Silicon photodiodes are known to decrease more rapidly in response to blue light than to longer wavelengths (Korde and Geist, 1987). Older quantum sensors thus appear to calibrate accurately using the LI-COR 1800-02 Optical Radiation Calibrator, but may under-report radiation in the 400 – 500 nm wavelengths (blue) due to degradation of the silicon photodiode in the sensor.
Hay, Alec and Bugbee, Bruce, "Downward Blue Drift of Quantum Sensors" (2007). Techniques and Instruments. Paper 9.