Page 62 - Computational Retinal Image Analysis

52 CHAPTER 3 The physics, instruments and modalities of retinal imaging

4 Polarization and birefringence

The polarization state of light used to image the retina, and the way in which po-
larization can be modified by ophthalmic instruments and by the eye can have a
profound impact on the appearance of retinal images. The polarization of light re-
fers to the direction of the electric field vector of light. In linear polarization, the
electric field oscillates in only one plane: for example, the vertical or horizontal
plane, while circular polarization can be produced by combining two orthogonally
polarized light beams with a 90° phase difference (or quarter wavelength path differ-
ence). Elliptically polarized light is produced by combining linearly polarized light
with a phase difference other than 90°. Natural light such as is produced by broad-
band sources, such as the LEDs, tungsten lamp sources and flash lamps suited in
direct and indirect ophthalmoscopes and fundus cameras, produce unpolarized light.
Unpolarized light can be polarized, however, by transmission through a polarizer—
most commonly a linear or circular polarizer. Similarly light that is linearly polarized
will be absorbed by a film polarizer aligned with the transmission axis orthogonal
to the polarization direction. Importantly a linear polarizer aligned with narrowband
sources, such as lasers or super-luminescent LEDs, such as are used in scanning laser
ophthalmoscopes or OCT systems respectively normally generate polarized light.
When light is reflected, such as from a glass surface, or from the cornea, or the
eye lens, the reflected light has the same polarization state as the incident light. If
then, the light that is transmitted through the pupil to illuminate the retina is linearly
polarized then the reflection from the cornea will have the same polarization state.
A linear polarizer, oriented to transmit only the orthogonal polarization, placed be-
tween the detector and the light reflected from the cornea, will then not transmit the
linearly polarized corneal light. As explained below, the linearly polarized illumina-
tion transmitted to the retina will tend to be depolarized by multiple scattering in the
retina and will be transmitted with an efficiency of 50% (for an ideal polarizer). In
consequence the use of this so-called crossed-polarizer technique can yield an image
with a strongly suppressed corneal reflection, albeit with a four-fold reduction in in-
tensity. Additional reflections deeper into the eye also exist, for example the Purkinje
images from the back of the cornea and from the two lens surfaces (see Section 2.3),
although these are much weaker than the front corneal reflection. Retinal images
are also commonly characterized by strong specular reflections, from, for example,
blood vessels and from the retinal nerve fiber layers and these can also be suppressed
by use of cross-polarized imaging. The suppression of the Purkinje reflections and
the retinal specular reflections is not very efficient: indeed the spatial characteristics
of the retinal specular reflections can vary significantly with orientation of the imag-
ing linear polarizer. As discussed below, this is due to the birefringence of the ocular
media.
In a uniaxial birefringent material, the refractive index experienced by light
depends on the orientation of the polarization of light with respect to a so-called
optic axis of the material. In general, linearly polarized light incident upon a bire-
fringent material will be converted to elliptically polarized light with a polarization