Full Spectrum LED(400nm~840nm) vs Grow Light LED (RED+BLUE)
None of the above.

All you want is Deep Red (650nm) and Deep (Royal) Blue (450nm).

You do NOT want Full Spectrum, UV, IR, Far Red, Only White or anything besides Deep Red and Deep Blue. Red White and Blue is always a safe bet.  Red and Blue come way before white.  White (aka Full Spectrum) is more like supplemental. 

I do some consulting work regarding grow lights and design for the University of Florida Horticulture LED Research.  I asked Professor Dr. T.A. Colquhoun who leads LED research at UF, "What colors would you recommend for a grow light?" and he replied "Deep Blue and Deep Red have always worked well for us". 

If you want some validity Google "thomas a colquhoun led".

The results of nearly every study ever done on growing with LEDs is Deep Red and Deep Blue worked well.

It's all about the  Action and Absorption Spectrum of Photosynthesis and its Quantum Yield.  

 

If you care to know more about chloroplast, types of chlorophyll, carotenoids read this:  Photosynthetic Pigments 

More: BIOLOGICAL ACTION SPECTRA 



Then there is the Quality of light.  The Quality is the result of light Absorptance, Reflectance, and Photosynthesis.  Quality is based on the Spectral Radiance.  More specifically the Spectral Radiant Flux, in wavelengths (or frequency), received by the leaf surface.  Often referred to as Photosynthetic Active Radiation (PAR) and Photosynthetic Photon Flux Density (PPF and PPFD) 

When you buy a grow light you want the most bang for the buck. The bottom half of the above chart clearly shows the Photosynthesis Rate is highest with Deep Blue and Deep Red.  That is where you put your money.   

The seminal paper describing the action spectra for 22 plant species was published by KJ McCree in 1972. This work was originally done in order to provide an accurate definition of PAR. McCree defined the efficiency or quantum yield of CO2 assimilation as a function of color wavelengths in 22 plant species. 

When getting advice from websites that refer to studies regarding UV, IR, and Far Red, beware of the difference between photosynthesis and phytochrome mediated responses. For grow lights photosynthesis is the important thing.  Many studies now use various colors to signal specific plant response.  

For example the ratio of red to far red is known to signal Shade Avoidance Syndrome which creates tall spindly plants.  If you are growing cannabis you do not want to signal SAS with far red and get tall spindly plants.  

UV in grow lights?  UV is commonly used to kill living things.  Its use requires some know how.  

Since 1972 this paper has stood up to extensive peer review. It still holds true today. 

McCree, K.J. (1972). The action spectrum, absorptance and quantum yield of photosynthesis in crop plants. Agric.Meteorol. 9 : 191-216. 

WATTS

Shopping for grow lights is a nightmare. It's the blind leading the blind out there. One of my pet peeves is the use of Watts when selling grow lights.  

When reading Reviews of LED grow fixtures, I often see the term Watt used.  Misused more than Used.  When someone uses the term “True Watts”, I know they do not know.  There is no such thing as a True Watt.  True Watt is a marketing term.  As with most marketing terms that use the word true or truth, it is neither. 

An LED grow fixture has many Watts. 


Wall Watts
Power Supply Watts
LED Driver Watts
LED Electrical Watt
LED Thermal Watt
LED Radiance Watt 


If there were a Watt of importance it would be the Wall Watt.  The main criteria for the evaluation of a grow fixture is how much electricity goes in, and how much light comes out.  In this evaluation the amount of electricity is typically specified in Joules, as in PPF per Joule, not Watts. Although that is just technical you can look at it as PPF/Watt.    

More specifically than just the amount of PPF, what is more important is the amount of light or phonton energy that reaches the plant leaves, PPFD measured in moles/m2/s, Watts/m2 /steradian, or Lumens/m2 /steradian.  

Although PPF is also unimportant.  PPFD, that is what is important when it comes to grow lighting.  It has become apparent there are very few that understand PPF and PPFD.   

In simple terms as related to grow lighting: 
 - PPF is the amount of light emitted from the LED.
 - PPFD is the amount of light that reaches the plant(s). 

The “F” in PPF and PPFD stands for Flux..  Flux is a measurement unit when measuring Energy per unit of Time and or Volume and or Area.   

Energy takes many forms in a grow fixture: 


Photometric, Luminous Energy (Lumen Second)
Radiometric, Radiant Energy (Joule)
Electrical Energy (Joule)
Thermal Energy (Joule)
Photon Energy (Moles)


Each form of Energy listed above there are many more measures.  Volume, Density, Intensity, Illuminance, Irradiance/Illuminance, and Radiance/Luminance. 

I could explain each one, but no one would remember.  There is only one that is important to grow lighting:  Photon Radiance measured in:  moles/m2/sec.  Or how much light reaches the plant. 

