The illuminance cone is used to illustrate the beam patterns of rotation-symmetrical luminaires (OPTOS downlights and spots). It represents the light bundle emitted at One-half-peak Divergence or Half-value Angle. This is the term used for the aperture angle at which maximum Luminance is halved (cf. Polar Diagram). In general the maximum value is achieved at a beam angle of 0°. 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 illuminance cone diagram also shows mean illuminance available within an area delimited by the half-value angle (light cone diameter) for various mounting heights (cf. Height above Working Plane). These data are used for approximative lighting planning calculations.