散光是当角膜变弯或者变平时表面不平坦，影响视近、视远时的聚焦。
When the cornea curves or flattens unevenly, the result is astigmatism, which disrupts focus of near and distant vision.
视向量能够在全局坐标系中通过视点坐标减去表面坐标来创建，并且要把结果转换到切线空间中。
The eye vector can be created in global coordinates by subtracting a surface position from the eye position and transforming the resulting vector into tangent space.
羟乙基纤维素（HEC）溶液的表面张力和视粘度很少受添加剂的影响，很稳定。
The surface tension and the apparent viscosity of hydroxyethyl cellulose (HEC) solution are hardly affected by the addition agents and are stable.
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