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A topographer is a newer version of the keratometer. Your doctor will have you sit at a table and look into a bowl. This bowl's inner surface is covered in a pattern of concentric rings. A digital camera sits at the base of the bowl and charts the cornea's surface by measuring the reflected pattern in the eye. This basically creates a contour map of your eye, and it's an excellent technique for measuring irregular astigmatism. Most corneal topography tests measure 8,000 to 10,000 points on the cornea, making it the most accurate keratometry test. Other advanced ways of measuring astigmatism include autorefractors that measure refraction, and the IOLMaster that makes extensive measurements of the interior and exterior of the eye. The IOLMaster is usually used before cataract surgery. The Placido disc is an old-fashioned, hand-held device that is shaped like a giant lollipop. The disc has several sets of concentric circles painted on one side, and is used to measure the regularity of your cornea's surface. Your doctor holds the disc up to their own eye with the rings pointed towards you, and they look into your eye. Sit still and look straight ahead. The image of your cornea reflects back, and where there is irregular shape it makes the concentric rings look distorted to your doctor. This old-fashioned method can help to diagnose keratoconus, a disorder that causes irregular astigmatism due to thinning and protruding of the cornea. The shape of your posterior cornea (the back surface of the cornea) can influence the severity of your astigmatism. Keratometry can only measure the front of your cornea, but Scheimpflug imaging uses tomography to get more accurate readings of how thick your cornea is and what the back of it looks like. This method can be helpful in predicting how successful surgery may be in correcting your astigmatism.
Use a corneal topographer for more precise measurements. Try a Placido disc for irregular astigmatism. Use a Scheimpflug device to measure the posterior cornea.