How are mere mortals supposed to know how much light is good, bad or indifferent? Lumens, watts, footcandles or whatchamacallits?
It's really pretty straight-forward, actually. There are two types of chlorophyll in plants which absorb different spectra of light. They have saturation points at which more light will not be beneficial, and may in fact be harmful. A plant can adapt to varying levels of light by producing thinner or thicker epidermal layers on leaves... but they cannot create light where none exists.
Therefore you want to generate light that saturates the plant, but not much more. When using artificial lighting for growing plants, you don't really care about LUX... but rather PAR (or Photosynthetically Available Radiation). This is a measurement of how much light being generated falls within the red and blue bands of spectra that chlorophyll uses to photosynthesize. You can get a high lumen bulb with low PAR, and a high PAR bulb with relatively low lumens. (LUX is simply a measurement of how many lumens are hitting an area. Light diminishes proportionally based on the square of the distance from the light source, so you can have a bulb putting out a fixed amount of lumens, and a plant 1 meter from the bulb will be getting 4x as many LUX as a plant 2 meters from the bulb).
That's about it. There are a few other considerations when dealing with artificial lighting, including photoperiod and moving the light source to avoid shadowing... but those have been pretty much solved through scientific studies and the adaptation of motorized light racks, or indirect/diffuse lighting.
(By the way, the second image here is a little misleading. They should have pulled out the plant icons, because I think they are a little misleading. Focus on the light circles being cast on the ground... and ignore the plants
I'm sure you will understand, that if you have a plant 12" under a light fixture (measured from the top of the plant) the bottom of the plant gets a very different level of light. Conversely... if you have a plant in a football stadium bathing under stadium lighting, the difference in light intensity between the top and bottom of the plant is much less different. And if you are dealing with the light coming from the sun, the difference in distance between the sun and the top of a plant, and the sun and the bottom of a plant, is so small as to be irrelevant).
