Visual acuity measures the ability to recognize small objects with high contrast. Often however, the visibility of objects in our environment is limited more by lack of contrast than by their small size. Consider the pedestrian who misses a curb or step, or the driver who did not detect a pedestrian on a dark road. In many surveys, contrast sensitivity is the visual parameter that is most closely related to problems experienced in activities of daily living (ADL).
Like visual acuity, contrast sensitivity loss is not disease specific. Like visual acuity, it can be affected by optical problems (refractive error, opacities) that prevent the projection of an adequate image on the retina. It can also be affected by disorders of the outer retina (such as macular degeneration), which prevent part of the image from being detected. Some longitudinal studies suggest that it is a good predictor of acuity loss in the next five years. Finally, contrast sensitivity loss can be caused by neural problems that interfere with adequate processing of the visual information. Neural problems might occur in the inner retina (glaucoma), the optic nerve (optic neuritis), or in the brain and visual cortex.
Regardless of the cause, the consequences of contrast sensitivity loss are significant, since many Activities of Daily Living (ADL) involve objects of less than optimal contrast. Patients with contrast problems often notice that something is wrong, but they cannot pinpoint it. They become frustrated patients when told not to worry because their high-contrast visual acuity is normal. A simple contrast test could have identified the problem. Once warned, the patient can take preventive action, such as avoiding night driving and other low-contrast situations. Warning the patient about steps and curbs may avoid a fall and thus a broken hip. Providing better contrast and better illumination in the home can also improve many tasks.
The relationship between visual acuity and contrast sensitivity (CS) can be depicted on a plot with visual acuity along the X-axis and contrast along the Y-axis.
On this plot, the Contrast Sensitivity Curve defines the limits of visibility. Points under the curve (large size, high contrast) can be seen; points beyond the curve (small size, low contrast) cannot be seen.
For letter recognition the top of the curve is practically flat. It is known as PEAK Contrast Sensitivity. It must be measured with rather large letters to be certain to be on the flat part of the curve.
The bottom of the curve indicates the familiar high-contrast letter chart acuity. The lower part of the curve is practically vertical. Improving contrast from around 25% to 100% usually has very little effect on visual acuity.
Measuring visual acuity at lower contrast levels yields increasingly lower visual acuity values.
The next diagram depicts various ways of determining points on the CS curve.
To determine point “a”, the PEAK of the CS curve, we use large letters of variable contrast. This can be done on a 1-meter wall chart, the well known Pelli-Robson chart (used in many research studies), or more easily on a handheld chart, the more recent Mars card.
Both charts are available through Precision Vision.
Point “b” represents high-contrast letter chart acuity.
To determine point “c”, low-contrast letter charts are used.
Precision Vision offers a wide variety of low contrast charts.
When “b” and “c” are known, the difference defines the SLOPE of the CS curve.
When the low-contrast level is set at 10%, a normal difference will be 1 or 2 lines. This difference is often independent of the visual acuity level. Greater differences point to a contrast deficit. In AMD patients differences of up to 10 lines have been found between high-contrast and low-contrast acuity.
The Colenbrander Mixed Contrast Cards are easy to use for this purpose, since the high and low contrast parts are printed side by side, so that the illumination and the viewing distance are always the same. The test takes little extra time, since the high-contrast part replaces an ordinary high-contrast reading card.
Since the two contrast levels are side by side, the test has a strong demonstration value both for the patient and for family members. If increased illumination improves performance, a strong case has been made for improved illumination at home or at work.
Point “c” could also be determined by offering small letters of varying contrast. Since point “c” is on the slope and not on the plateau of the CS curve, it is sensitive to variations in visual acuity as well as to changes in contrast. If point “c” is on a steep part of the CS curve, a small change in visual acuity (X-axis) will be equivalent to a large change in contrast (Y-axis). Thus, the test may be particularly sensitive to follow subtle acuity changes after refractive surgery. It is not a good test to determine peak CS. Such a chart is known as the Small Letter Contrast Sensitivity test developed by Rabin.
