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Brightness Values: Magnavox Electronics Reference

Light SourceLux Watts/ft2Watts/m2
Office fluorescent Lighting300-500.04-.0744-.73
Halogen lamp750.1021.10
Sunlight, 1 hour before sunset1000.1361.47
Daylight, Cloudy sky5000.6807.35
Daylight, Clear sky10,0001.3614.7
Bright Sunlight> 20,0002.7229.4

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 filled20
tungsten vacuum lamp10
Fluorescent: (includes ballast loss)
standard cool white46.5
standard warm white51.2
de luxe cool white29.0
de luxe warm white31.0

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.