我们可以把一个实际空间，看作是由很多个小立方体组成的（和FDTD里面经常用到的YEE网格是一致的）。任一实际物体，都可以通过许多小立方体组合近似，即用许多立方体对实际物体进行建模，称之为阶梯近似。-We can put an actual space, as many a small cube, composed of (and frequently used inside the FDTD grid YEE is consistent). Any one of the actual objects, to pass through the combination of many small cube approximation, which uses many of the actual object cube model, known as the ladder approximation.