(Term of photometry)
The Beam Angle is the angle between the two directions opposed to each other over the beam axis for which the luminous intensity is half that of the maximum luminous intensity.
The luminous intensities are measured in a plane normal to the nominal beam centerline. If the beam is not rotationally symmetric, then the beam angle is usually given in two planes at 90° of each other, possibly the maximum and minimum angles. Other angles (eg. at 45°) may also be given.
The Field Angle is the angle between the two directions opposed to each other over the beam axis for which the luminous intensity is 10% that of the maximum luminous intensity. Note that in certain fields of applications the field angle was formerly called beam angle.
The Beam Lumens are the total luminous flux that is emitted within the region of space, where the luminous intensity is more than half the maximum luminous intensity. With most light source, this region can be described as a cone shaped by all beam angles.
The Beam Spread is a general term, describing the angle between the two directions opposed to each other over the beam axis for which the luminous intensity is a certain fraction of that of the maximum luminous intensity. The amount of that fraction needs to be given in each specific case.
References: 
beam axis luminous intensity luminous flux photometry 

 
English  German  
beam angle  Der Strahlwinkel  
field angle  Der Feldwinkel  
beam spread  Die Strahlbreite Die Spreizung 