Hi Linda, Maybe this will

Hi Linda,

Maybe this will help…

“The actual ‘BEAM ANGLE” of a fixture is defined as the angle at
which central intensity (in candelas/candlepower) drops to 50 percent.”

from

Stage Lighting Design PART 6 – Lighting Mechanics by Bill Williams

http://www.mts.net/~william5/sld/sld-600.htm – every durn thing you want to know about beams and angles etc.