Quote Originally Posted by KnowledgeIsPower
How should i apply PAR?
If par is so important, why is intensity measured in lumens?
Or does PAR only refer 2 spectrum, and lumens to intensity?
:wtf:
Lumens is a measurement of how bright a light looks to the human eye - not necessarily how well that light can grow a plant. When measuring HID lighting, lumen is used because the spectral output happens to be reasonably close to what a plant needs for photosynthesis *and* it looks bright to the human eye. Lumen output is not a suitable measurement for LED grow lights because much of what a plant needs to grow is in the blue and red wavelengths that do not look bright to the human eye (when compared to the yellow/green/orange prevalent in HID).

This is where PAR comes in. PAR measures the amount of light energy from a given source that can contribute directly to photosynthesis. This is where the comparisons come from with HID vs LED grow lights. For example . . . a 100W LED grow light (from the right manufacturer) may have the same PAR (photsynthetically active radiation) as a 400W HPS, meaning that the HPS light is wasting about 75% of its output as heat.

Now, some LED grow lights use infrared in their designs . . . infrared light is not used directly by a plant for photosynthesis but does work with secondary pigments which then relates to plant growth. So the PAR reading of an LED with infrared chips will read lower than a light with no infrared but the grow results can be vastly different. . . . the point here is that PAR is not a true indicator of plant-growing power of a light either (depending on the light).