Chemistry Reference
In-Depth Information
5.2.3 D
ICHROISM
-B
ASED
P
HOTOMETRY
An orderly arrangement of carotenoid molecules in the Henle i bers is responsible for an entoptic
phenomenon similar to Maxwell's spot, and provides the basis for a method of measuring MP opti-
cal density (Bone 1980, Bone and Landrum 1984, Bone et al. 1992). The entoptic phenomenon,
Haidinger's brushes (Von Haidinger 1844), appears at the i xation point if a surface, uniformly
illuminated with light in the 400-500 nm range, is viewed through a polarizing i lter. The brushes
appear as a dark, hourglass-shaped object against the background, the main axis of which is per-
pendicular to the plane of polarization of the light entering the eye. The dimensions of Haidinger's
brushes match those of Maxwell's spot. In order to account for the brushes, it has been proposed that
a fraction of the long-chain lutein and zeaxanthin molecules are aligned transversely with respect
to the cylindrical membranes of the Henle i bers (Bone and Landrum 1984, Bone et al. 1992). Since
the i bers are themselves arranged radially from the foveal center like the spokes on a wheel, the MP
molecules would be oriented perpendicular to the “spokes.” A second requirement is the dichroism
of the chain-like lutein and zeaxanthin molecules, meaning that they absorb light preferentially that
is polarized parallel to the chain.
Exploiting these properties, Bone et al. devised a method for quantifying the MP (Bone and
Landrum 1984, Bone et al. 1992). A monochromatic stimulus (460 nm) was presented to the subject,
as shown in Figure 5.3. The two triangular areas were polarized either parallel or perpendicular to
the main axis of the i gure whereas the surrounding i eld was unpolarized. Using central i xation,
the subject adjusted the intensity of the triangles to match that of the surround for each of the polar-
ization conditions. Because the colors were identical, the task was relatively easy. When the plane of
polarization was perpendicular to the main axis of the i gure, the triangles appeared relatively dark
because they coincided with the dark sectors of Haidinger's brushes. When the plane of polarization
was rotated by 90°, the triangles appeared lighter because they coincided with the light sectors of
the brush pattern.
The quantity that was reported was the log ratio of intensity settings (log
G
) for the two polar-
ization conditions. It was shown theoretically that this quantity was proportional to the MP optical
density. The constant of proportionality included the fraction of preferentially aligned MP mol-
ecules. Additional measurements of the subjects' MP optical density by HFP indicated that this
fraction was essentially the same for all subjects. If this is generally true, the method could be added
to the list of other psychophysical methods for measuring MP optical density. The validation of
the method was carried out by repeating the measurements at multiple wavelengths. The resulting
spectrum of log
G
was virtually identical in shape to the optical density spectrum obtained by HFP.
There is, however, an additional determination that must be made. The cornea is birefringent (Bone
and Draper 2007) and, as such, transforms incident plane-polarized light into elliptically polarized
light except when the former is polarized parallel to either of the principal axes of the cornea. It is
(a)
(b)
FIGURE 5.3 (See color insert following page 336.)
Appearance of the visual i eld in dichroism-based
photometry. The background i eld is unpolarized and is of wavelength 460 nm. The triangles are also of wave-
length 460 nm, but are polarized. In (a), the plane of polarization is horizontal causing the triangles to appear
darker; in (b) the plane of polarization is vertical causing them to appear lighter. In each case, the subject
adjusts the luminance of the triangles until they match the luminance of the unpolarized background.
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