NIR instruments need to be trained, or calibrated, for the measurements you want to perform. Often instruments come pre-calibrated from the factory, but understanding some basics of NIR calibration techniques can be helpful for users.
NIR is a secondary form of analysis and to calibrate an instrument you need to measure a number of samples both by the NIR instrument you want to use, and a reference method. The data from these measurements is entered into a software which finds the relationship between the infrared information from the samples, and the analysis results from the reference method. This relationship which is called the calibration model, is expressed as an algorithm, or mathematical equation, which the instrument uses to predict measurement results when analyzing unknown samples.
Several mathematical techniques are used to derive these algorithms, ranging from very simple ones to very advanced. There is no calibration technique which is inherently better than others, but rather it depends on the situation.