Borescopes with diameter less than or equal to 2mm, but larger than 1mm. 
One application of these small diameter borescopes is to exam the internal channels of medical endoscopes, especially for reprocessing endoscopes to identify debris and damages inside working channels and cleaning verifications. More info here:   ​​https://www.mobileaspects.com/the-importance-of-visual-inspection-for-endoscope-reprocessing/