What are they and why should we care?
First off, full spectrum is not a purely scientific term, but more of a generalized descriptor of the properties of a light source. Essentially it means that the grow light puts out significant radiation in all bands of the visible light spectrum. Perhaps a more accurate phrase for what we are trying to emulate would be balanced full spectrum, with the average human eye responding to wavelengths from about 390 to 700 nm.
Plants have evolved for millions of years to best adapt to drawing the most energy possible from solar radiation to convert CO2 and H2O into sugar for growth (photosynthesis). Because of this, the absolute best light source known for horticulture is the sun; and for growing marijuana indoors, it will be a grow light that best mimics the intensity and spectrum of the sun.
LED grow lights – More detail
HID grow lights – More detail
High pressure sodium (HPS) grow lights – More detail
Metal halide (MH) grow lights – More detail
Fluorescent (CFL) grow lights – More detail
Chlorophyll and Cartenoids
These are the pigments key to photosynthesis and are found in chloroplasts; primarily in stem and leaf cells.
Chlorophyll is a green pigment and is the main converter of energy to sugar. This molecule absorbs mostly red and blue light while reflecting much of the green light; hence why plants appear green.
There are two distinct chlorophyll molecules aptly named chlorophyll A and chlorophyll B.
Chlorophyll A is the most important molecule in photosynthesis as it easily passes its energized electrons onto molecules that manufacture sugar.
Chlorophyll A is sensitive to spectral peaks at 430nm and 660nm while chlorophyll B is sensitive to spectral peaks at 460nm and 630nm.
In order to absorb and transmute as much of the sun’s energy as possible, pigments other than green are also required. These ancillary pigments cannot directly convert sunlight into sugar and hence must pass their energy to the chlorophyll receptors for final conversion. Essentially they capture the light bands that chlorophyll cannot.
- Beta-carotene is the most abundant with a strong preference for blue light with other carotenoids absorbing bands of yellow, orange and red;
- Just because these receptors are less in number than chlorophyll does not mean that hold less importance as some are used for triggering and regulating plant growth cycles and other non-photosynthetic functions.
That is why the leading manufacturers of LED grow lights are targeting these secondary receptors by adding full spectrum white LEDs (see method for producing white LEDs) to a multiband mix or switching to white LEDs entirely.
First Generation Two-Band Lamps
The visible light spectrum for a normal, healthy human eye is roughly 390 nanometers to 700 nanometers. A nanometer (nm) is the measure of the wavelength, or frequency, of light in billionths of a meter; the lower the number, the shorter the wavelength.
Marijuana can detect and respond to light in a somewhat wider range than humans do, dipping down into the UV range (<390nm) and up into the far-red range (700nm+). And while yellow and green appear brightest to us, red and blue appear ‘brightest’ to our green brethren. By this, I mean that the greatest conversion of light into photosynthesis occurs in these regions. This range is known as Photosynthetically Active Radiation (PAR).
Don’t let this term PAR scare you. It simply refers to the band of radiation emitted from a light source that a plant can utilize for growth.
NASA started with this knowledge and found that most land plants could grow to fruition merely on two narrow spectral bands; one with a central peak at 630nm (red) and one blue (460nm).
Following this research is what led earlier LED manufacturers to come out with two-band LED grow lights to target two of the four chlorophyll receptors; usually the more abundant and efficient chlorophyll A.
While functional and capable of growing our favorite plant through flower, these early two-band lamps had several limitations:
- Low conversion of energy to light. Early models were probably a modest 10-15 LPW (lumens per Watt), while current models are likely to be in the 50-60 LPW range. LPW is a measure of how much energy put into a lamp is actually converted to light; also known as efficiency. Energy not converted to light will be emitted as radiant heat which will then require additional cooling.
- The narrow beam angle (30-45 degrees) could cause bleaching.
- Having different color emitters spaced some distance apart meant that color mixing (giving each receptor equal amounts of red and blue) would not be uniform.
- Only two of the four chlorophyll receptors were being targeted and few of the non-chlorophyll (carotenoid) related receptors.
Knowing that plants have four distinct chlorophyll receptors led to three and four band LEDs with the addition of deep-red (660nm) and royal blue (430nm), but still ignored the minor receptors not used for energy conversion, but still essential to optimal plant health. These somewhat harder to manufacture bands (color) added more cost to the units.
3rd and 4th generation LED manufacturers added even more bands, particularly in the UV and far-red spectra. Even with multi-band lights, yellow and green were largely absent, so some engineers added white emitters to fill in the missing colors.
This graph above, based on meticulous research, shows the absorption of green land plants relative to the spectrum of light received. Obviously, a lamp best mimicking or following this curve would make for the best spectral output for a grow lamp. To the best of my knowledge, the Spectrum King™ (SK450) comes closest with some added emphasis in the red region specifically designed for extra-large, full buds. After all, this is what we are after!
The sun gives off black body radiation in which there is a continuous band of light in all spectra in the visible and near-visible (infra-red and ultraviolet) range. There are no spikes such as with most artificial light sources. A spike is the over emphasis of a narrow band of the spectra. This generally will not harm your plants, but neither is it optimal.
