Abstract: With incident light of  different angles,the conditions for the phenomenon of spectral line absence occuring and being observed are theoretically deducted.By the means of numerical simulation,the condition for the phenomenon of spectral line  absence under the vertical incidence  is verified.And  with incident light of with different angles the spectral line  absence   is compared.The results show that the phenomenon meets  j=kd/b.The difference between the two incidences is that the observed range of the spectral line’s absence striation is different to some extent under the same experimental conditions.

Key words: grating diffraction, spectral line absence, conditions for absence