Sorry I meant to include this part as well about the experiment, useful if you a situation with a light leak or whatever, sorry early and brain isn't working.
In the 1940's it was discovered that a flash of light during the skotophile phase prevented flowering in short-day plants.51 52 What this tells us is that the length of uninterrupted darkness is critical, not length of light. Thus, scientists deduced that florigen is created by long-night plants during the skotophile phase and that it degrades during the photophile phase, which explains why interrupting the photophile phase is not harmful but interrupting the photophobe phase
More experiments were conducted--this time, using different light colors. It was then hypothesized that red light during the skotophile phase will convert the newly (and spontaneously) created Pr back into Pfr instantly, and prevent the plant from flowering.53 But--and here's the kicker--a second flash of light (this time a far-red light so that the plants are exposed first to a flash of red and then to a flash of far-red during the dark period) will undo the damage! In other words, the far-red flash corrects the first interruption. (The lag time varies by species.) In fact, you can do this as many times as you like. If you finish with a flash of red light, long-night plants will not flower (but short-night plants will). If you finish with a flash of far-red light, long-night plants will flower (despite the interruption), but short-night plants will not.54 Called photoreversibility, this phenomenon does have its limits. If too much time elapses between flashes, the plant is said to have "escaped from photoreversibility" and it can no longer be influenced thusly.55