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Monday, August 25, 2014

Calibration: It's important, but am I doing it right?

Our primary customer contact at a facility in the U.S. was retiring. He called and asked if we could come in and provide a training session on color measurement. At the same time, he felt it would be good to review calibration and use of their Color Touch PC.

During the session some of their product was measured on their instrument. It was a blue sample. The reflectance curve looked like the red curve to the right.  It was obvious to me that something was wrong with the curve since it seemed 'chopped' off at the longer wavelengths. Normally, there would be at least some very low reflectance values beyond 540 nm.

When we asked for the black cup, they didn't know what that was. After explaining that the black cup is a machined black cavity which yields a very low reflectance. They looked around in the lab and found the black cup (left).  We used fresh calibration standards and the black cup to calibrate the instrument. 

This calibration procedure ensures that the following are correct: 
  • photometric linearity - the amount of reflectance (zero, 100% and points in between are correct)
  • spectral response - the energy level at specific wavelengths
  • fluorescence level - the amount of fluorescing resulting in whiteness levels for UV Level D65 and C

After calibration the standards were reread to verify that the calibration achieved the correct results for the standards. Then the customer product was measured again. The spectral curves below represent the correct reflectance curve (in blue) and the incorrect reflectance curve (in red).

This customer was not calibrating with the black cup. They were probably calibrating with the sample plunger in place (which has a higher reflectance than the black cup).  Also, they were using outdated paper calibration standards.  These are common mistakes for users.  Monthly calibration standards typically cost between US$1,000-2,000. This is far less than any customer claim on product that is sent back or rejected because the measured values are incorrect.

Make sure your standards are current and your procedure is correct by walking through this with a certified Technidyne Service Technician.