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