Musa Mutali J., Zeppieri Marco
University of Benin, Benin City, Edo State
University Hospital of Udine, Italy
Astigmatism is defined as a condition where there is an unequal curvature of the front surface of the eyes. Astigmatism can occur in the cornea (corneal astigmatism), the crystalline lens (lenticular astigmatism), or, less frequently, in the retina. Astigmatism is usually congenital but may change in location and amount as the individual ages. A rare occurrence is when excessive congenital astigmatism results from a toric growth of the crystalline lens onto the underlying capsule in a condition known as lenticonus. It is commonplace to see the terms "meridians" and "axes" when dealing with this topic. An astigmatic axis is the exact location of astigmatism on the 360-degree scale, while a meridian is a plane along which that power exerts its optical characteristics. (See Images) Simple Myopic astigmatism - One axis is on the retina while the other is in the front of the retina (in the hypothetical vitreous space). Compound myopic astigmatism - Both axes are in front of the retina (in the hypothetical vitreous space). Simple hyperopic astigmatism - One axis is on the retina, while the other is behind the retina. Compound hyperopic astigmatism - Both axes are behind the retina. Mixed astigmatism - One axis is in front of the retina, and the other is behind the retina. are instruments used for the detection and quantification of ocular astigmatism. They usually consist of black lines arranged radially (i.e., Fan dial, Sunburst dial, and Clock dial) or in a cross, as in the Cross dial (see image). The dial may be stationary and fixed in one position or rotate around an axis. They are usually made of plastic as an accessory to a distance wall-mounted chart, which can have a digital chart component. Twelve radial lines comprise the astigmatic dial. The lines may be single in some models or consist of three parallel lines per clock hour. The sunburst dial consists of a semi-circle facing upwards like a sunrise, with the radiating lines 5 degrees away from each other, running from 0 to 180 degrees. The Lancaster dial consists of two lines perpendicularly bisecting each other similar to the cross dial.
散光被定义为眼睛前表面曲率不均的一种状况。散光可发生在角膜（角膜散光）、晶状体（晶状体散光），较少见的是发生在视网膜。散光通常是先天性的，但会随着个体年龄增长在位置和程度上发生变化。一种罕见的情况是，在一种称为圆锥晶状体的病症中，先天性散光过度是由于晶状体呈环曲面生长到下方的晶状体囊上所致。在处理这个主题时，经常会看到“子午线”和“轴”这两个术语。散光轴是散光在360度范围内的确切位置，而子午线是该屈光力发挥其光学特性的平面。（见图）单纯近视散光——一个轴在视网膜上，而另一个轴在视网膜前方（在假想的玻璃体空间中）。复合近视散光——两个轴都在视网膜前方（在假想的玻璃体空间中）。单纯远视散光——一个轴在视网膜上，而另一个轴在视网膜后方。复合远视散光——两个轴都在视网膜后方。混合散光——一个轴在视网膜前方，另一个轴在视网膜后方。 是用于检测和量化眼部散光的仪器。它们通常由呈放射状排列的黑线组成（即扇形表盘、旭日形表盘和时钟表盘），或者像十字表盘那样呈十字形排列（见图）。表盘可以是固定的并处于一个位置，或者绕轴旋转。它们通常由塑料制成，作为壁挂式远距离视力表的附件，视力表可能有数字图表组件。散光表盘由12条放射状线组成。在某些型号中，这些线可能是单线，或者每个钟点由三条平行线组成。旭日形表盘由一个向上的半圆组成，类似日出，放射状线彼此相隔5度，从0度到180度。兰卡斯特表盘由两条相互垂直平分的线组成，类似于十字表盘。
