Since I now know you love to read HD (and you thought HD meant Harddon ) here's some for you.

"Lumens are a measure of light output in wavelengths to which the human eye is sensitive. It is not a meaningful or useful measure for plant lighting. Interestingly, the SI lumen is defined in terms of the candela, which is in turn defined in terms of watts. The wavelength defining the lumen is approximately 550 nm, a wavelength that is not very efficient for growing plants.

The watt is the SI unit for power and also for radiance. The first is the measure of electicity used by a lamp (e.g., 400w). The second is the light emitted from the lamp at all wavelengths.

It can be very difficult to obtain ratings for lamp output in terms of radiant flux (watts). Different lamps will have different radiant efficiency (how many radiant watts are produced per input watt). The exact spectrum produced also determines how useful that light will be for plant growth.

That still doesn't make lumens useful. 50-60 watts (minimum) of HID lighting (double that for flouro) is a rule of thumb that is at least as useful as any lumen recommendation, and has the further advantage of not further promoting misunderstanding of a very misunderstood subject, or further propogating the misinformation that lumens are at all useful for our purposes."

and

"The lumen is not a useful unit of measure in this regard. In fact, for any two lamps of the same technology and wattage, the one with higher lumens very likely produces less useful light for plant growth than one with lower lumens. Why? Because lumens are a measure of how close the light emitted comes to 555nm. But light between 500 and 600nm is not very useful to the plant. Low Pressure Sodium is the perfect case in point: it has the highest luminous efficiency (lm/w) currently available, but is useless for plant growth (otherwise everyone would have LPS lights in their grow room). GE Plant & Aquarium flourescent bulbs are another case in point: they have very low lumen ratings, but provide much more energy for plant growth than other kinds of flourescents with higher lumen ratings.

Lumens are used as a proxy for "intensity" by people who don't understand what a lumen really measures, or the difference between photometry and radiometry."

and

"Because the overwhelming majority of lamps are designed for human vision applications, lumen ratings are required for all lamps. However, the SI defintion of lumen is 1 lumen = 1 candela * steradian. One candela is the power in a given direction, of a source that emits monochromatic radiation of frequency 540 x 1012 hertz (approximately 555 nm wavelength) and that has a radiant intensity in that direction of 1/683 watt per steradian. So we see that lamp output ratings really do start with watts and then derive lumens (according the the CIE photopic vision curve)."