From a NIST report titled Measurement Science Needs for Real-time Control of Additive Manufacturing Powder Bed Fusion Processes:
Finally, metallic debris from the [heat affected zone] can coat a window or viewport used in an AM imaging system, and disturb temperature measurements by changing the radiation transmission through the window. This is particularly troublesome in electron-beam melting (EBM) systems, and prompted Dinwiddie et al. to create a system to continuously roll new kapton film over the viewport in order to provide new, unsullied transmission.
This is a very important and totally nontrivial challenge. Measuring process signatures (which this report defines as "the dynamic characteristics of the powder heating, melting, and solidification processes as they occur during the build") is key to the industrialization of additive manufacturing. If the systems we have for measuring those factors are unreliable, machine manufacturers need to develop improvements for them ASAP.