Illumination device, optical element and luminaire

An illumination device comprising a support structure having a first side with mounted thereon:

• a plurality of spatially separated solid state lighting elements disposed in an array, and
• an optical element made of a solid, light transmissive, light diffusing, foam material,
• wherein the optical element is provided with a pattern of concaves, essentially each concave being formed by:
• a recessed portion of the concaves comprising a closed bottom wall,
• a circumferential side wall extending from the closed bottom wall to a light exit window, the light exit window bordering an apex of an elevated portion of the concaves and opposite to the closed bottom wall, and
• wherein the plurality of spatially separated solid state elements is arranged outside and upstream of the concaves and conformal to said pattern of concaves.