Quote Originally Posted by growbe
A very good post when it comes to comparing CFL to HID in terms of Lumens and such but, plants "see" light differently than human beings do. As a result, lumens, lux or foot-candles should not be used to measure light for plant growth since they are measures used for human visibility. More correct measures for plants are PAR watts, PPF PAR and YPF PAR, although each in itself does not tell the whole story. In addition to quantity of light, considerations of quality are important, since plants use energy in different parts of the spectrum for critical processes.
Plants are sensitive to a similar portion of the spectrum as is the human eye. This portion of the light spectrum is referred to as photo synthetically active radiation or PAR, namely about 400 to 700 nanometers in wavelength. Nevertheless, plant response within this region is very different from that of humans.
The human eye has a peak sensitivity in the yellow-green region, around 550 nanometers. This is the "optic yellow" color used for highly visible signs and objects. Plants, on the other hand, respond more effectively to red light and to blue light, the peak being in the red region at around 630 nanometers.
Red light provides the most efficient food for plants. However, a plant illuminated only with red or orange light will fail to develop sufficient bulk. Leafy growth (vegetative growth) and bulk also require blue light. Many other complex processes are triggered by light required from different regions of the spectrum. The correct portion of the spectrum varies from species to species. However, the quantity of light needed for plant growth and health can be measured, assuming that all portions of the spectrum are adequately covered. Light for plants cannot, however, be measured with the same standards used to measure light for humans.
First, how do we measure light quantity for humans? The obvious way is based on how bright the source appears and how "well" the eye sees under the light. Since the human eye is particularly sensitive to yellow light, more weight is given to the yellow region of the spectrum and the contributions from blue and red light are largely discounted. This is the basis for rating the total amount of light emitted by a source in lumens.
The light emitted from the source is then distributed over the area to be illuminated. The illumination is measured in "lux", a measurement of how many lumens falls on each square meter of surface. An illumination of 1000 lux implies that 1000 lumens are falling on each square meter of surface. Similarly, "foot-candles" is the term for the measure of how many lumens are falling on each square foot of surface.
Clearly, both lumens and lux (or foot-candles) refer specifically to human vision and not to the way plants see light.
How then should the rating for plant lighting be accomplished? There are two basic approaches to develop this rating: measuring energy or counting photons.
PAR Watts for Plants
Watts is an objective measure of energy being used or emitted by a lamp each second. Energy itself is measured in joules, and 1 joule per second is called a watt. A 100 watt incandescent bulb uses up 100 joules of electrical energy every second. How much light energy is it generating? About 6 joules per second or 6 watts, but the efficiency of the lamp is only 6%, a rather dismal number. The rest of the energy is dissipated mainly as heat. Modern discharge lamps like high pressure sodium (HPS) and metal halide convert (typically) 30% to 40% of the electrical energy into light. They are significantly more efficient than incandescent bulbs.
Since plants use energy between 400 and 700 nanometers and light in this region is called Photo synthetically Active Radiation or PAR, we could measure the total amount of energy emitted per second in this region and call it PAR watts. This is an objective measure in contrast to lumens which is a subjective measure since it is based on the response of the subjects (humans). A PAR watt directly indicates how much light energy is available for plants to use in photosynthesis.
The output of a 400 watt incandescent bulb is about 25 watts of light, a 400 watt metal halide bulb emits about 140 watts of light. If PAR is considered to correspond more or less to the visible region, then a 400 watt metal halide lamp provides about 140 watts of PAR. A 400 watt HPS lamps has less PAR, typically 120 to 128 watts, but because the light is yellow it is rated at higher lumens (for the human eye).
Since plant response does "spill out" beyond the 400 nanometer and 700 nanometer boundaries, some researchers refer to the 350 – 750 nanometer region as the PAR region. Using this expanded region will lead to mildly inflated PAR ratings compared to the more conservative approach in this discussion. However, the difference is small.
While HPS and incandescent lamps are fixed in their spectral output, metal halide lamps are available in a broad range of color temperatures and spectral outputs. With this in mind, the discriminating grower can choose a lamp that provides the best spectral output for his specific needs.
found this one in the same thread as roughrider
dutch i actually seen those thumbnails in the same thread 2. thnx 4 postin em here 2.

but anyway...nanometers is the actual meaning. (so im assuming)
srry 4 makin another thread on the same topic btw.
i kno how annoyin it can be n i will definately look harder in the future.