Contrast sensitivity testing
What is it for?
Measuring your ability to distinguish objects in low contrast situations.
Assessing your ability to see contrast between images in low lighting conditions provides an important measure of your visual function at night.
It gives your surgeon an understanding of how you see things in the real world as opposed to just black-on-white (100% contrast) letters on a chart. Occasionally, people complain of poor vision in dim lighting conditions yet have normal or very near normal black-on-white visual acuity. This is often because they have reduced contrast vision.
What actually happens?
You are shown images of grey stripes, shapes or letters. The images are shown in sequences, and the contrast (the difference between the colour of the image and the colour of the background) reduces each time so that it becomes more difficult to distinguish the image.
You are challenged to recognise the stripes and letters as the contrast decreases until it is so faint that you can no longer recognise whether there are stripes, or what the letters are.
How does it feel?
You feel nothing.
How does it benefit you?
Night vision disturbances may be induced by laser eye surgery.
This is more likely in patients with high prescriptions or large pupils. Measuring your contrast sensitivity provides your surgeon with a further measure of how well you already see at night, giving an indication of how best to plan your treatment in order to avoid night vision disturbances.
In general, lasers using ‘aspheric treatment profiles’ will be safer with respect to night vision. Some laser systems are so well designed that they can even improve contrast sensitivity and night vision in some patients.