Believe it or not, the light source that most closely mimics the sun is the lowly incandescent bulb. Then why would we not use them to grow with? The answer is simple: incandescent lights are inefficient and put out far too much energy in the unused infra-red spectrum (heat) for our plant’s needs.
Compact Fluorescents (CFLs)
Compact fluorescent lights are spiral wound fluorescent lights capable of growing marijuana, but are somewhat low in intensity at roughly 45 to 65 LPW, emitting much of the electricity converted as heat due to inefficient ballasts and light trapping in the coils. More on CFLs.
Here is a spectral graph of a typical cool white or 6500K CFL:
Note that while somewhat full spectrum, it has four spikes, a bit too much yellow and green, and not enough red to keep our cannabis plants happy all the way through flower.
High Pressure Sodium
Below is the spectral graph of one of the new breed of ‘optimized’ HPS lamps, the HiLux Gro™ model by Ushio.
Note that while the red and blue spectrums are superior to the previous generation of HPS lamps, the spectral output is spiky and uneven with minimal blue and excessive amounts of green and yellow. Due to sodium being the main element of such bulbs and fluorescing at 589nm, there are severe limits to how much this technology can change. These lamps have a CRI of about 85 and give objects a yellow/orange tint.
HPS lamps are still very popular due to their field-leading output of up to 150 LPW on top-shelf 600W models. This makes them the most efficient bulbs available. More on HPS grow lights.
Metal Halide lamps evolved much more dramatically due to the wide variety of halides that may be added to the sealed gas to customize the spectral output. More on metal halide grow lights.
This is the spectral output of a previous generation with lots of blue, violet and UV; too much yellow and insufficient red. These are fine for vegging marijuana, but are far from optimal.
The current generation has grown leaps and bounds largely due to pressure from indoor medical marijuana growers clamoring for better lights.
The Hortilux Blue™ from Hortilux has an output that very closely matches the sun, and discounting cost, is far superior to any metal halide lamp that has come before. Note that the lumens per watt (LPW) have dropped from about 115 LPW in the standard MH to 80 LPW in the Hortilux Blue™. This does not necessarily mean a decreased radiant flux, but it does mean less output in the yellow and green, which is a plus for horticulture. The down side is high cost and a fairly short lifespan making these lamps good for only two or three grow cycles.
Similar lamps would be the Ceramic Metal Halides (CMH) from GE and Philips. All three of these bulbs have a very high Color Rendering Index (CRI) of 92+ meaning that objects viewed under such lamps look very natural in color.
Full Spectrum LED lights
As I stated in the opening of this article, full spectrum is not a scientific term but it is used to help buyers understand that the lights they are buying are not in bands with gaps but offer a ‘full spectrum’ band of light right through the visible spectrum. Many of the LED lamps out there use this descriptor for their 7-10 band LEDs.
A typical 9-band lamp might contain the following emitters:
Yellow – 580nm
Orange – 610nm
Red – 630nm
Deep red – 660nm
Blue – 460nm
Royal blue – 430nm
UV – 410nm
Infrared (IR) – 730nm
White – 14,000K
Manufacturers such as Black Dog, Apollo, Agromax, and the myriad of Chinese no-name UFO-style and panel lamps offered on eBay.com and AlibabaExpress.com, fall into this category.
While superior to earlier two-band lamps, they still suffer from two problems: color-mixing (giving each photoreceptor on a leaf the exact same ratio of colors) and banding. The color output is spikey and non-contiguous so would more correctly be called broad spectrum lamps.
Next up is the new generation of LED lamps using many colored emitters on a single chip called COB (Chip-On-Board).
These chips are surrounded by a small parabolic reflector. This technology is a notable improvement in that it does away with any color mixing issues. However, that still leaves us with the problem of color banding.
Finally, we shall examine one of the latest entrants in the field, the Spectrum King from SuperGrowLed.com. These technology-leading lamps use nothing but top bin white LED emitters from Bridgelux and Epistar on custom, high-output 45mm dies covering the full range of 420nm to 750nm making them a truly fully spectrum luminaire. These square chipsets focused by 90 degree lenses overcome the common problems of poor color mixing and color spotting (or banding) by putting out a more even pattern than those using discrete 3W and 5W unfocused monochromatic dies.
Just one of these 250W (actual draw) grow lights will match the growing power of a 500W HPS and will cover a 16 square foot are (4’ X 4’) in bloom and an even larger area (25 square feet) during veg. With an unmatched peak photon delivery of 1000μmol @ 12”, a lifespan of 12+ years or 60,000 hours and a five year warranty, it just might be time to put your HID grow lights up for auction.
How does the Spectrum King™ compare to the competition? Let’s take a look. Manufacturers such as BlackDog™ and Haight™ have been popular as a mid-level staples in the grow light market and have upgraded their lights over previous generations, but are still using last generation multiband technology. They both claim full spectrum lamps, but what we actually find is that they have output spikes in every band of color, but are not contiguous as with the Spectrum King™ (SK450). These spikes exaggerate light at some light frequencies while skipping others all together. As covered earlier, our sun has a smooth spectrum throughout – and this is what marijuana wants most for lush, healthy growth.
Give full spectrum LED grow lights a try when growing indoors and you may be pleasantly surprised at the results!