Learning about retinal illuminance
and reduced color contrast and saturation helped me understand the added layer of difficulty the elderly face in navigating space.
FDT is quick, (33-42) relatively inexpensive and fairly robust to optical blur, (43) though it has also been found to be less robust to reduced retinal illuminance
, (44) a limitation when examining those with pupil miosis or cataract.
Dense scotomas were mapped with a 12 arcminute square target with a 50,000 Td retinal illuminance
. Relative scotomas were also mapped with the use of targets at the retinal illuminance
level that was determined to represent the threshold sensitivity of the fovea or eccentric PRL.
The relationship between retinal illuminance
and contrast sensitivity to sine-wave gratings is summarily described as obeying DeVries-Rose and Weber's laws (Graham, 1989, Section 13.9.1; Shapley & Enroth-Cugell, 1984): At low illuminance, sensitivity increases with the square root of illuminance (over the so-called DeVries-Rose range) and then becomes constant and independent of illuminance (over the so-called Weber range).
A significant portion of this loss can be attributed to a decrease in retinal illuminance
caused by the age-related reduction in pupil size and the loss of lens transparency (Owsley, Sekuler, & Siemsen, 1983; Sloane, Owsley, & Alvarez, 1988).
The retinal illuminance
of a typical 60-year-old person is only one third that of a 20-year-old." Further, on pages 10-16, "Older persons tend to require higher task illuminances for the same retinal illuminances
and greater attention to sources of glare within the field of view is more important for older persons."
FFFs were found to increase approximately linearly with the logarithm of the time-averaged retinal illuminance
. This is in accordance with the Ferry-Porter law derived for 100% modulation of spatially uniform fields (Le Grand, 1968, pp.