in partial fulfil]ment of the - MSpace - University of Manitoba

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centrate on the test light as
throughout the test.
it is dimmed and increased continuously
8.2.2 Rapid Dark Adaptation Test (RDÀT)
The Rapid Dark Àdaptation Test described by Thornton (1977 ) is based
on separating white, blue and red plastic discs from a randomly mixed
pile under scotopic conditions, a dark condition where color vision
(cone function) is non-functional. The time taken to complete the test
h'ith 100% accuracy determines the individual's dark adaptation ability
and hence, Vitamin À status, since dark adaptation ability is influenced
by Vitamin À status.
The physiological basis for the Rapid Dark Adaptation Test is the
measurement of the time of occurrence of the purkinje-shift. The
purkinje-shift is a phenomenon whereby the peak wave Ìength sensitivity
of the retina shifts from the red towards the blue end of the visual
spectrum during the transition from day vision (photopic or cone mediated)
to night vision (scotopic or rod-mediated). This shift causes the
intensity of blue color to appear brighter Lhan that of red under dim
scoptic lighting condition where color vision is non-functional.
Therefore, a brue object is seen as a brighter shade of gray than a red
object.
The Rapid Dark Àdaptation Test described by Thornton (1977) is sim-
PIe, inexpensive and easy to used wiLh adults under field conditions,
This test has also been shown to correlate with other reliable and valid
measurÊs of dark adaptation and has been validaÈed as a screening method
for Vitamin A deficiency in adults (vinton and Russell, 1981). The