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One part of the assimilation irradiance curve that currently gets little attention is its light-limited slope - the region where the quantum efficiency of assimilation is at its maximum, setting an upper boundary for assimilation at higher irradiance. In general, this light-limited quantum yield is as good as assimilation gets in terms of its light-use efficiency. While other photosynthetic parameters, like Pmax, Jmax and Vcmax, are considered to be very variable both within and between genotypes and species, and are therefore interesting and deserving of further investigation. The light-limited quantum yield is usually viewed as quite boring. The value of the yield will depends on oxygen and carbon dioxide partial pressures affecting the kinetics of rubisco, so the light-limited yield also depends on temperature. The maximum value of the yield is not often discussed - the right answer for an exam is probably 0.125!  The light limited yield is, in fact, an interesting parameter, and I do not think we can fully account for all the phenomena that are revealed when it is measured. I will explain how we measure the light-limited yield and discuss some of our results showing the spectral dependency of quantum efficiency of assimilation (which may vary between species), the effect of state transitions on assimilation, and if there is enough time, what might be evidence of cyclic electron transport.