What is Contrast Sensitivity?
Reprint
from http://www.lea-test.fi/en/vistests/instruct/contrast/csensiti/csensiti.html
Contrast
sensitivity measures the ability to see details at low contrast levels. Visual
information at low contrast levels is particularly important:
1. in communication, since
the faint shadows on our faces carry the visual information related to facial
expressions.
2. in orientation and
mobility, where we need to see such critical low-contrast forms as the curb,
faint shadows, and stairs when walking down. In traffic, the demanding
situations are at low contrast levels, for example, seeing in dusk, rain, fog,
snow fall, and at night.
3. in every day
tasks, where there are numerous visual tasks at low contrast, like cutting an
onion on a light colored surface, pouring coffee into a dark mug, checking the
quality of ironing, etc.
4.
in near vision tasks like reading and writing, if the information is at
low contrast as in poor quality copies or in a fancy, barely readable
invitation, etc.
Contrast
sensitivity is the reciprocal of the contrast at threshold, i.e., one divided
by the lowest contrast at which forms or lines can be recognized.
If a
person can see details at very low contrast, his or her contrast sensitivity is
high and vice versa. Depending on the structure of the stimulus used in the
measurement - either gratings of different size or symbols - contrast
sensitivity of a person gets different values.
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What is Contrast? |
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Contrast is
created by the difference in luminance, the amount of reflected light,
reflected from two adjacent surfaces. It can be defined in slightly different
ways. In clinical work, we usually use the Michelson formula:
There
is also the Weber definition of contrast:
Lmax
= Luminance on the lighter surface When
the darker surface is black and reflects no light, the ratio is 1. Contrast
is usually expressed as percent, then the ratio is multiplied by 100. The
maximum contrast is thus 100% contrast. The symbols of the visual acuity
charts are close to the maximum contrast. If the lowest contrast perceived is
5%, contrast sensitivity is 100/5=20. If the lowest contrast perceived by a
person is 0.6%, contrast sensitivity is 100/0.6=170. There
is no international recommendation on how contrast of visual acuity charts should
be defined. Therefore there are differences in the contrast of tests of
different manufacturers. |
Which
Luminance Level Should Be Used?
An
international recommendation does not exist on the luminance level for contrast
sensitivity testing, but there is a recommendation for visual acuity testing.
It recommends a luminance level equal or higher than 85 candelas per square
meter.
In the
United States and in a number of other countries, measurement of visual acuity
for research purposes is done by using the back illuminated ETDRS light box
with the luminance level adjustable from 220 to less than 1 cd/m² by using
layers of filters. In the small light box the maximum luminance level is 125
cd/m².
Measurement of Contrast Sensitivity
Measurement
of contrast sensitivity resembles audiometry: a pure tone audiogram depicts
which are the weakest pure tones at different frequencies that the person can
hear. Contrast Sensitivity Curve or visuogram shows the faintest
contrasts perceived by the person. If the stimulus is a sine wave grating,
then the curve depicts similar function as does the pure tone audiogram. If the
stimuli are optotypes (letters, numbers or pediatric symbols),
recognition is required and the test resembles speech audiometry. As in
audiometry, the result of the contrast sensitivity measurement is not one
single value but a diagram.
A
B
Contrast sensitivity curve. A. Visual acuity is plotted along the
horizontal axis and contrast sensitivity along the vertical axis (Figure A).
The size of the symbols decreases along the horizontal axis and they become
paler and paler in the vertical direction (Figure B). The boundary
between symbols perceived and those that are too small or too pale and thus not
seen, is depicted by a curve, called Contrast Sensitivity Curve. Its declining
right-hand slope is the most interesting part of the curve in clinical cases.
To define the slope of the contrast sensitivity curve, we need two or three
measurements. The first one defines the point at the x-axis, the visual acuity
value determined in the usual way. The second is the definition of the upper
end of the straight part of the slope usually located in the 1-5% contrast
area. An additional measurement at lower contrast is often of interest.
The
threshold values can be measured with two different techniques when using
optotype tests:
- By using low contrast
visual acuity charts, or
- By using tests with one
symbol size and several contrast levels.
Test Procedure
When Using Low Contrast Visual Acuity Charts
Testing
is identical to the measurement of visual acuity at high contrast level, i.e.,
we measure the smallest size of the optotypes that the person can recognize.
The threshold is defined as the line on which at least 3 out of the 5 optotypes
are correctly recognized. The 2.5% test is the most practical test in clinical
use. The resulting threshold point on the curve is far enough from the high
contrast value so that the declination of the slope of the curve can be
defined. In severe low vision, the test must be quite close, which may require
use of reading lenses.
Move
quickly down the chart and ask the person to identify the first or the last
symbol on each line. When the person hesitates or makes an error, recede one
line and ask the person to read the entire line. To record the result
carefully, record the number of optotypes read correctly, i.e., if on the 2.5%
chart one of the symbols was read incorrectly on line 20/63 (6/18, 0.3) record
the visual acuity value as 20/63 (-1) at 2.5%.
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#253800 |
#271600 |
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Translucent Low contrast visual acuity charts for the small light box.
The optotypes are printed at 2.5% (the old 25%, 10%, 5%, and 1.25% contrast
charts are still available). Low contrast chart is available also for the ETDRS
cabinet.
