This technology relates to a full spectrum broad wavelength emission white light source fabricated using a graded composition optically clear substrate that enables high efficiency, high flux, narrow or wide spectral width, large area, low cost LEDs with peak emission wavelength in the range of visible wavelength range from 400-750 nm. The technology also provides a platform technology and pathway for rapid development of LEDs with a multitude of emission wavelengths across the entire visible range at significantly lower costs using inexpensive equipment (versus direct emission LEDs based upon epitaxial technology). This device structure is based on wavelength conversion principle like that used in traditional phosphor LEDs where photons from a blue or a UV emitter will be absorbed by a phosphor material resulting in emission of longer wavelength photons. The phosphor substrate, however, is optically clear as opposed to phosphor layers used in LEDs. Unlike phosphor powders, the phosphor substrate is grown as a crystal from melt as single crystallographic phase poly-crystalline and single crystalline ingots or thick films. Crystallographic purity enables spectral width narrowing, highest possible quantum efficiency and optically transparent (scattering free) films and substrates. Design and fabrication of high color rendering index sources, high color gamut sources or high efficacy light sources are enabled.