Improving Low Vision Performance by Enhancing Contrast
Thomas I. Porter, O.D., FAAO
Contrast sensitivity loss is a common problem with low vision patients regardless of the underlying pathology or congenital condition. In some pathologies such as glaucoma contrast loss is a very early problem that has a functional impact far beyond what one would predict based solely on acuity or visual field loss. Unfortunately contrast enhancement is an area that is often overlooked by new low vision professionals as they formulate a treatment plan.
In the simplest sense contrast sensitivity is the ability for the eye to visually separate an object from the background. Patients will often describe their loss of contrast by saying things look foggy or by describing difficulty identifying low contrast objects such as facial features. These same patients will often report improved visual performance under certain lighting conditions.
There are several techniques that are commonly used in low vision to enhance contrast appreciation. These include increasing illumination, using contrast enhancing lens filters, modifying foreground and background colors, and electronically altering images with a device such as a CCTV.
It has been estimated that patients with a loss of contrast sensitivity function (CSF) require 3 to 10x more illumination when reading when compared to an age matched normally sighted population. By using specific task lighting a low vision practitioner can both enhance contrast and minimize reflected glare. In many low vision practices the most commonly used optical low vision aids have built-in illumination systems. It is recommended that various types of illumination such as halogen, LED, and incandescent be all demonstrated to better evaluate the impact each type of illumination has on overall performance.
Many patients with low vision problems also complain of glare sensitivity. Unfortunately fashion sun lenses commonly used to control glare tend to further reduce the already compromised contrast sensitivity. Through experience low vision practitioners have discovered that by selectively filtering the blue end of the visual spectrum, contrast appreciation can be enhanced. This means that lenses in the yellow, orange and dark amber colors range will work effectively to enhance contrast and reduce glare on most low vision patients. Due to individual differences each patient’s subjective and objective response to filter colors should to be evaluated. From your history determine how significant glare sensitivity is as a problem to the patient. If glare sensitivity is a big issue then dark amber filters would be a reasonable starting point. In cases were patient’s describe their vision as being “very dim” such as in optic nerve problems, then a better starting point would be with a yellow lens. Keep in mind that the darker lenses cut more glare and the lighter and more yellow filters tend to give the greatest contrast enhancement. The challenge for the practitioner is to balance contrast enhancement with glare reduction.
Don’t overlook “low tech” contrast enhancement. By altering the visual relationship between an object and the background contrast appreciation can be improved. A patient with reduced CSF would report that an aspirin virtually disappears if placed on a white tablecloth. However, that same aspirin can be seen easily when the background is black or navy blue. Counsel your patients to use highly contrasting backgrounds wherever possible. This could include simple everyday objects such as the color of dinner plates, types of writing devices, paper color, and markers on stair steps.
Most electronic low vision aids such as CCTV’s provide contrast enhancement by allowing the user to change the image on the screen from natural color to a totally black and white image. When using a black and white image glare can be significantly reduced by “reversing” the image to a black background with white text.
Evaluating and discussing contrast enhancement should become a regular part of your normal low vision exam protocol. As you address contrast issues you will find that both patient’s general visual performance and overall satisfaction will improve.