CONTRAST SENSITIVITY CHARTS

Contrast Sensitivity 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.

Which Contrast 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.)

As 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.)

The 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 curve.

The Functional Acuity Contrast Test (FACT™)

More information on FACT sine-wave grating test modalities...

 

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