Richard

Watts refers to the entire spectrum of light delivered -- at some efficiency.

A 600W HPS is typically 90% efficient, so you are getting about 540 Watts (Joules of energy per second) total output at the immediate outside surface of the lamp. At one meter (a little over a yard) from the lamp in a humid atmosphere, you can expect another 10% to 15% diffuse loss in parts of the spectrum. If there is a wall (or floor) 1 meter away from the lamp, then the total energy per second being delivered to the illuminated surface is about 390 Watts. If that area is 4 square meters (a square a bit wider than 2 yards by 2 yards) then on average each of the 1 x 1 square meters is receiving about 100 Watts. This is what I was referring to as net watts.

Lumens on the other hand is normalized to human perception and tied to specific wavelengths of light. 1 lumen in the green wavelength (555 nm) does not equal 1 lumen in the blue wavelength (e.g., 480 nm). Lumen ratings on light bulbs are calculated by one or another standard of measurement -- the best of which is an equivalent energy standard. To make it more confusing, sometimes the Lumen rating (e.g., consumer light bulbs) is at the immediate outer surface of the lamp, while ratings on instrumentation that say "nominally 4000 lumens" often mean 4000 lumens per square meter at 1 meter distance from the source.

The often cited conversion of Watts to Lumens is only valid at 555 nm, that is 1 Watt @ 555 nm = 680 Lumens @ 555 nm. So for example, a green light source of 400 Watts output will have a consumer rating of 272000 lumens. The 90,000 lumens quoted below refers to only the green wavelengths of the HPS lamp.

To simplify all of this, I find it much easier to first identify light sources with the desired spectrum, and then examine what it will take to obtain the desired Watts per square meter. A good rule of thumb is that if the objects (plants) are about 1 meter from the source, then I'm looking for a Wattage per square meter which is 25% larger than my needs.

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