Testing :: Several contrast test systems are available -- the key
difference is target type. For example, the Pelli-Robson chart determines
the contrast required to read large letters of a fixed size. With the
Pelli-Robson chart, the contrast varies while the letter size remains
constant. The Regan chart, a low-contrast letter chart having different
size letters, reduces the contrast levels of a standard Snellen-type
letter acuity chart resulting in several charts. The Functional Acuity
Contrast Test (FACT) developed by Dr. Arthur P. Ginsburg uses
sine-wave gratings, which measure specific visual channels.
Test Is Best? A
Comparison of Contrast Sensitivity Tests :: Research shows that
the contrast sensitivity curve provided by sine-wave grating tests is
more sensitive and informative than the results obtained from low-contrast
letter acuity systems. Consider the findings below.
Plotted in Figure
3 are the average contrast sensitivity values of charts using sine-wave
gratings (VCTS, SWCT, FACT charts), the Pelli-Robson chart, the Regan
chart, and a curve of the identification contrast threshold for a Snellen
letter chart. The identification contrast threshold shows the upper
limit of sensitivity that can be obtained using low contrast letters.
As readily seen, each contrast test measures a different range of spatial
frequencies and contrast levels. The sensitivity of the letters fails
to reach the sensitivity of the gratings. (In references,
see: Ginsburg AP. Next generation contrast sensitivity testing.)
Figure 3 - A
comparison of contrast sensitivity tests.
Even after the sine-wave
grating charts are graphed on the same spatial frequency and contrast
test space (as shown in the above figure), it may be difficult for the
lay person to translate these test spaces into an everyday visual experience.
To better understand this concept, consider the example below.
When a street scene
is filtered using the full spatial frequency ranges of the three test
types (Pelli-Robson, Regan, sine-wave gratings), the resulting filtered
images allow a direct comparison with the original-image size and contrast
information being tested. The results of such filtering are shown below.
(Note: It is difficult to know what range of spatial frequencies the
Pelli-Robson chart tests because letter identification depends upon
the relevant letter spatial frequencies reaching threshold, which can
vary considerably from one person to another. Therefore, a generous
range (0.0 to 3 cpd) was used for the Pelli-Robson filtering.)
you can see, the Pelli-Robson chart tests a size too large to be relevant
to the scene. It may be useful for predicting the threshold visibility
of large trucks in the fog but not for determining the presence of small
objects such as the little girl on the street. The sine-wave grating
charts test a size and contrast range relevant to the complete scene
information. The Regan charts test the contrast and size range most
relevant to the sharp edges of the scene, which reveals little of the
image quality and the little girl. The grating chart obtains and measures
the most relevant information for evaluating one's ability to view this
scene clearly. (In references, see:
Ginsburg AP. Next generation contrast sensitivity testing.)
Functional Acuity Contrast Test (FACT) :: Developed by Dr.
Arthur Ginsburg, the FACT sine-wave grating chart tests five spatial
frequencies (sizes) and nine levels of contrast. The patient determines
the last grating seen for each row (A, B, C, D and E) and reports the
orientation of the grating: right, up or left. The last correct grating
seen for each spatial frequency is plotted on a contrast sensitivity
Functional Acuity Contrast Test (FACT)
information on FACT sine-wave grating test modalities...