The beam angle of a light source is defined by the size of the angular region where luminous intensity is more than or equal to 50% of it's peak luminous intensity or CBCP (Centre Beam Candelas). Beam angle is not to be confused with field angle, which refers to the angular region where luminous intensity is more than or equal to 10%. Based on these definitions, a light's field angle can be significantly greater than it's beam angle, since it includes a greater region of weaker, unusable light, in it's calculation. Occasionally manufacturers may specify their product's field angle or even it's overall light emission angle rather than it's beam spread, confusing and disappointing customers who were expecting a wider distribution of usable light.
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