Keratometer Optical Design

In this article, we will continue our description of different optical instruments used by optometrists. Last time we talked aboutOphthalmoscopes, today we will talk about another very common optical instrument called the Keratometer.

角膜仪于1851年开发，用于测量前角膜前角膜的曲率。那是角膜的外层。角膜仪使用已知大小的对象，并通过修改角膜的距离来创建特定大小的图像。然后从凸镜配方获得前角膜曲率，并使用Snell的折射定律和简化的光学元件计算角膜功率。考虑到由角膜厚度和后表面角膜仪的曲率引起的误差，使用的折射率为1.3375，而不是更准确的值为1.376。

When utilizing a keratometer, two images of a target mire are created. Crosses or circles with marks along the outside might be used as the mire. These photos may be out-of-focus, in which case an optical professional will adjust the object’s distance until the image is sharp. The target is denoted by a circle with a cross symbol in the center. The cross should be in the center of the circle. The curvature is measured using the upper and left markings. We can quantify that curvature by changing those photos until the lateral markers are superimposed. We can observe the three different markings before and after alignment in Figure 1.

It is important to mention that in the case of an astigmatic eye, both marks wont be able to be superimposed simultaneously.

In figure 2 we show a basic layout of a keratometer. We will describe the basic elements, although we won’t be able to mention any of the specific details about the optical components, we will be able to describe their basic functions.

In the figure we are showing a test sphere instead of a patient’s eyes. Several of the components shown are used to control the illumination. The mires are placed on its own mechanical fixture that allows for the control of their position as explained before. One problems when measuring the cornea curvature, is that the eyes are in constant movement. In order to help stabilize the image long enough to perform the required measurements it is common to use a double prism. Finally we have optical elements that are used to create an image that can be captured by an eyepiece and the operator.

In modern systems it is possible to replace the eye of the operator by a camera. One of such devices is shown in Figure 3. Figure 3 was developed by OpticsForHire, although we can go into details of the system, the basic operation is pretty similar to the one that we just described. Notice that the mires and mechanical positional components are not shown.