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 Traffic Highway Safety Commission
discovered in 1990 that the nighttime auto accident rate was 3.7 times
higher than the daytime rate. Some contributing factors are:
- Difficulty seeing
contrast in poor light.
- Decreased peripheral
vision when contrast sensitivity is diminished.
- Rapid recognition
ability deteriorates under poor contrast.
- Visual attentions
falls off at night.
- Many younger
myopic drivers (20-40 years old) experience decreased nighttime vision.
poor night vision as a major problem. Herve M. Byron, MD. Ocular Surgery
News, March 1, 1993.)
License Testing & Renewal A 1990 study of Pennsylvania driver
renewals found that the use of contrast sensitivity testing during vision
screenings revealed instances of potentially blinding diseases in older
drivers. The study also found that using contrast sensitivity testing
in addition to standard visual acuity and visual field criteria testing
resulted in a decrease in driving accidents and violations.
The study was conducted
on a random sample of 12,710 license renewals. 73 percent of the drivers
who failed the visual screening went for an eye exam and completed a
follow-up questionnaire. Of those in the follow-up group, 54 percent
indicated that they were unaware of their vision problems before the
screening. Some of the common problems found during the contrast sensitivity
screening included out-of-date corrective lenses, cataracts, glaucoma
and macular degeneration. The study concluded that standard visions
tests should be supplemented with contrast sensitivity testing.