Dear Ishwar,

Internal quantum efficiency (IQE) is defined as the proportion of the total number of photons emitted by the phosphor (fluorescence) vs the number of excitation photons (typically 405/455 nm) that are absorbed and not reflected by the phosphor.

external quantum efficiency (EQE is defined as the proportion of the total number of photons emitted by the phosphor (fluorescence) vs the number of excitation photons that are emitted by the excitation light source.

Formulaically, we define IQE as:

IQE = fluorescence / excitation – reflection

EQE = fluorescence / excitation

Fluorescence: total number of photons re-emitted by the phosphor

Excitation: total number of photons emitted by the excitation light source

Reflection: total number of photons reflected and not absorbed by the phosphor.

As such, we see that the IQE measures the efficiency performance of the phosphor post-absorption and is not concerned with the amount of light that is reflected, while EQE measures the efficiency performance of the phosphor including absorption efficiency. IQE can be considered to beinternal since it is concerned with the efficiency of the phosphor performance within the fluorescence process, while EQE can be considered to be external because it also considers the efficiency of the phosphor performance in absorbing an external excitation light. Of course, as a phosphor manufacturer, a higher, as well as a more consistent, quantum efficiency is desired. This distinction is very useful for phosphor performance measurement because IQE provides information about the quality of the phosphor itself, while EQE provides further information about the absorption performance, which is typically concerned with the composition of the phosphor and possible changes that may be required. Together, these two metrics provide a standardized method of evaluation for any phosphorescent material.

http://www.yujiintl.com/nims-standard-reference-phosphor

Hoping this will be helpful,

Rafik