Arteficial light and some rough calculations.

I use as reference light source the sun it gives on a clear midsummer day at noon about 800W/m2 but only halve of that energie is in the PAR range 400-700nm. The plant photon efficency for sun light is ~0.7 and the daily intensity is a sinusodale curve which has a average intensity of 0.707 times the max intensity. As result we have 800x0.5x0.7x0.707 ~ 200W/m2 usable for our plants, that is not bad at all we take a 200W HPS, MH or LED lamp and we imitate the sun do we really?
No we don't because the calculated 200W is photon power and the 200W of the grow lamp is electrical power, a grow lamp has an efficency of about 20%, then for 200W photon power we need a 1000W grow lamp.
Isn't that nice, it is roughly an 1 to 1.25 ratio to the initial sun power which make the calculation very easy because the next step is to try to calculate needed Watts from Lux light intensity.
Again on a clear sunny summer day (in Holland) the intensity at noon is something like 80.000+ Lux and a grow lamp have to be 1000W that give

80.000/1.000 = 80Lux/Watt.

So when we divide Lux by 80 we have an idea what power lamp we need to illuminate 1m2 of our plants
I have an orchid collection and it is common to divide those in three groups, high, medium and low light species what give the next table:

Light ---------- Lux ------ W-Lamp/m2 --- Orchd species
High ------- 40.000-------- 500 ----------- Cattleya, Cymbidium
Medium--- 20.000 ------- 250 ----------- Oncidium, Phalenopsis,
low --------- 10.000 -------- 125 ------------ Paphiopedium, Macodes

When you know amount of Lux or Candela your plants need it is easy to calculate the power of the required grow lamp to illuminate 1m2. Of course the plamt will maybe do also fine at 1/2 or 1/4 of that power but it gives a kind off ball park figure for the optimal illumination.

Lux/80 or Candela/7.3 = Watts grow lamp.