Precision Vision offers the Rabin contrast sensitivity test. The original test fits the small illuminator cabinet. A second version of this Rabin test is available for the large ETDRS Illuminator Cabinet.
Others have advocated the use of sine-wave gratings, rather than letters. Sine-wave tests are preferred for lens quality calculations, but may not appear as relevant to the patient as do letter tests. Letter tests may also be faster to administer with higher test and re-test repeatability.
The clinical implications of contrast sensitivity losses are not well understood. One of the reasons is that contrast tests are rarely included in a routine eye exam, because they involve doing two tests and comparing the results. This means that little clinical experience is available, which in turn does not stimulate testing.
The Mixed Contrast cards were developed to remedy this situation. Because the high and low contrast text or symbols are printed side by side, only one card is needed. Because the cards are handheld, no repositioning of the patient (as with the Pelli-Robson wall chart) is needed. The high contrast part of the test replaces a high-contrast-only test, so the extra time needed is only the time for the patient to look at the low contrast part. It is hoped that this will encourage more clinicians to include contrast tests in their routines, so that more clinical experience can be gained.
Mixed Contrast cards are available in several versions.
The small mixed contrast reading card is meant for routine use, especially in the elderly.
It has a cord for the fixed 40 cm reading distance. The appropriate visual acuity values for that distance are listed on the card.
To detect early AMD changes the use of a reading card is recommended since reading involves a larger retinal area than does single letter recognition.
The mixed contrast card set is meant specifically to compare the effectiveness of various presbyopia solutions at 40 cm, 63 cm and 100 cm.
Cords are attached for the various distances.
Since this is an optical problem in patients with a normal retina, the use of letter cards is acceptable.
The large mixed contrast reading card is meant for use with low vision patients, especially those with AMD.
It has a wider measurement range than the small card and can be used at any distance, using the included ruler and the modified Snellen formula as discussed in the low vision section.
Contrast Measurement Scales
The contrast on a chart can be labeled in different ways.
“Contrast” identifies the difference between the brightest and the dimmest parts of the target.
“Contrast Sensitivity” (CS) identifies the sensitivity of the visual system. It is the reciprocal of the threshold contrast. Compare: visual acuity is the reciprocal of the threshold magnification requirement.
“Log(CS)” converts the CS values to a linear scale, suitable for comparisons between levels of contrast sensitivity. Compare: MAgnification Requirement and logMAR.
When contrast measurements are always done with the same test, it is not important to know how the initial contrast value is calculated. However, when contrast values need to be compared between different tests or with literature data, it is important to know that there are two contrast scales: Michelson contrast and Weber contrast.
Early CS tests considered mainly optical factors and used sine wave gratings, since this is the preferred target for measurements of optical lens quality. The contrast of such repetitive patterns is expressed with Michelson’s formula: brightness amplitude / average brightness. Sine wave tests always use Michelson contrast; based on this tradition, the Precision Vision charts are also labeled with Michelson contrast.
When Pelli and Robson introduced the Pelli-Robson letter chart, they chose to use Weber’s formula for non-repetitive patterns: brightness difference / background brightness. Weber contrast is also used for the Mars chart and some other charts.
On both scales 0% indicates the absence of contrast and 100% indicates the theoretical maximal contrast. For values in between the two scales differ. In the range from 1% to 10%, which is the range most used for clinical measurements, the Weber contrast values are about twice the Michelson values; the log(CS) values differ by 0.3, as indicated in the following table.
|Michelson contrast (PV charts)||0.6 %||1.25 %||2.5 %||5 %||10 %||25 %|
|log(CS) for Michelson||2.2||1.9||1.6||1.3||1.0||0.6|
|Weber contrast (Pelli-Robson, Mars)||1.25 %||2.5 %||4.9 %||9.5 %||18.2 *||40 %|
|log(CS) for Weber||1.9||1.6||1.3||1.0||0.7||0.4|
Precision vision Low Contrast charts are labeled with Michelson contrast.