Beam Angle Diagram. Choosing the right type of beam angle can give your building the right type of ambience and visibility. In other words we find the center of the beam and take an intensity measurement.
The gamma is the elevation angle and gamma00 corresponds to a ray of light pointing downwards. That angle is the half maximum angle. That is how i remember it.
As a mnemonic remember that a field of corn for example is much bigger than a beam of wood in a barn on the same farm.
Here is an example from oxytechs. The beam angle specifically refers to any light that falls within 50 of the maximum intensity the bulb is capable of producing. The tables below give equations for the deflection slope shear and moment along straight beams for different end conditions and loadings. That angle is the half maximum angle.