FAQ: “We take the results from the Didymium Filter Test on our HunterLab ColorQuest XT and plot them in an SPC chart over time to look at trends. We are seeing the 430 and 570 nm filters slowly decrease. Can you help us understand what may be occurring?”
The Didymium Filter Test is an instrument diagnostic test designed to allow the user to self-certify that the wavelength alignment of the instrument optical path is consistent over months and years since the sensor was last wavelength calibrated at the factory.
Typically two points are monitored for the HunterLab ColorQuest XT, ColorQuest XE and UltraScan VIS – one at the blue end (430 nm) and one at the red end (570 nm). As long as the results are within test norms, the wavelength calibration of the instrument is considered to be within manufacturer’s specification.
The CQXT tolerance at 430 nm for the didymium filter is +/- 1% represents the signal from a single pixel on the detector when reading the saw tooth pattern of the didymium filter. This 1% tolerance represents a shift of about 0.2 nm which is very sensitive in verifying wavelength consistency over time on an instrument that reports 10 nm spectral data.
I notice that your SPC chart indicates rule violations but the results are still well within the +/- 1% tolerance at 430 nm for this test. I assume the rule is based on a statistical violation of multiple data points going slightly down in one direction indicating a lack of randomness even if the amount is too low for significance in the instrumental measurement of color.
I suggest you clean the didymium filter on both sides with a Kim wipe moistened with IPA (isopropyl alcohol) and continue to monitor both the 430 nm and 570 nm points over time. If the shift is systemic and becomes significant, your SPC results will continue towards the lower limit.