Lumens.

Hi,

I'm by no means an expert. but from what I can tell lumens is not really the measurement in question. this guy explained it quite well. http://forum.grasscity.com/advanced-growing-techniques/417864-very-important-info-about-lumens.html

Lumens is a measure of light flux, so is independent of the area. It is essentially the amount of light available. So think of a bright light. It has a fixed amount of lumens, regardless of how near or far you are from it.
imo the PAR watts and visual spectrum is far more important. not sure if I helped in any way. gl with it bro.

 

I don't use lumens to determine whether I have enough light or not. I go by watts per square foot of growing space. I try to get at least 50 watts per square foot. Those pictures tell me that your plants aren't getting enough light.

 

Lumens are for humans, PAR (photosynthetically active radiation) is for plants.   Each of these measurements measure the number of photons (amount of light) in different spectrums.  Lumens measure the light that's detectible by the human brain (via the eyes).  PAR is a measurement of the amount of light that's within the spectrum useable by most plants.   There is a bit of overlap between the two, and therefore you can measure the number of lumens of light that you're providing for a plant, but it isn't technically the key factor you're after. 
 
For our purposes, what you're after is the amount of power (measured in watts) used by a light that has an output within the useable range of our plants.   Note that watts are watts are watts.  a 13w light that "replaces a 60w light" is still a 13w light.  A watt is a watt is a watt. fluorescent, LED, or HID, watts are the measurement of power that a light uses.  Only so much light can be generated by x number of watts.  Some are more efficient than others, but it's mostly negligible. 
 
The next most important factor is the distance from a light source.  The cool thing here is that you don't have to worry about lumens or PAR.  Most artificial light sources cannot provide too much light for your plants.  So you must use heat to judge a good distance for your plant, from the light source.   Use the back of your hand under a light.  If it feels a bit uncomfortable to you, it will be uncomfortable for your plant, and the plant will begin to dry up and die.  HEAT (also measured by watts used) has to be considered when placing your plant under a light.
 
If you're using CFL lights, your plants can nearly touch the bulb, just be sure to have a fan moving air around it regularly. 
 
Buy lighting that uses the most wattage that you can afford to control the heat output of.