An illumination system, comprises a radiation source and a monolithic ceramic luminescence converter comprising at least one phosphor capable of absorbing a part of light emitted by the radiation source and emitting light of wavelength different from that of the absorbed light; wherein said at least one phosphor is an alkaline earth metal sulfide of general formula AE1-zS1-ySey:Az , wherein AE is as least one earth alkaline metal selected from the group of Mg, Ca, Sr and Ba, 0 = y <1 and 0.0005 = z = 0.2, activated by an activator A selected from the group of Eu(II), Ce(III), Mn(II) and Pr(III). is highly efficient, especially if a blue light emitting diode is used as a radiation source, and provides excellent thermal and spectroscopic properties. The invention is also concerned with a monolithic ceramic luminescence converter comprising at least one phosphor capable of absorbing a part of light emitted by the radiation source and emitting light of wavelength different from that of the absorbed light; wherein said at least one phosphor is an alkaline earth metal sulfide of general formula AE1-zS1-ySey:Az, wherein AE is as least one earth alkaline metal selected from the group of Mg, Ca, Sr and Ba, 0 <= y <1 and 0.0005 <= z <= 0.2, activated by an activator A selected from the group of Eu(II), Ce(III), Mn(II) and Pr(III).