A method of diagnosing optical inhomogeneity of a volume scattering object is provided. The method includes the steps of providing a substance coated with phosphorous materials, adapting the substrate to a surface of a volume scattering object to be diagnosed, and trans-illuminating the volume scattering object with radiation. A dental tool including a substrate is also provided. The substrate includes a transparent flexible film or metallic foil with an applied coating of upconversion phosphors, afterglow particles or both.