It is a concrete thing, except that it's the amount of light and the length of that amount. Over time.

And light is never constant (take a week of rainy weather). It all has to be figured in. That's why a light meter is only a start, and not a quick answer.

If you were hyped up anal-retentive, you'd measure foot candles hourly every day for a season and compute your average per day on a seasonal basis. Go for it! (I'm bad, but not that bad...)

Don't despair - paying attention to what the orchid is telling you is actually easier than figuring out a formula. Somewhere betwixt the two we all seem to manage.

Julie