I think what most people are missing in this argument is this.....
Spurrs point is to give the plant the maximum amount of photons it can process, per day, for 18 hours. When you do as such, and let it rest for 6 hours, you will get maximum growth.
Now, when you DO NOT utilize maximum photon admission in those 18 hours, running 24/0 is more beneficial because the plant is basically starved for light and hasn't had enough light to store energy for night growth, which in turn makes 24/0 appear more beneficial.
Now, the proper way to test would to give two identically sized/strain/phenotype plants maximum photons (slightly short of saturation) per second, one plant for 18 hours, the other for 24.
This is when you will see 18/6 more successful than 24/0.
That is at least the way I understand what Spurr is saying, breaking it down to laymans terms of course.
If I'm missing something, I'm sure Spurr will set me straight.
Yup.
With the caveat that both 18/6 and 24/0 would need to provide the same DLI; that's the sticking point. If we don't account for, and measure and equalize DLI for both 18/6 and 24/0 then the side by side is not about what daylength is better, it's about what DLI is better.
To test what daylength is better we must account for, and equalize DLI because it's the major variable that affects growth in terms of photosynthesis.