An inspection apparatus is provided that comprises an optical target including a solid host material and a fluorescing material embedded in the solid host material. The solid host material has a predetermined phonon energy HOSTPE. The fluorescing material exhibits a select ground energy level and a target excitation (TE) energy level separated from the ground energy level by a first energy gap corresponding to a fluorescence emission wavelength of interest. The fluorescing material has a next lower lying (NLL) energy level relative to the TE energy level. The NLL energy level is spaced a second energy gap FMEG2 below the TE energy level, wherein a ratio of the FMEG2/HOSTPE is three or more.