Wide-area solid-state illumination devices and systems

An illumination system, comprising:

• a thin and flexible sheet of an optically transmissive material having a first broad-area surface, an opposing second broad-area surface extending parallel to the first broad-area surface, a first edge, and an opposing second edge;
• a first plurality of compact solid-state light sources optically coupled to the first edge;
• and
• a two-dimensional pattern of first discrete light extraction features formed on the first broad-area surface with variable spacing, at least one of the first discrete light extraction features comprising a semi-opaque layer of a light scattering material,
• wherein the light scattering material comprises an optically transmissive binder material and submicron-sized particles embedded into the binder material and having a greater refractive index than the binder material, wherein the semi-opaque layer has a variable thickness characterized by a curved cross-sectional profile, and wherein a ratio between a longest dimension of said at least one of the first discrete light extraction features and a maximum thickness of the semi-opaque layer is greater than 5.