Contrast sensitivity is very sensitive to subtle changes in vision caused by small refractive errors or a mismatch between the contact lens power and patient visual requirements. Contrast sensitivity is most useful in establishing whether a specific lens provides the best visual performance. Examples include evaluation of lens aging, comparing a toric versus a spherical equivalent lens and evaluation of a multifocal lens.
Examples of Contact Lens Evaluation
Contrast sensitivity of a patient wearing a new set of soft contact lenses, and nine months later after protein build-up. Note the loss in contrast sensitivity without a change in acuity.
Contact Lens Aging
The visual quality of lenses degrades over time due to protein build-up and lens surface damage. Contrast sensitivity can be used to determine the extent to which visual capability has been affected by lens aging. Compare the contrast sensitivity measured when the lenses were newly fit to the measurement taken on the most recent visit. If a loss of 2 or more contrast levels is noted, the lens should be replaced.
Contrast sensitivity provided by a toric versus spherical equivalent lens in a patient with moderate astigmatism.
Spherical Equivalent Versus Toric Lens
Many borderline astigmatic patients can be fit with either a spherical equivalent or toric lens. When visual acuity is the same with both lenses, it is difficult to determine which is best for the patient. Compare the contrast sensitivity results of the two lenses. If the sensitivity provided by the toric lens is 2 or more contrast levels higher than that provided by the spherical equivalent lens, the patient should wear the toric lenses.
Contrast sensitivity can be used to determine if a bifocal contact lens affects the patient’s functional vision. Here the lens reduces the contrast sensitivity outside the normal range
Bifocal Contact Lens
Many of the "baby boomer" population have developed presbyopia (age-related far-sightedness) and want the convenience of the bifocal contact lenses. When bifocal lenses are used for the treatment of presbyopia, in some cases, it is difficult to determine whether the patient will be able to tolerate a visual degradation associated with the multifocal lens. Contrast sensitivity can be used to determine the contrast loss, if any, caused by the lens and aid in determining the compatibility between lens and patient.
Compare the contrast sensitivity provided by the bifocal lens to that provided with best correction through spectacles or the phoropter. If the bifocal lenses cause more than a two contrast level drop, the patient will most likely be unhappy with the visual performance provided by the lenses. Comparison of various brands and types of bifocal lenses can also be accomplished to determine which lens provides the best visual capability.