There is an analysis group in the Object library for calculating the far field from a closed box (scat_ff). This type of monitor is particularly useful for particle scattering. For example, if you have some structure in a homogeneous medium and you want to look at the far field for the light scattered by the structure, you can simply enclose it in the scat_ff box monitor. However, you must be careful to enclose everything that is important for the scattered fields in the box monitor. Therefore, in some cases this is not the right monitor for looking at the far field. For example, if you have a substrate that extends infinitely, the right monitor to capture the far field is a monitor on top or below your structure as you can see here:
In this example we used a TFSF source (grey box in the diagram), which is ideal for particle scattering; see:
The simulation span in the x direction (parallel to the substrate surface) is quite wide compared to the scattering object in order to collect light scattered at steep angles. For the same reason we also bring the monitor as close as possible, making sure the monitor does not overlap with the source region (marked by the gray area).
Some care is required when using TFSF sources. As you can see here, it is important that the box representing the source encloses the scattering object completely and that there is no overlap between the object and the gray area . Also, the box must be large enough in the injection direction so that the box includes part of the substrate.