在低级晶族光学特性的研究中，光性符号的确定对未知晶体及矿物的鉴定有着重要的意义。从折射率面、光轴角出发，借助数学工具理论上分析了低级晶族光性正负判别的两种方法在临界点的不一致性，从而说明在一定范围内用这两种方法判断光性会得出不同的结论。并给出修正后利用主折射率判别方法在临界点附近的精确表达式及其判别条件，结果表明，在临界点附近要用修正后的表达式判别。

In the study of optical properties of low class crystal system, the identification of optic sign has important meaning to unknown crystal and mineral determination. From the refractive index and optical angle, with the mathematics tools, two methods are theoretically analyzed in this paper. They are not consistent at the critical points of biaxial crystal. So there would be different conclusions by using the two methods to judge the optical properties in certain ranges. A precise expression near the critical point after revision is given as well. The result shows that the revised expression should be used near the critical point.