A lighting device of the type mentioned in the opening paragraph is known from the patent publication U.S. 2010/0096974-A1. This document describes a device comprising a LED having an encapsulant, which acts as a wavelength converting element. Said encapsulant is formed as a shell of a suitable plastic or glass, in which a phosphor or a phosphor blend in particle form is dispersed. Said phosphor (blend) acts as a luminescent material that absorbs radiation energy in a portion of the electromagnetic spectrum and emits radiation energy in another portion of the electromagnetic spectrum. In this manner, (part of) the light with a primary wavelength emitted by the LED is converted into light with another (secondary) wavelength.
The inventors have found that phosphors with metal-ion activators which are to be excited via a partially forbidden absorption band show limited interaction with light of an appropriate wavelength. Such forbidden absorption bands are especially found in phosphors having a transition metal-ion activator in the host lattice. In these metals, specific d-d electronic transitions are forbidden. Such phosphors show relatively low absorbance upon interaction with light having a suitable wavelength. Nevertheless sufficient light conversion of the primary wavelength can be achieved when using relatively long interaction paths of the light with the primary wavelength through the converting element. In practice this solution implies rather thick elements. However, such long interaction paths usually gives rise to undesired large scattering losses.