Special filters for medical applications

Retinitis pigmentosa, or more correctly retinopathia pigmentosa, is a hereditary, progressive disease of the eye. Due to a malfunction in the metabolism of the retina, the sensory cells (rods and cones) are not sufficiently supplied with blood and hence gradually degenerate. The many forms which this disease takes make it impossible to give an exact description of the symptoms.

Initially, the function of the rods is impaired. Increasing difficulty is experienced with twilight and night vision, extending to total night blindness. There is a gradual contraction of the visual field from the periphery inwards until the patient is left with "gun barrel" vision. Orientation then becomes very problematic.

A further symptom of RP is reduced cone function. Reduced visual acuity, a deterioration in visual contrast and colour, and a high degree of sensitivity to glare are the result. Special filter lenses often provide enhanced wearing comfort.

The damage caused by diabetes is usually focused on the middle and inner layers of the retina which are responsible for processing image contrast.

In a retina damaged by diabetes the sensitivity to contrast is impaired. In bright surroundings, relatively dark details generate the maximum signal possible in the brain. Brighter details do not generate a sufficiently strong signal and cannot therefore be distinguished from darker zones. Particularly patients in the advanced stages of diabetic retinopathy are easily dazzled by glare and find light/dark adaptation difficult.

Cut-off filters such as F 540, F 560 and F 580 absorb the part of the spectrum to which the cones are sensitive and hence shield them. The special filter lenses therefore reduce glare and facilitate adaptation to changing light.