The ability to see at night requires visual skills that are not
tested adequately with standard visual acuity tests. Many people
who have 20/20 acuity on an E-chart test do not test nearly as
well in dim light. More importantly, people who have excellent
Snellen acuity scores may still experience difficulty in
perceiving contrast, have poor peripheral vision as the light
fades, or have trouble dealing with night-time glare. Changes
With Light Levels This chart illustrates contrast sensitivity
functions for two different levels of mean luminance. The upper
curves were measured with gratings having a mean luminance of
500 cd/m2. The lower curves were measured with gratings having a
mean luminance of 0.05 cd/ m2 squared. (In references, see:
Campbell FW, et al. Application of Fourier analysis to the
visibility According to National Safety Council statistics,
there are 5.4 deaths per million vehicle miles driven at night,
whereas there are 1.6 deaths per million vehicle miles driven
during daytime. The National T