Lumens are for humans, PAR is for plants.

Lumen is a measurement of visible light output. While helpful in comparing LEDs and fixtures, the lumen output is not necessarily indicative of the energy that is absorbable by a plant during photosynthesis and other correlative developmental cycles. PAR is a better measure of the energy that is accessible by the plant.

PAR is a measure of photonic light energy.  This is energy in the spectrum of light from 400-800 nM.  Plants use energy from this entire spectrum for growth during it's lifecycle.  Cannabis and other flowering annual plants respond well to PAR measurements of 500-800 µmols. 

A PAR footprint map will show the PAR output at a given point in the area.  Maintaining consistent PAR across the coverage area will help to eliminate hotspots and provide enough energy for the plant to grow and thrive.

 

Why is white LED light better for plant growth than only red/blue or "targeted spectrum" LEDs?

It is important to have wavelengths of light from the entire 400-800 nM spectral range available to the plant in order for all its biological processes to take place.  Targeting only Chlorophyll A & B peaks does not provide the entire photosynthetic range required for proper plant growth.  Cree CXB3590 LEDs provide a spectral distribution that closely approximates outdoor sunlight.  These LEDs provide more red and blue photonic energy than HPS to help with chlorophyll production, general growth and flowering.  Full spectrum white light produced by COBs contain parts of the spectrum missing from companies using "targeted spectrums".

This is also called the spectral output curve.  We use CREE CXB3590 LEDs because the most closely approximate the spectral distribution curve of natural outdoor sunlight.  This is known as the McCree spectrum curve.

And then there is the added benefit of sight.  The pinkish light emitted from a "targeted spectum" light can be very hard on the eyes, creating difficulties in noticing crop quality shifts and for maintaining and diagnosing plant health.   We use Cree LEDs that put out white light with similar color temperatures and color renderings of those used in high end retail displays which allow for better visibility and crop management.

 

Is wattage important?  What are the different wattage ratings?

LEDs are very efficient compared to traditional HPS/HID lights.  HPS/HID lights put out a large amount of energy, but much of it is converted to heat, and is not usable and can harm plants if not properly cooled.  LEDs require much less wattage to produce similar PAR values due to less energy being lost to heat. 

That said.....

An LED may draw 200 watts from the wall, but due to heat dissipation, it may only put out 86 W of usable energy, or PAR watts.  This is the energy that can be absorbed by the plant across the entire light spectrum.  PAR wattage, or PPFD, is the truest comparison of LED grow lights. 

 

What is PAR, PPF and PPFD - and what are the differences?

PAR simply put is the amount of wattage output that is energy that is usable in plant (within the 400-800nM spectral range).  The difference between "actual wattage" and PAR wattage is usually heat and energy required by the light to operate.  This "excess and unusable" energy is dissipated through cooling.