Spatial Coherence or Lateral Coherence: To understand spatial coherence consider two points P1 and P2 which, at time t = 0, lie on the same wave front of some given electromagnetic wave and let E1(t) and E2(t) be the corresponding electric fields at these points. According to definition the difference between the phases of the two fields at time t = 0 is zero. If this difference in phase remains zero at any time t > 0, we would say that there is perfect coherence between these two points i.e., we will say that the wave has perfect spatial coherence. Thus if the electric fields at any two different points on the e.m., wave front has constant phase difference over any time t, then perfect coherence is said to occur. The concept of spatial coherence can be understood by Young’s double slit experiment as shown in Fig. 2. Here S is a point source, S1 and S2 are equally spaced pinholes.