The ideal average photosynthetic response of plants.
The McCree Curve quantifies the average photosynthesis response of plants to light from 350nm
to 750nm, and shows the average sensitivity of plants
to light (the plant sensitivity curve) in this region of the
spectrum. The McCree curve can be used
to compare the spectral output of a plant/grow-light with the
average sensitivity curve for plants.
The McCree Curve
Dr. Keith. J. McCree
To quote from the abstract of the paper: "The action spectrum, absorptance and spectral quantum yield of CO2 uptake were measured, for leaves of 22 species of crop plant, over the wavelength range 350 to 750 nm. The following factors were varied: species, variety, age of leaf, growth conditions (field or growth chamber), test conditions such as temperature, CO2 concentration, flux of monochromatic radiation, flux of supplementary white radiation, orientation of leaf (adaxial or abaxial surface exposed). For all species and conditions the quantum yield curve had 2 broad maxima, centered at 620 and 440 nm, with a shoulder at 670 nm. The average height of the blue peak was 70% of that of the red peak."
From his study data, Dr McCree was able to create a generalized plant light absorption curve (an average curve for all the plants he measured), which is known as the McCree curve. On the left below, you can see the original McCree curves (for all 22 species of plants) as published in Dr. McCree's paper , and on the right is a more colourful version of the averaged McCree Curve.
For More information about plant light sensitivity and the McCree Curve, see see
What Light do Plants Need?
 "The action spectrum, absorptance and quantum yield of photosynthesis in crop plants" - McCree, Keith J.
Agricultural and Forest Meteorology 9: 191-216. doi:10.1016/0002-1571(71)90022-7
 "The action spectrum, absorptance and quantum yield of photosynthesis in crop plants" - McCree, Keith J. Agricultural and Forest Meteorology 9: 191-216. doi:10.1016/0002-1571(71)90022-7