Differential Interference Contrast Microscopy (DIC) - Biomedical Optical Phase Microscopy and Nanoscopy - page 22

Thus, the shear angle could be found by measuring a derivative of the retardance with
respect to the coordinate along the shear direction:

λ
360

d d x

ε 5 (2.6)

Retardance can be determined by a number of techniques. In particular, we employed the
Senarmont compensator [26] . A schematic of the used setup is shown in Figure 2.3 . A laser
radiates the collimated narrow beam with horizontal linear polarization. Here, angle 0
corresponds to the horizontal direction (the X -axis). Then, the first quarter-wave plate
QWP1, with fast axis oriented at 45 , transforms the linear beam polarization into the
circular polarization. Polarizer, the second quarter-wave plate QWP2 and analyzer are
mounted into rotatable holders with angular scale each. The DIC prism under investigation
is placed on a linear stage with a Vernier micrometer. The stage allows movement of the
prism in the horizontal direction. The minimum intensity of the passed laser beam was
visually determined by observing the laser spot brightness on a screen.

The measuring procedure of retardance derivative consists of the following steps. Before
placing the DIC prism under investigation in the setup, the prism is positioned between the
crossed polarizers. The shear plane is perpendicular to the observed color fringes or/and the
black fringe. The shear plane orientation is noted.

Next, the second quarter-wave plate, QWP2, is removed from the setup, cross polarizer and
analyzer, and the DIC prism is positioned in the optical path. By rotating simultaneously

DIC prism ( ψ )

QWP1 (45º) Analyzer ( χ )

Laser (0º)

Polarizer ( ψ + 45º) QWP2 ( ψ +45º)

x

Screen

y Linear stage

z

Figure 2.3
Setup for measuring shear angle of DIC prism by employing the Senarmont compensator. Laser
with horizontally linearly polarized beam; QWP1: first quarter-wave plate at 45 azimuth;
polarizer at 45 azimuth; DIC prism under investigation with shear plane azimuth at angle ψ 1 ψ ;

linear stage for shifting DIC prism in the horizontal direction; QWP2: second quarter-wave plate
at 45 azimuth; rotatable analyzer at ψ 1 χ azimuth.