The importance of choosing the ratio of the width of light transmitting slit to that of interference fringes is dealt with.It is also pointed out that the largest signal contrast and the largest signal amplitude can be obtained when the width of interference fringes approaches infinite.Besides,a relation between the signal contrast,the signal D.C.shift and the error of counting pulse interval is given Accordingly,the quality requirments for photoelectric output signals can be determined through the tolerance of counting pulse interval.