted vibration and motion artifacts. These
issues may be addressed by the application of an electri-
cally tunable lens (ETL). An ETL is a transparent device
whose optical power can be rapidly tuned by adjusting the
drive current. Fast axial scanning has been successfully
demonstrated in other optical systems by using ETLs with
a speed of ∼1 kHz
[16–19]
.
In this Letter, we present a real-time three-dimensional
(3D) fluorescent microscope system based on structured
illumination and an ETL. In the system, a DMD is used
to rapidly generate structured images, achieving high-
speed 2D imaging. The ETL is used to perform fast axial
scanning, realizing high-speed volumetric imaging. The
speed of 3D imaging is only limited by the speed of cam-
eras, i.e., tens of kHz. Figure
1 presents the optical con-
figuration of the system. The light source is a light
emitting diode (LED) (wavelength ¼ 455 nm; M455L3,
Thorlabs), which provides incoherent illumination free
of speckles. Next, the light is collimated by a lens, L1
(f ¼ 30 mm), and projected to the DMD (DLP4500,
Texas Instruments). The DMD-modulated light is guided
by a high-reflectivity mirror, M, and a collimating lens, L2
(f ¼ 100 mm), to enter and fully fill the back aperture of
the objective lens (Apo 40 × 1.15 NA, Nikon). Note that
the system is aligned to satisfy Köhler illumination that
has a uniform illumination at the front focal plane of
the objective lens. The collimating lens, L2, and the
objective lens together form a 4-f system that ensures
COL 15(9), 090004(2017) CHINESE OPTICS LETTERS September 10, 2017
1671-7694/2017/090004(4) 090004-1 © 2017 Chinese Optics Letters