Light Energy for Plant Growth
Plant Development is dependent on the specific spectrums of the light source and the usable light energy

 Only 37% of the energy in sunlight is within the wavelength (colors) useful for photosynthesis, while 62.4% is infrared (thermal energy) and the remaining 0.6% is ultraviolet. Photosynthesis in the plant leaf is powered by 1% of the sunlight that falls on the plant, 10% of the sunlight is reflected and 10% passes through the leaf. The leaf will retain 80% which is used for transpiration. Some of the light is re-radiated, while the fraction that remains is used for building building food from the carbon dioxide, minerals and water.1

Percentage of usable of  light energy (0-100% y-axis)  / Light spectrum range (x-axis). For photosynthesis the most important spectrums of the light are blue and red.1   Germination, flowering and stem growth are influenced by red to far red.  In artificial environments it is important to keep these factor in mind when programming the light source for the plants life cycle.  For the associated light spectrum consult your bulb's manufacturer.

1. The Solar Greenhouse Book
 

Chart of  Usable Light Energy for Artificial Lighting
Useable light energy for plant growth is measured in Micro-Einsteins ( micro-mols of photons per meter squared per second). The sunlight reaching a plant is approximately 2,200 micro-einsteins on a cloud-less day  and 170  micro-einsteins on a cloudy day.

For indoor growing under artificial lighting a range of  395 to 500 micro-einsteins is considered by experts to be minimal for plant growth.  Note:  The higher the bulb wattage the further away the plant must be from the light source to prevent the plant from transpiring too quickly. Thus, the  further the plant is from the light source the less available useable energy is delivered to the plant.  For optimum usable energy, select a bulb that has the lowest wattage with the highest usable energy coefficient.

 Here is chart that can help you in determining the useable light energy for your plants.
 
BULB Type:  Distance:  Micro-Einsteins:  Usable Energy Coeffecient 
  (micro-einsteins per watt)
Incandescent
100 watt 
3 inches 310 3.100
Incandescent
100 watt 
6 inches 190 1.900
Incandescent
100 watt 
12 inches 65 0.650
Incandescent
100 watt 
18 inches 27 0.270
Incandescent
100 watt 
24 inches 18 0.180
Incandescent
100 watt 
36 inches 9 0.090
Halogen
20 watt 
3 inches 1279 63.950
Halogen
20 watt 
6 inches 413 20.650
Halogen
20 watt 
12 inches 145 7.250
Halogen
20 watt 
18 inches 122 6.100
Halogen
20 watt 
24 inches 65 3.250
Halogen
20 watt 
36 inches 32 1.600
Sodium
250 watt
3 inches 250 1.000
Sodium
250 watt
6 inches 99 0.396
Sodium
250 watt
12 inches 30 0.120
Sodium
250 watt
18 inches 14 0.056
Sodium
250 watt
24 inches 10 0.040
Sodium
250 watt
36 inches 7 0.028
Gro-Lite
100 watt
3 inches 420 8.400
Gro-Lite
100 watt
6 inches 135 2.700
Gro-Lite
100 watt
12 inches 60 1.200
Gro-Lite
100 watt
18 inches 31 0.620
Gro-Lite
100 watt
24 inches 18 0.360
Gro-Lite
100 watt
36 inches 9 0.180
Fluorescent
40 watt
3 inches 52 1.300
Fluorescent
40 watt
6 inches 36 0.900
Fluorescent
40 watt
12 inches 13 0.325
Fluorescent
40 watt
18 inches 7 0.175
Fluorescent
40 watt
24 inches 5 0.125
Fluorescent
40 watt
36 inches 2.5 0.063