Straylight as Additional Indicator for Visual Function Assessment and Clinical Decision Making

For decades, visual function (VF) was defined by visual acuity (VA) and visual field. VA is a measure of perception of fine detail. In ophthalmic practice, corrected distance visual acuity (CDVA) is routinely tested using a letter chart and appropriate spectacles or contact lenses. Visual field examination is used to determine retinal luminance sensitivity at various retinal locations, and to determine the boundaries of peripheral vision. Then it was realized that other aspects of VF are also important for visual performance in the “real world”: quite frequently, patients report subjective visual symptoms despite having good CDVA (and normal visual field). In other words, it was realized that good CDVA does not imply good VF. Ophthalmic interventions were usually evaluated using VA as an outcome parameter. Real-world visual scenes however, comprise complex combinations of contrasts with large luminance differences; including luminance differences that are more subtle than the high contrast letters used for CDVA testing. To evaluate this part of VF, contrast sensitivity (CS) tests were introduced. CS is a measure for the ability to detect luminance differences, in particular the ability to detect the luminance difference between an object and its surroundings. CS is tested using a chart with sinusoidal gratings or low contrast letters. However, the aspect of VF assessed with CS testing is quite similar to that assessed with VA testing. The more independent aspect of VF that could still not be adequately assessed is straylight (see “Straylight” section). Straylight causes glare sensitivity. Glare sensitivity can be described as loss of visual quality, or even loss of VF, that is particularly experienced in the presence of a distant bright light source and low-contrast surroundings. Glare symptoms include blinding, perception of halos (ring of light) around bright lights, “hazy vision”, and lowered contrast perception. Glare sensitivity enhanced scientific curiosity for straylight. After it was found that glare sensitivity is caused by intraocular straylight, glare sensitivity was officially defined as straylight. The straylight parameter was introduced as a well defined measure for this aspect of VF (see “Clinical straylight measurement” section). A typical example of glare sensitivity is the blinding effect of headlights of oncoming traffic during night driving.

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