Definitions to describe process control

off-line -  Manual sampling from the process sent to the laboratory for NIR  analysis. In this case (normally) one or a few samples are analyzed for every batch, and when we get the result it can be late to change  the regulation of the process, with all the disadvantages that it means (re-process all the batch,.....).

at-line - Manual sampling from the process analysed in a NIR, sited close to the production line. It is normally used by the plant operators. It can be installed in the process control room, and the operators scan samples when they (based on their experience in the process) consider. Response to problems in the process is quicker than in the off-line case.

on-line - Automated sampling sent  through a sample line (by-pass) to be analyzed by a process NIR. This system is more used in liquids where there are a lot of issues related to the pressure, vacuum, and requires of a well design of the bypass to remove bubbles and other problems to get a representative spectrum.
We can use this system to carry flour or feed, by gravity to a bypass cell (when a valve opens) this way it can be analyzed in transmittnce or reflectance  after the analysis another other two valves can open, one in case we want ro return the sample to the process pipe or other to take out the sample for laboratory reference analysis. This can be a way to get a more representative sample with more correlation with the spectra acquired, but the disadvantage is that the percentage of sample analyzed for the batch  is less than in the in-line system.

in-line - A fiber optic probe connected to a process NIR, and inserted into the process line (Reactor, pipes,..), it can work in transmitance or transflectance . It can be also a process  NIR window placed in the process line  to work in difusse reflectance.