# VS Card

This card specifies known visibility information (required when using physical optics with multiple reflections) to reduce the calculation time.

On the Solve/Run tab, in the Rays group, click the  Physical optics icon. From the ドロップダウンリスト, select the  PO visibility (VS) icon.

To accurately compute multiple reflections Feko needs to determine which basis functions are visible to each other. Since this applies to all the PO triangles it may be very time consuming for large problems. The time required to determine the visibility may be greatly reduced if the user can specify which triangles are visible or hidden from each other.

## Parameters:

Triangles labelled
The label of the source triangles.
are visible from
All triangles with the label specified in the field Triangles labelled are visible from all triangles with label(s) indicated in the fields below.
are not visible from
All triangles with the label specified in the field Triangles labelled are not visible from all triangles with label(s) indicated in the fields below.
(specify range of labels)
If this item is unchecked, only a single label is specified (in triangles with label). If checked, the card applies to all triangles with labels in the range from the value specified in triangles with label to those in to triangles with label.
Note that visibility is reciprocal, meaning if all triangles with label n are visible from all triangles with label m, all triangles with label m are visible from all triangles with label n as well.

Basis functions cannot illuminate each other if all the triangles they are attached to lie in the same plane.

The VS card should only be used if the user can specify the visibility beyond any doubt and if it applies to all triangles of that label. If no information is specified for a specific combination of labels/triangles, full ray tracing will be executed.

The figure below depicts a structure consisting of four flat plates and a cylindrical section. The two plates orientated at 45 degrees to the coordinate system (labelled 1 and 3), are half as wide as the plates with labels 0 and 2. Thus a subset of the triangles with label 2 are visible to triangles with label 0, but not all.

Visibility information should be specified starting at label 0, then label 1, and so forth. VS cards should be specified as follows:
• Triangles labelled 0 are not visible from triangles with label 0.
• Triangles labelled 0 are visible from triangles with label 1.
• Triangles labelled 0 are not visible from triangles with labels 3 to 4.
• Triangles labelled 1 are not visible from triangles with label 1.
• Triangles labelled 1 are visible from triangles with label 2.
• Triangles labelled 1 are not visible from triangles with labels 3 to 4.
• Triangles labelled 2 are not visible from triangles with label 2.
• Triangles labelled 2 are visible from triangles with label 3.
• Triangles labelled 3 are not visible from triangles with label 3.
• Triangles labelled 3 are visible from triangles with label 4.

The VS cards are realised with code section below.

VS: 0 : 3 :  : 0
VS: 0 : 1 :  : 1
VS: 0 : 4 :  : 3 : 4
VS: 1 : 3 :  : 1
VS: 1 : 1 :  : 2
VS: 1 : 4 :  : 3 : 4
VS: 2 : 3 :  : 2
VS: 2 : 1 :  : 3
VS: 3 : 3 :  : 3
VS: 3 : 1 :  : 4

Since all the triangles with label 0 lie in the same plane, they cannot illuminate each other. Thus the first card states that label 0 is hidden from label 0.

All triangles with label 1 are visible from all triangles with label 0. This is specified by the second VS card. Since a subset of triangles with label 2 are visible from triangles with label 0 while others are hidden, any information for this combination of layers cannot be specified. However, the plate with label 2 shadows all triangles with labels 3 and 4 and it can be specified that these are hidden. This is done with the third VS card. Note that this card specifies a range of hidden labels.

Next specify which triangles are visible (or hidden) from all triangles with label 1. As for label 0, triangles with label 1 are not visible to each other, specified by the fourth VS card. All triangles with labels 0 and 2 are visible from all triangles with label 1. Since the visibility between labels 0 and 1 has already been specified, it does not have to be specified again. The fifth VS card then specifies that label 2 is completely visible from label 1. As for label 0, both labels 3 and 4 are hidden completely which completes the first six VS cards.

Next consider label 2. As before labels lower than 2 need not be considered. Also the label is hidden from itself as indicated by VS card number seven. Next it can be stated that label 3 is visible, but nothing can be specified about label 4 as not all of these triangles will be visible.

Similarly VS cards 9 and 10 state that label 3 is not visible to itself and fully visible to label 4. Finally consider the case for triangles with label 4. All visibility with layers 0 to 3 has been specified and may not be specified again. Unlike the previous flat plates, layer 4 is curved and triangles may indeed illuminate other triangles with the same layer. However, not all other triangles will be illuminated (this is only possible for a doubly concave surface), so no information can be specified for label 4.