In various species of plants it has been shown that it is the total photon flux a plant receives over a period of time that matters, not the peak illumination. When we say a plant needs 1,500 foot candles we are talking about the peak light the plant receives in the wild or in a greenhouse with natural daylight. Again, in various plant species, lengthening day length with lower illumination intensity can compensate. Under natural daylight, we forget that a good portion of the day the light intensity is fairly low.
A number of under lights growers have reported using even 18 hour days for some or much of the year. A rough calculation shows that 18 hours of 500 foot candles will give equivalent photon flux to a well positioned greenhouse with peak 1,500 foot candles over the course of a year. There are other variables involved, but it means that under lights growers can be successful with various intensities of light and day lengths.