CONTRAST
SENSITIVITY MEASURED BY USING LOW CONTRAST VISUAL ACUITY CHARTS
Test results are marked on the recording sheet at the level used, e.g.
in this picture 2.5% and 1.2%, going along that level toward the right until
the visual acuity value, measured at that contrast (A' at 1.2%, B' at 2.5%), is
reached. If the person's visual acuity was 20/20 (6/6, 1.0), the line
connecting these three points, A', B' and X, depicts the slope of the contrast
sensitivity curve of this person. Record the results as 20/50 (6/15, 0.4) at
2.5% and 20/100 (6/30, 0.2) at 1.2%.
The Range of Normal Contrast
Sensitivity
Among
the normally sighted people, both visual acuity and contrast sensitivity have a
wide range of variation. In visual acuity, 20/25 (6/9, 0.8) is a low normal
value; the highest normal values are three times higher, 20/8 (6/2.5, 2.5).
Similarly, the range of normal variation in contrast sensitivity values is
great. Therefore, a value within the range of normal may or may not mean that
that particular person has normal contrast sensitivity. If his or her contrast
sensitivity was previously high, it may decrease to less than one-half or
one-third of its original value and still be "normal."
The range of normal variation in both visual acuity values (horizontal
arrow) and in contrast sensitivity (vertical arrow) is great. If a person's
contrast sensitivity was previously A and has then decreased to A', the value
is still within the range of normal values but is highly pathologic to this
person.
A
change in contrast sensitivity is the diagnostically important feature that
will be watched in the future. Because of the large variation in the normal
values, we need to have an older value to compare with to notice a change.
Ideally,
contrast sensitivity and visual acuity should be measured when children leave
their high school/ secondary school or in young adulthood. These values should
be recorded and saved as part of the basic information related to each person's
health. A change warrants an examination to find out the cause of the change.
Although the most common cause would be a small change in the refractive power
of the eye, which is a benign finding, repeating the measurement of contrast
sensitivity would be beneficial as a part of routine health examinations to
rule out changes in the visual pathways.
Measurement
of contrast sensitivity would also help us to better understand the complaints
of a person whose visual acuity at high contrast has not changed but whose
vision has decreased at low contrast levels. Then we would not annoy him/her by
saying that his/her vision is as good as before, a situation which is now
experienced by all too many patients/clients.
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If
occupational tasks require good visual function at low contrast levels, visual
acuity alone does not select the most suitable persons for that particular
task. For example, if the task is to notice airplanes approaching within low
clouds, these planes are best seen by a person with good visual acuity in the
contrast range of 1-5%. Since the declination of the slope varies even in
normal individuals, it is possible that a person with lower visual acuity at
high contrast has better function at the lower contrast levels than a person
who has higher visual acuity at high contrast. This is important to remember
in all such occupational tasks that require exceptionally good visual
function at low contrast levels. |
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Figure 12. Variation in the
declination of the slope in normal subjects. |
The Types of Contrast Sensitivity
Changes
Usually
the loss of visual function is roughly equal at high and at low contrast
levels. The slope of the curve moves toward the left without a change in the
declination (Type I). When there is a small circumscribed lesion in the center
of the macula, visual acuity may decrease several lines, yet in the low
contrast vision there is slight or no loss (Type II). Type III change in the
transfer of visual information is characterized by moderate to no loss of
visual acuity at high contrast and a greater loss of visual function at low
contrast. This is often caused by diabetic retinopathy, cataract, glaucoma, or
optic neuritis, to mention some of the most common causes.
Types of changes in contrast
sensitivity.
Clinically,
it is well known that there can be three people with different types of
contrast sensitivity losses even when they have similar visual fields and
visual acuity values. They can have very different functional vision. The three
people whose contrast sensitivity curves are in figure all have visual acuity
of 20/63 (6/18, 0.3). Person A has high normal function at low contrasts and
functions like a normally sighted person. Person B has somewhat decreased low
contrast function and the typical behavior of a person with low vision
(bringing texts closer and moving slightly slower on stairs, etc). Person C has
lost visual functions at low contrast and is severely visually impaired. Of
these three people with the same visual acuity, one is normally sighted, one
has low vision and one is severely visually impaired.
Contrast sensitivity curves of three persons with visual acuity 20/63
(6/18, 0.3).
Lower Visual Acuity May Mean Better
Vision
If
occupational tasks require good visual function at low contrast levels, visual
acuity alone does not select the most suitable persons for that particular
task. For example, if the task is to notice airplanes approaching within the
low clouds, these planes are best seen by a person with good visual acuity in
the contrast range of 1-5%. Since the declination of the slope varies even in normal
individuals, it is possible that a person with lower visual acuity at high
contrast has better function at the lower contrast levels than a person who has
higher visual acuity at high contrast. This is important to remember in all
such occupational tasks that require exceptionally good visual function at low
contrast levels.
Summary
Assessment
of visual function at low contrast adds an important dimension in the
evaluation of a person's capabilities. It should be a part of evaluation of
vision in occupational health and in low vision services as well as in all
diagnostic work. With the easy-to-use optotype tests, it is possible to assess
visibility of low contrast details. A person's ability to see low contrast
lines requires grating tests, which presently are under construction.