The message "Clean Prism" appears when the prism surface of your SmartRef is not clean enough for an accurate measurement.
SmartRef measures the refractive index directly on the prism surface. If there are any residues such as dried sample, sugar, oil, fingerprints, or scratches on the prism, the measurement can be negatively affected. In this case, the device automatically detects the issue and displays the "Clean Prism" message to ensure reliable results.
To resolve this, clean the prism and sample well thoroughly. Use distilled water or a few drops of ethanol and wipe the prism gently with a soft, lint-free cloth. Additionally, check your prism for any visible scratches.