As TAPPI Test Method T 452 (Brightness of pulp, paper and paperboard) states:
- "Obtain reflectance standards at monthly intervals and calibrate the test instrument with them as soon as they are received."
- "Check the test instrument readings at least monthly against the assigned values of all the standards."
|Graph 1 - Good Calibration|
In Graph 1, the calibration was performed with the 70-range standard. Then it was compared against the other standards at the different points e.g. 50, 60, 80 and 90. It is right on all of these points. This verifies calibration across a large range.
|Graph 2 - Bad Calibration Using 70-range|
|Graph 3 - Bad Calibration Using 80-range|
If single-point calibration is used and there is no means to verify the linearity of the measurement, we may run into a situation like Graph 2 or 3. In these cases the instrument responds in a linear manner, but the calibration is not correct. This could be due to a number of other issues such as spectral response, failing electronics or failing optics, to name a few.
|Graph 4 - Bad Calibration with Non-linear Response|
Also, as seen in Graph 4, the calibration was performed with the 80-range standard. Then it was compared against the other ranges. This example shows a non-linear response. This would not be seen if only the 80- and 50-range standards were measured.
ALWAYS use more than one range to verify your measuring devices. If you see responses like those in Graphs 2, 3 and 4:
1. Recalibrate you device
2. Get fresh calibration standards (or get them recertified)
3. Contact a factory-certified service technician for support