The plane angle of diffusion describes the rotation-symmetrical light distribution of a spot or flood (DIN 5037, Sheet 1). It is formed by two beam angles of equal Luminous Intensity within a plane. Luminous intensity within this angle of diffusion must be higher than the luminous intensity selected.
In many cases the beam pattern of a spot or flood is described in terms of one-half peak or one-tenth peak divergence. This is the term given to the aperture angle at which luminous intensity sinks to half or one-tenth maximum (cf. Polar Curve, Illuminance Cone). In this context, DIN 5037, Part 4 describes one-half-peak divergences of up to 5° as ultra-narrow beam, from 5° to 10° as narrow-beam, from 10° to 50° as wide angle and above 50° as extra-wide angle.
The useful angle of diffusion is the plane angle characterised by the Luminous Intensity, Luminance or Illuminance required for a defined task. (DIN 5037, Sheet 1).