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Prior to collecting ultrasonic data for a selected test specimen , an optical flat reference standard was used to characterize the ultrasonic probes and to quantify the system ’ s mechanical stability , repeatability , and accuracy when measuring the physical dimensions of features .
 data, optical, standard, probes, s, mechanical, repeatability, accuracy, dimensions
Please list all entity words in the text that fit the category. Output format is "word1, 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} 
Answer:
 data, optical, standard, probes, s, mechanical, repeatability, accuracy, dimensions