在研究双折射现象中非寻常光行为及折射率的新结果的［1，2］基础上，建立了准确的光程差模型，给出了无Δn＝no-ne≤no(或ne)近似的双折射滤光片产生位相差的精确公式。新光程差模型正确地描述了调谐滤光片时各段光程的位置和大小。新公式不仅适用光轴平行表面的也适用光轴倾斜表面的滤光片。由于调谐滤光片时光轴倾斜的滤光片折射率变化比平行的大，所以，前者具有比后者更宽的调谐范围。应用新公式，设计了一块调谐钛宝石激光器的光轴倾斜表面的双折射滤光片。

Based on the new results of studying extraordinary rays′ behavior and the refractive index of the extraordinary wave in uniaxial crystal , an exact model of the optical path difference yielded by birefringent filters is set up. Then the versatile and precise formula for the filters is derived without the approximation Δn=n o-n e≤n o (or n e). The new model shows the correct position and dimension in each path while the filter is tuning. The formula is applicable not only to the filter made of the crystal with the optical axis parallel to the surface (FAPS) also to the one with the axis incline to the surface (FAIS). It is pointed out that the FAIS enables to tune laser wavelengths in a wider range than the FAPS. The origin of the wide range is interpreted as the variation in the index of the FAIS is larger while tuning the filter. With the help of the formula, a FAIS for tuning a Ti:sapphire laser is presented.