An instrument may appear to give an inaccurate reading. Many times the error can be caused by the sample presentation. It should be noted that a bad calibration can also influence instrument readings.
The calibration plaque should be cleaned regularly and stored when not in use. Clean the white calibration plaque with a clean lint free cloth. If needed use deionized water. It should also be noted that the instrument and the calibration plaque are married to each other and care should be taken to keep the correct plaque with the correct instrument.
When reading samples make sure that the presentation of the sample is consistant. For example keep the unit in the same direction when taking measurements, Variations in the sample surface, coating and shape can cause variations in color, or appearance of the sample.
The alignment of the target window is very important. It determines where and how you "look" at your sample. It can also be an obstruction to the viewing area if it isn't aligned.
Consistency cannot be over stressed. Good practices will save time, and money.