Multimodal Biomedical Imaging Systems - Biomedical Optical Imaging Technologies: Design and Applications - page 327

9.3.3.2 Polarization Effects in Optical Systems

When light propagates through an optical system, the transmission, reflection, and
optical path length are polarization dependent because all optical components in
the system change the polarization state to some extent due to refraction, reflection,
coating, and birefringence.
At nonzero angles of incidence, the s and p components of the light have
different transmission and reflection characteristics for refractive and reflective
surfaces. Generally, for a nonflat lens surface and noncollimated light, the angle of
incidence varies across the lens surface; therefore, the lens surface acts as a weak,
spatially varying partial polarizer. Large variations in the angle of incidence could
be an indication that polarization effects should be considered [ 34 ].
Most coatings in optical systems are designed to improve light throughput,
either to reduce reflection on refractive surfaces or to enhance the reflection on
reflective surfaces, without considering phase retardance. Generally, all coatings
have polarization effects to some extent. A coating on the lens surface may result in
either more or less polarization effect compared to an uncoated surface. Maximizing
light throughput and minimizing phase shift may work against each other. To
optimize light throughput with minimum phase shift requires some trade-offs.
Another consideration is birefringence which is the difference in refractive
indices of an optical material between two orthogonal polarization states. When
unpolarized light is incident onto a birefringent material, it is split into two paths
with orthogonal polarization states, and the phase of each path accumulates at a
different rate. This results in different optical path lengths and different focal lengths
for light with different polarization states, thus degrading imaging performance.
Most optical glass elements are isotropic and have no natural birefringence.
However, because optical elements are usually mounted using mechanical parts,
the interface between the optical element and the mechanical element can produce
stress. Stress can introduce anisotropic material birefringence, resulting in a change
in the optical path. Birefringence is an inherent property of plastic optical materials
and can be categorized into orientational birefringence and stress birefringence.
In addition to birefringence generated from mechanical loads during standard
operation, birefringence can also be generated in optical components by a change in
temperature. Due to the mismatch of coefficients in thermal expansion of cemented
elements and thermal mismatch between the optical element and the mounting
material, uniform temperature changes can produce mechanical stress in optical
components.

9.3.3.3 Design Considerations of Polarization Imaging Systems

Optical designs consist of lens and coating designs. Typically, lens designs optimize
wavefront performance and image quality without considering coating effects; coat-
ing designs only maximize transmittance or reflection within the working spectrum