NIR instruments working in reflectance analyze first a reference (you can consider it as a blank) and after, the reference move from the path and the light goes to the sample to get an spectrum corrected versus the reference. So the reference is an important part of the instrument, so we must check their performance in the future respect to the present or the past.
In a spectrum of the reference we have included many things, like the state of the optics, the detector´s response, state of the lamp,.....
In some instruments the reference is exposed to external conditions (humidity, dust,...) more than others. NIR Companies are conscious that the reference spectra must be as much stable as possible during the NIR instrument life, so now the reference ceramics are more protected and they are more stable all along the instrument life.
An external reference will help us to check the spectrum of the internal reference, and correct, when necessary, the differences. This external reference has a file with the spectra of the difference between the external reference itself and a master standard. So all the instruments can be reference corrected to have the same reference spectrum. This is known also as reference standardization or External Reference Correction. This technique will help to transfer in a better way the calibrations from an instrument to other.
An example is after an instrument repair (changing for example the monochromator which include the optics and detectors), if we dont run the ERC the spectrum of a sample, like the Check Cell (which is quite stable over time), will look different before and after the repair, and the predictions are quite different for some of the parameters. If we scan the external reference and correct the spectrum using the file, the spectrum of the check cell will change to overlap their shape previously to the repair (same for the predictions).