# is Farfield fileter is spatial filter? and can i use it in periodic boundary condition?

#1

I’d liked to get color angular distribution in multilayer structure like OLED.
one of encapsulation layer is thick enough to make interference pattern in color spectrum.
To avoid interference pattern, I put my monitor into encapsulation layer and I set material index in farfield script.

but in the result, there is intensity at 90 degree, and I applied farfield filter=0.5 to the monitor and the result seemed reasonable. but I can’t understand that I can use farfield filter in periodic boundary.

Please explain about farfield filter in periodic boundary and let me know the way to get rid of the interference pattern in farfield result (angular distribution).

#2

Hi @kilbock.lee,

Please note that the far field filter option should not be operated for periodic structures. This option has to be zero when you activate the ‘assume periodic’ option. You can refer to this KX page. If your model has periodic nanostructures like a grating, then you’d better take into consideration the grating projection functions instead of the far field projections.

The periodic boundaries is not recommended for the OLED simulation with a dipole source. Although the structure has the periodic geometry, the periodic BCs indicate that the dipole sources are periodically located to radiate in a coherent manner. As a matter of fact, the sources irradiate incoherently, which requires several simulations with a single dipole source for each simulation. For more information on the OLED simulation, please refer to this KB website.

I wish it would be helpful for you.

#3

Thank you for reply.
I didn’t use periodic structure. it is a just multilayer structure but I applied periodic boundary to reduce computing resource and to detect high angle distribution.
So, Periodic boundary condition isn’t appropriate far field projection even the structure is not periodic structure?

#4

Hi @kilbock.lee,

To apply the PBCs in a FDTD model, both structure and light source should be periodic. In OLED application, the underlying nanostructures are periodically arranged, whereas in reality the dipole source is incoherent without a periodic nature.

As I mentioned, “field filter option” should not be used for the periodic system. Therefore, it is recommended to employ the PML BCs instead of PBCs in the OLED application, with considering the use of “field filter option”. Although you don’t apply the PBCs in your model, in this case you might need several periodic structures because the OLED system has the periodic architectures physically.

 A far field projections are typically used for isolated structures rather than periodic arrays. However, it is possible to use the projection functions to calculate the approximate far field distribution of finite sized periodic arrays.
So, you can use the far field projection for a periodic structure, but the “field filter option” is not allowed to use for the periodic system.