Next step is to place a detector object at some distance from the source to study the irradiance distribution at that location.

Make the 3rd object in the editor a “Detector Rect” and enter the following parameters.

Z position:   800
Material: Blank (do not type the word "Blank" but leave the cell empty)
X Half Width:  150
Y Half Width:  150
# X Pixels:  150
# Y Pixels:  150
Color:   1 (detector displays inverse greyscale)
All other parameters as default

The YZ plane view (default) of the layout shows:

Observe that the layout shows the rays going though the detector. The detector is totally transparent since the material type is air (blank in the editor).