Contrast Sensitivity Experts
CONTRAST SENSITIVITY BACKGROUND
How is Contrast Sensitivity Different than Acuity?:
In 1862, Dr Snellen, from Utrecht, Holland developed the Snellen visual acuity test primarily for the purposes of prescribing glasses. This Snellen Chart, "The Big E Chart," presents a series of high contrast black-on-white letters in different sizes. Relatively small changes in refractive status can be detected by this test and it is very useful as a standard for describing changes in vision caused by spherical blur. Unfortunately, many types of vision loss are not caused by spherical blur and for these types of vision changes, e.g. cataracts, glaucoma, irregular astigmatism, etc., Snellen acuity is an inadequate measure.
Contrast sensitivity determines the lowest contrast level which can be detected by a patient for a given size target. Normally a range of target sizes are used. In this way contrast sensitivity is unlike acuity. Contrast sensitivity measures two variables, size and contrast, while acuity measures only size. Contrast sensitivity is very similar to auditory testing, which determines a patient's ability to detect the lowest level of loudness of various sound frequencies. The patient is asked to depress a button when the tone is just barely audible and release the button when the tone can no longer be heard. This procedure is used to test auditory sensitivity to a range of sound frequencies. If auditory testing were evaluated in a similar way to visual acuity, all the sound frequencies would be tested at one high level of loudness.
The Contrast Sensitivity Curve:
Contrast sensitivity is presented as a curve, which plots the lowest contrast level a patient can detect for a specific size target. The x-axis of the curve is for spatial frequency, while the y-axis is for contrast sensitivity. Low spatial frequencies are fat gratings and high spatial frequencies are thin gratings. Contrast sensitivity is the inverse of contrast level. The higher the contrast sensitivity, the lower the contrast level at which the patient can detect a target.
The example graph to the left shows a contrast sensitivity curve. Note that Snellen acuity measures only a narrow range of the curve.
Plotting a Contrast Sensitivity Curve:
Most commercially available contrast sensitivity tests provide measures for 4 or 5 size bar patterns (spatial frequencies). Each of these spatial frequencies is presented at 8 to 10 contrast levels. To plot the curve, simply determine the highest contrast sensitivity level a patient can detect for each spatial frequency. The curve on the right shows an example for the CSV-1000 contrast sensitivity test.
On row A of the CSV-1000, the highest contrast sensitivity level the patient can detect is level 5. On row B, the highest level is 7. On row C and D the highest levels are 6 and 5, respectively. To plot the curve, check the number 5 in column A, 7 in column B, 6 in column C and 5 in column D. Connect these check marks and the result is a contrast sensitivity curve.
Visit other parts of our website to determine how this curve can be used to assess cataracts, glaucoma, diabetes, refractive surgery, contact lenses, etc.
Immediately after LASIK refractive surgery, contrast sensitivity falls such that the patient's quality of vision is well below the normal range. After several weeks, or in some cases several months, the quality of vision and contrast sensitivity recover to normal levels.
For PRK refractive surgery, the amount of time required to recover completely to normal levels may be longer, up to six months.
