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Contrast Sensitivity

Contrast sensitivity charts used for LASIK evaluations.

During the pre-operative evaluation we’ll test your contrast
sensitivity with a special eye chart that has light gray letters
on the bright background.

When people speak about vision, they often refer to what they can see using “20/20” notation. Eye doctors refer to this as “Snellen Acuity” after the Dutch doctor who developed the eye test chart in the mid-1800’s. This method to measure vision can be thought of as a measure of the “quantity” of what you can see, but it doesn’t reflect the “quality” of what you can see. Contrast sensitivity is a better measure of “quality” or crispness of vision.

In its most basic terms, contract sensitivity describes our ability to detect subtle differences between objects that are not black and white. Contrast can be “high” like a black dog in a snowy field on a sunny day or the standard Snellen Chart or it can be low, like a white rabbit in that same snowy field or a person wearing a dark coat walking along the side of the road at dusk. Our ability to detect contrast is best when there is very bright lighting and it decreases as the light becomes dimmer. Most of our world falls in the medium to low contrast range. This is why Snellen charts do not always give the best measure of how well we can actually see.

Many of the visual problems that you hear about as being negatives with regard to refractive surgery are related to changes in contrast sensitivity. It is well known that refractive surgery causes a temporary mild decrease in contrast sensitivity immediately after surgery. This is due to the normal swelling and healing of the cornea. It tends to resolve significantly within the first few weeks after surgery and recovers to near-normal in the first few months.

Certain factors can result in more persistent losses in contrast sensitivity. These include treatment using older broad beam lasers, treating an area of the cornea that is too small as compared to the size of the pupil under dim light and the presence of significant higher order optical aberrations, which are very subtle imperfections in the visual system.

During your pre-operative evaluation at Flaum Eye Institute Refractive Surgery Center, we’ll carefully measure your pupil size, analyze the subtle optics of your visual system and test your contrast sensitivity with a special eye chart that has light gray letters on the bright background. If we feel that significant contrast sensitivity problems could develop after treatment, we might recommend customized ablation with the Bausch and Lomb Zyoptix system or we might recommend that no treatment be performed at all.

The use of contrast sensitivity testing by the Flaum Eye Institute Refractive Surgery Center team allows us to detect subtle nuances about your vision, making refractive surgery safer for our patients.