Quantitative Methods for the Determination of Carotenoids in the Retina - Carotenoids: Physical, Chemical, and Biological Functions and Properties - page 77

0.8

0.6

0.4

0.2

0.0

400 420 440 460 480 500 520 540 560

Wavelength (nm)

FIGURE 5.1 Absorbance spectrum (relative) of human MP.

LEDs tends to be relatively large and corrections must be applied to the data in order to be able to
report the MP optical density at the test wavelength.
The instrument's visual i eld typically subtends an angle of about a degree at the subject's eye
thus ensuring that the corresponding stimulus on the retina falls within the area of the yellow spot.
While i xating on the center of the visual i eld, the subject adjusts the intensity of the blue light
until the sensation of l icker is either eliminated or minimized (see Figure 5.2). In what has been
termed “customized HFP (cHFP),” the critical frequency is customized for the individual subject
(Stringham et al. 2008). Older subjects may be less sensitive to l icker and require a lower frequency
compared to young subjects. The intensity setting for the blue light that eliminates l icker will, of
course, depend on the optical density of the subject's MP. Subjects with a high MP optical density
will require a higher intensity to compensate for attenuation by the MP compared with subjects
having a low optical density. However, other factors will affect the intensity setting, such as lens yel-
lowing that increases with age (Weale 1963) and, like MP, will attenuate the blue, but not the green

Central
fixation

MP

Peripheral
fixation

MP

FIGURE 5.2 (See color insert following page 336.) Illustration of the method of HFP. On viewing the
stimulus directly (upper), MP attenuates the blue component of the stimulus whereas with peripheral viewing
(lower), no such attenuation occurs. In each case, the subject adjusts the luminance of the blue component until
it matches the luminance of the green component, which is unaffected by MP.