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What meter did you use? Is it calibrated for LEDs? The problem is that the primary meter on the market is an Apogee, and that particular manufacturer in no way envisioned the advent of LEDs and has built their meters using a smoothing algorithm that anticipates sunlight or fluorescent light. LED light is not properly measured. Plus we have little knowledge on what happens when we flood corals with specific wavelengths instead of a broad spectrum of wavelengths.
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I agree with your statement but I disagree with where it leads. The Apogee sensor has a smoothing algorithm to correct defficiencies in the sensor itself. The meter will report low for any light source with a strong component of light below 500nm. It will also read any light over 600nm as more powerful than it should. However, the differences only equate to 10% of the light from those sources. If you take par readings of an LED fixture you only need to correct the readings from your blue LEDs by 10%. This means the overall error factor will generally be around 5-7%. If you have red LEDs present in the fixture the error will be even smaller. This is a trivial difference when you factor in other sources of error. It will also be a factor when measuring high kelvin metal halide bulbs and actinic/blue T5 bulbs.
The last sentence I agree with completely.