beam angle tolerance in glideslope indicators
 

Hi I am researching glideslope indicators for ship borne helipads. there are typical angles given in such devices e.g. 8.5 to 4 degrees , 4 to 3 degrees and 3 to 0.5 degrees. I believe the beam centreline is defined as the centre of each group of these angles. I have searched the web but cant find any reference that defines these angles or gives the tolerance on such angles. I have a background in optics and its not possible for angles to have top hat profiles i.e. drop to zero at each beam boundary . I anyone can clarify this for me or can point me someone who can I would appreciate it. many thanks in advance . Best regards
spaceman