The optical image can tell you about problems, for example, with the condition of the prism.
Y-axis is light intensity and X-axis the position of the shadow edge. RI calibration follows the X-axis. The position of RI measurement on X-axis is at the steepest curve drop, being the highest angle at the corner.
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Optical image with a reference RI liquid 
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Erosion of the prism 
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No sample 
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Process liquid is leaking into the refractometer 
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Prism coated 
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CCD card is disconnected or faulty, or LED card is disconnected or faulty, or the optical fiber is broken 
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Good
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Compromised
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Heavy scaling
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