An ophthalmometer is a medical instrument used to measure the curvature of the cornea during an eye exam, diagnose astigmatism, and determine corrective prescriptions. It was invented in the 1880s and uses optical sensors and computer technology.
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 measurements can be used to diagnose the presence and degree of astigmatism, a vision problem usually caused by a deformed cornea. This tool is also used for contact lens fitting and to monitor corneal changes due to contact lens wear. Ophthalmometers can also be used during eye surgery to allow for precise incisions in the cornea. An eye exam with an ophthalmometer is usually performed in the 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 the eyes”, deals with the anatomy, physiology and diseases of the eye, for example astigmatism. Astigmatism is usually caused by an uneven or misshapen cornea. This defect causes a refractive error that makes it difficult to see fine details and gives blurry vision. More rarely, astigmatism is caused by deformation of the lens behind the cornea.
An ophthalmometer is commonly used to diagnose astigmatism and determine what corrective prescription is needed to treat it. During the vision test, the eye doctor will look at the eye through the instrument as a light shines on the pupil, obtaining the necessary measurements. These measurements of the cornea are called keratometry values and are used to determine what treatment is needed. Depending on the type of astigmatism, glasses or contact lenses can be used to restore vision.
The ophthalmometer was invented by German physiologist Hermann von Helmholtz in the 1880s, and while modern versions of this instrument use optical sensors and computer technology, its basic design is still quite similar. To get an accurate measurement of the curvature of the cornea, a light pattern of images called the mire on the ophthalmometer is reflected off the cornea. The size and pattern of this reflex 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 real images on the instrument and their reflection in the cornea, the curvature of the cornea can be determined using a mathematical formula.
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