Red histogram: stability of the LED flux (i.e. the NIST photodiode current) after correction for the cooler-brighter effect (and for the backend board temperature variations). Blue histogram: stability of the LED channels after correction for the control photodiode measurements. The two outliers above 5 × 10^-3 are LEDs in the blue histogram that correspond to channels that are intrinsically unstable. The outliers in the red histogram correspond to the same LEDs, plus one LED whose control photodiode is faulty.