Email from Apogee Instruments on Measuring LED with Quantum Sensor:
"In regards to measuring LEDs with our quantum sensor, there are some caveats to doing so. The following link shows the spectral response of our quantum sensor (http://www.apogeeinstruments.com/quantum/spectralresponse.html). As the graph shows, Apogee quantum sensors underweight blue light, and as a result, photon flux measurements for blue LEDs will be too low. They also overweight red light up to a wavelength of approximately 650 nm, above which they do not measure, and as a result, photon flux measurement for red LEDs will either be too high (if the LED output is all below 650 nm) or too low (if a non-negligible fraction of the LED output is above 650 nm). Additionally, LEDs often have a very narrow spectral output, with a sharp peak of only a few nanometers. So, unless the quantum sensor has a perfectly flat spectral response, meaning it weights all wavelengths of light exactly the same, there will be errors. Electrically calibrated Apogee quantum sensors will likely provide a reasonable measurement for white LEDs because they are broadband, and because electrically calibrated quantum sensors are calibrated under CWF lamps. However, for narrowband LEDs, like red and blue, Apogee quantum sensors will not provide an accurate measurement.
As a less accurate method you can use the same spectral response graph as mentioned above to get a relative idea of the error. For example, a 450nm blue LED will have a relative response of approximately 0.8. Therefore, you can figure that the photon flux reading from the sensor is reading approximately 20% low. Just remember, this approach is only relative so give yourself a wide margin of potential error. A blue/white configuration should give you reasonable accuracy, particularly from the broadband spectrum of the white.
To sum up, quantum sensors can be used to measure the relative output of an LED or bank of LEDs, in order to track variability in output with time or temperature for example. However, quantum sensors should not be used to characterize the absolute output of LEDs (except for the possibility of white LEDs), to compare one LED to another, or to determine photon flux for plant growth for example. Ultimately, a spectroradiometer is the best instrumentation for characterizing LEDs."