Not to be, as often is, confused with Flux (moles/s), Intensity (moles/m3), Density (moles/m 3/steradian), or Irradiance (moles/m2).  Note about steradian:  A steradian is a unit of volume.  Because light spreads as it travels and takes on a cone shape, the steradian is used.  The steradian is just that, the volume of a  cone.   

Photon Radiance is the number of photons (moles) hitting (incident) a surface area (m2) in a given amount of time (seconds).   

PPFD (Photosynthetic Photon Flux Density), is a measurement of Radiance, not Density.  Why is it called Density?  Because someone didn't think this lighting measurement was confusing enough?  No, it is more likely someone was already confused enough that they used the term Density where it was actually Radiance.  Happens all the time.   

PPFD measures the Transmittance of light.  There is still more than just  Transmittance.  Transmittance being the effectiveness of transmitting the Radiant Energy.   

As Watts relate to LED grow lighting, the Electrical, Thermal, and Radiant Watts are too dynamic to be meaningful in the evaluation of a grow fixture.   

The Electrical Watts are divided in to a ratio of Thermal and Radiant Watts.  A lighting electrical engineer will choose an efficient LED, with good efficacy, with the highest Radiance Watt to Thermal Watt ratio. 

No more watts please. 



LUMENS

Lumens is not a measurement of light or photons. It is a calculation based on the measurement of light photons with respect to the sensitivity of the human eye. 

The problem with Lumens is it not only irrelevant to photosynthesis, higher Lumens often means less PAR.  A light source with high lumens may very well be the worst choice for grow lighting. 

An LED can radiate more PAR and have less lumens when compared to another LED. 

The following image shows an LED Luminous Flux in lumens.

This LED looks horrible  for grow lighting.  
Mostly Green and very little Red or Blue.  





The following LED looks PERFECT for an LED grow light.

Plenty of Deep Red with a perfect ratio between Red and Blue for most plants. 



The difference between the above two images is:


The first shows the Luminous Flux of the LED.
The second shows the Radiant Flux of the LED


They are the SAME LED!

Lumens overemphasizes Green and de-emphasizes Blue and Red.  Just the opposite of what is important in grow lighting.  

As shown above, Deep Blue and Deep Red are the most efficient colors for photosynthesis. But are under emphasized in lumen calculations. 

The human eye is much more sensitive to green than red or blue.  Lumens is a calculation of human sensitivity to light at each visual wavelength.   Lumens is not a measurement of light as in the measurement of photons.  

Lumens is a calculation that takes the measurement of photons and the actual wavelengths are multiplied by a factor as determined by CIE (International Commission on Illumination).

General illumination lighting is a $75 Billion market, so LED datasheets are photometrically oriented for this market.  Typical LED emitted visual light in datasheets are Lumens, Lux ,and Candella.  When the LED's lightwave spectra is not all visible to the human eye milliWatts (mW) are used.  This presents a problem in horticulture lighting as Red, White, and Blue LEDs are specified in Lumens and Deep Red, Far Red, and Royal Blue in mW.   

There are three common methodologies used in the measurement of LED light.
Photometry, lumens, lux, and candelas 
Radiometry, milliWatts 
Photonics, photons, moles  (1 mole of light = 6.022×1023 photon particles)
The three basic methods of light measurement are photometric (photometry), radiometric (radiometry), and Photonic (quantum). 

Radiometry is the study of optical radiation of light, ultraviolet radiation, and infrared radiation.   Radiometric is a measurement of  the actual flux emitted from the light source.   

Photometry, is about human optical visual response to light (luminous spectral response).  Photometric is a measurement of the flux emitted from the light source and then adjusted for its sensitivity to the human eye.  Photometric measurements only includes light visible to humans.  The human eye is most sensitive to green at a wavelength of 555nm (Lime Green) as determined by the .CIE (International Commission on Illumination).  This 555nm wavelength is the international photometric standard to which all other wavelengths are compared. 

Photonics measures light as quantum particles called photons.  Photons are elementary particles of light which carry light's electromagnetic force.  Photons, travel in straight lines at the speed of light, and carry a fixed amount of energy.   Photons have two distinguishing characteristics, direction and wavelength.  Photons are a quantum measurement of light particle energy measured in the quantity of photons per second.  Typically measured with some sort of photon detector which counts the number of photons that hit the detector's surface within a period of time. 









Photometry and Luminosity

To compare the actual (not perceived) amount of light being radiated by two LEDs of different wavelengths you must consider the  the luminous efficacy which overemphasizes the actual radiated flux.

The perceived brightness, or luminous efficacy, of light is technically specified by the International Commission on Illumination (CIE).  Each wavelength efficacy is  related to a wavelength of 555nm (Lime Green).









To convert other colors to the 555nm equivalent output flux, multiply the LED's published lumens by the reciprocal of the CIE factor.













The top number is the reciprocal of the CIE efficacy.