Maximum Efficiency

Brightness Values: Magnavox Electronics Reference

Light Source	Lux	Watts/ft2	Watts/m2
Office fluorescent Lighting	300-500	.04-.07	44-.73
Halogen lamp	750	.102	1.10
Sunlight, 1 hour before sunset	1000	.136	1.47
Daylight, Cloudy sky	5000	.680	7.35
Daylight, Clear sky	10,000	1.36	14.7
Bright Sunlight	> 20,000	2.72	29.4

Note that the above watts/sq. ft and watts/sq. meter is the amount of energy in the light and not the amount of watts that it took to make the light. Many light bulbs are inefficient. Typically only 10% of the energy goes to make light in vacuum tungsten filament light or possibly up to 20% with halogen bulbs. Florescent are about 70-90% efficient. I pulled the following quote from Lite Manufacturing:

Under natural conditions, maximum rates of Photosynthesis are attained in single leaves of many species at 25-35% of full sunlight intensity and in some shade species at even lower intensities.

If one now takes 25-35% of 14.7-29.4 watts/sq. meter one gets 3.7 to 10.3 watts/sq. meter or an average of 7 Watts/sq. meter. Note that this is very close to 6 watts/sq. meter. Now lets start from scratch and calculate an estimate amount of light hitting your plants. From Handbook of Engineering Fundamentals by Eshbach second edition. Efficiency of light sources table 4 page 10-33.

Light source efficiency	(lumens/watt)
tungsten gas filled	20
tungsten vacuum lamp	10
Fluorescent: (includes ballast loss)
standard cool white	46.5
standard warm white	51.2
de luxe cool white	29.0
de luxe warm white	31.0
daylight	41.3

Depending on the bulb used you could be putting out

29lumens/watt*40watts = 1160 lumens or
51.2*40= 2048 lumens of light

Offered by Mike.