The Insert Photometric Mask command is used to define an area in a calculation area where the illuminance values should not be calculated.
To insert a mask on the drawing, go to
Ribbon: DM Electrical->Point-by-Point Photometrics-> 
Insert Photometric Mask
Pulldown Menu: DM Electrical->Point-by-Point Photometrics->Insert Photometric Mask
You will be prompted to specify the starting point of the mask.
Specify points for mask:
Specify start point:
You will then be prompted to specify the next point of the mask.
Specify next point:
You can specify additional points or press ENTER to complete the mask. The mask will be displayed on the drawing. The illuminance levels inside the area will not be calculated or displayed.