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The actual weld thickness is observed directly by optical microscopy ( OM ) characterization of cross sections of PE-AM samples .
 process parameter: weld thickness; process characterization: optical microscopy; process characterization: OM; concept or principle: cross sections; material: PE-AM
Given options, please tell me the categories of all the listed entity words.Output format is "type1: word1; type2: word2". 
Option: manufacturing standard, machanical property, manufacturing process, material, engineering feature, process parameter, machine or equipment, application, process characterization, enabling technology, biomedical, concept or principle 
Text: {0} 
 Entities: weld thickness, optical microscopy, OM, cross sections, PE-AM 
Answer:
 process parameter: weld thickness; process characterization: optical microscopy; process characterization: OM; concept or principle: cross sections; material: PE-AM