# 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.