Light source with diffractive outcoupling

An illumination device comprising:

• a plurality of solid state light sources and
• a concentrating wavelength converter arranged to inject light from said light sources through at least one entrance surface and to extract wavelength converted light from at least one exit surface,
• wherein said wavelength converter is configured to provide total internal reflection of converted light when an incidence angle at a side surface is between an angle A and 90 degrees, where the angle A is determined by the refractive index of the converter and the surrounding media, respectively, and wherein the geometry of said wavelength converter is such that an incidence angle of converted light at said exit surface, after total internal reflection in said side surface, is in a range including angles greater than the angle A, wherein the angle A is defined as A=a sin(nsurr/nconv) where nsurr is the index of refraction of the surrounding media of the wavelength converter and where nconv is the index of refraction of the material forming the wavelength converter,
• a structured layer provided on said exit surface, wherein said structured layer has a structure period less than 5 micrometers, so as to enable out-coupling by a combination of refraction and diffraction, and the index of refraction of the material forming said structured layer is equal to or greater than nconv−0.3.