Optical element, lighting device and luminaire

A lighting device comprising:

• a plurality of solid state lighting elements in a circular pattern;
• and
• an optical element comprising a central cavity extending from a first aperture to a second aperture of the optical element, said central cavity gradually widening in a direction from the first aperture to the second aperture, said central cavity being delimited by a stack of transparent frustums including a first frustum defining the first aperture and a final frustum defining the second aperture, each frustum having a prismatic outer surface having a first surface portion tapering in the direction from the second aperture to the first aperture and a continuous inner surface, wherein the respective continuous inner surfaces combine to delimit said central cavity;
• wherein the first frustum is placed over the solid state elements such that the solid state lighting elements are arranged to emit light into the first frustum;
• and
• wherein the width of the respective transparent frustums as defined by the average distance from its continuous inner surface to its prismatic outer surface decreases in the direction from the first aperture to the second aperture.