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An ophthalmometer, also known as a keratometer, is a medical instrument commonly used to measure the curvature of the cornea during an eye exam. The cornea is the clear front part of the eye, covering the iris and pupil, and the measurements can be used to diagnose the presence and degree of astigmatism, a vision problem that is usually caused by a misshapen cornea. This instrument is also used when fitting contact lenses and to monitor changes to the cornea as a result of wearing contact lenses. Ophthalmometers can also be used during eye surgery to enable precise incisions in the cornea. An eye test using an ophthalmometer is usually performed in-office by an optometrist or ophthalmologist, the procedure is non-invasive and does not require any part of the instrument to touch the eye.
The word ophthalmometer comes from the Greek word for eye, ophthalmos. Ophthalmology, "science of eyes," deals with the anatomy, physiology and diseases of the eye, for example astigmatism. Astigmatism is usually caused by an irregular or misshapen cornea. This defect causes a refraction error that makes it difficult to see fine details and gives blurry vision. More rarely, astigmatism is caused by the lens behind the cornea being misshapen.
An ophthalmometer is commonly used to diagnose astigmatism and determine what corrective prescription is needed to treat it. During the eye test, the eye specialist will look at the eye through the instrument while a light shines into the pupil, obtaining the needed measurements. These measurements of the cornea are called keratometric values and are used to determine the necessary treatment. Depending on the type of astigmatism, either eyeglasses or contact lenses can be used to restore vision.
The ophthalmometer was invented by German physiologist Hermann von Helmholtz in 1880, and though modern versions of this instrument use optical sensors and computer technology, its basic design is still fairly similar. To obtain an accurate measurement of the curvature of cornea, a luminous pattern of images called mires on the ophthalmometer is reflected onto the cornea. The size and pattern of this reflection in an area in the center of the cornea is then measured. This reflection is compared to the actual size and pattern of the images on the ophthalmometer. Knowing the distance between the actual images on the instrument and their reflection in the cornea, the curvature of the cornea can be determined using a mathematical formula.