Tomographic Phase Microscopy (TPM) - Biomedical Optical Phase Microscopy and Nanoscopy - page 228

GM

L1

BS1

C

He-Ne laser

Sample

AOM1
AOM2

OL

Tube

BS2

CMOS camera
: Photron 1024PCl

Figure 12.1
Tomographic phase microscope. GM, galvanometer scanning mirror; L1, lens (f 5 250 mm);

C, condenser lens (NA 1.4); OL, objective lens (NA 1.4); Tube, tube lens (f 5 200 mm); BS1 and

BS2, beamsplitters; and AOMs, acousto-optic modulators. The frequency-shifted reference laser
beam is shown in blue [13] .

(wavelength λ 5 633 nm) was divided into sample and reference beams. The propagation

direction of the sample beam is controlled by a galvanometer mirror, and the sample image
of the transmitted beam is delivered to the camera by objective and tube lenses. Two
acousto-optic modulators are used to shift the frequency of the reference beam by 1.25 kHz,
and the frame rate of a CMOS (complementary metal-oxide semiconductor) camera
(Photron 1024PCI) is adjusted to take images at 200 s intervals. For each angle of

illumination, four successive interferogram images are recorded in 800

μ

s, and phase-
shifting interferometry is used to produce a pair of quantitative phase and amplitude
images. To maximize the range of illumination angles, a high NA condenser (Nikon, 1.4
NA) and an objective lens (Olympus UPLSAPO, 1.4 NA) are used. The sample beam is
rotated using a galvanometer mirror to cover from 2 70 to 70 in 0.23 steps. It takes
about 10 s to record a set of angular complex E-field images.

μ

12.3.2 Data Processing by Inverse Radon Transform

To reconstruct a 3D refractive index tomogram from the projection phase images, a
procedure based on the filtered back-projection method [21] is applied. A discrete inverse