LED Grow Lights
Plants convert light energy into plant energy via a process called
photosynthesis. There are two primary compounds that drive
photosynthesis: Chlorophyll A, and B. These compounds absorb primarily
blue and red light, while nearly all other spectra are reflected. The point
at which Chlorophyll
converts light energy into
plant energy most
efficiently, is known as an
absorption peak. These
peaks can be measured in
units called nanometers
(nm). Peak absorption
points for Chlorophyll A
occur at 439nm and 667nm,
while they occur at 469nm
and 642nm for Chlorophyll
B. Beta-carotene 450nm
480-485nm dual peak.
Phycoerythrin 590nm
single peak. Phycocyanin 625nm single peak. 670nm and 700nm for the
Emerson effect. The photosynthetically active radiation (PAR) contains
the wavelengths between 400 and 700 nanometers, and falls just within the
so-called visible spectrum (380-770nm).
Only part of the light that comes from the sun is used by plants for
photosynthesis. The rest of it is reflected. The photosynthetically active
radiation (PAR) contains the wavelengths between 400 and 700
nanometers, and falls just within the so-called visible spectrum (380770nm). The total visible spectrum is perceived by our eyes as white light,
but with the aid of a prism, “white” light is actually separated into a
spectrum of colors from violet to blue, to green, yellow, orange and red.
Plants use the blue to red light as their energy source for photosynthesis.
Kelvin Color Temperature is not how hot the lamp is. Color temperature is
the relative whiteness of a piece of tungsten steel heated to that temperature
in degrees Kelvin. For example, High Pressure Sodium lamps (HPS) have a
warm (red) color temperature of around 2700K as compared to Metal
Halide (MH) at 4200K, which has a cool (blue) color temperature. Daylight
spectrum bulbs most closely replicate the sun’s temperature at 6500K.
Lumen is a measurement of light output. It refers to the amount of light
emitted by one candle that falls on one square foot of surface located at a
distance of one foot from the candle. Traditionally, lumens have been the
measurement of a lamps ability to grow plants; meaning the brighter the
lamp the better the plant growth. However, studies have shown that a
broader color spectrum lamp will perform much better than a lamp with
high lumen output, especially when it comes to plant growth. Lumens and
Lux are measurements of how bright a light source appears to the human
eye. Since the human eye is most sensitive to colors plants don't need, and
least sensitive to colors plants prefer, Lumens can't be used to accurately
compare the plant growing capability of grow lights. If a grow light
manufacturer rates their grow light output in Lumens they are only telling
you how bright the grow light will appear to you and light your room, not
how well it will grow your plants. The most accurate unit of measurement
for comparing grow lights is the micro Einstein, which measures how many
photons of light strike an area per second. But, while this is a much better
way to estimate a lamp's plant growing ability than Lumens or Lux, it is still
very difficult to directly compare two different types of grow lights. All
grow lights except the properly matched LED grow lights emit large
amounts of light that plants don't use very efficiently, so including the nonPAR light output in a light's plant growing measurement is misleading.
Why LED’s? As you can see, by selecting the PAR portion of the spectrum
most of the energy used is available to the plant for growth and fruiting.
LED’s are also a step ahead because they are much more efficient at
converting electrical energy into photons than other light sources. Many of
the other grow light options produce large amounts of heat, a waste of
energy, and a potential risk to your plants. LED’s also have a fantastic
lifespan (around 50,000 hours), making them a good long term solution to
efficient plant growth.
The cons. Buying prefabricated light boards is expensive. Most of the
commercial LED’s light boards are priced to account for the long term
energy savings that are realized when using LED’s, not the real cost of the
product. LED’s (Light Emitting Diodes) are used in a DC (Direct Current)
circuit, and if you design your own you will need to understand the voltage
drop and current draw of your array and provide a DC power supply for it.
Heat, despite the fact that LED’s produce far less heat than other light
sources, heat is produced. The semiconductor junction must be kept within
it’s operational limits or the life of the LED will be greatly diminished. This
becomes very important when you have boards with 100’s of LED’s or if you
use the high power LED’s.
Types of LED’s
Inside the LED you will find a few
interesting things. If you have a water
clear casing, you can easily see what’s
inside. Like you see in the picture, the
first thing you will notice is the lead
posts. The LED is in an epoxy casing
and the lead posts are held secure by
the epoxy casing. Then there is the
semiconductor assembly. Inside there
you will find that there is a reflector
cup. This is to direct the light from
the semiconductor chip up and out of
the casing. The way you can tell the
positive of the LED is if you look carefully inside, you can see the small
internal post is the positive 'Anode' and the bigger internal post is the
negative or the 'Cathode'. The semiconductor chip is a piece of silicon
crystal with a chemical coating that provides the different colors. The
picture below shows a typical light board circuit using the 5mm round
LED’s. As you can see resistors need to be added to the circuit, and will vary
with the type of LED you use and the length of the string in the array. There
are online calculators and tutorials available help with this.
High Power LED’s (HPLED) are very good choice for grow lights. They
drain more power than standard LED’s but they offer high light output. The
original problem with the small LED’s was this. If
you wanted LED’s to produce lots of light you put a
lot of standard LED’s (5mm) in a big LED array.
These were very common in flashlights and grow
lights. But when you crammed so many LED’s into
one light, problems arose. The first was the
problem of size. The head of the flashlight was very
big and the light output was not increased that much per LED added. A well
known fact is "One 100watt light bulb emits more light than three 33watt
light bulbs". So the High Power LED was developed. It requires special
drivers (power supply) but once you have a driver, they are almost as robust
as a standard LED’s. High Power LED’s require a heat sink because they get
warm and if there wasn't a heat sink, the LED semiconductor junction
would burn out very quickly. These LED’s are popular in flashlights because
they are very efficient and they have very high light output for their size.
They can be found in many different power ratings and sizes. This is the
type of LED that we used in our grow light construction. We used three 10
watt LED’s in each of our fixtures, two blue and one red. The LED’s are
available on the web, you just need to decide the wavelengths that you want
and shop around.
Here is a list of wavelengths (nm) that can be used for selecting LED's when
building your own LED Grow Lights or Solid State Plant Lighting:
200 – 280 nm UVC ultraviolet range which is generally harmful to plants.
LED’s in this spectrum are non-existent or very expensive.
280 – 315 nm Includes harmful UVB ultraviolet light which causes plants
colors to fade. UV LED’s in this range are now available and coming down in
price.
315 – 380 nm Range of UVA ultraviolet light which is neither harmful nor
beneficial to most plants.
380 – 400 nm Start of visible light spectrum. Process of chlorophyll
absorption begins. UV protected plastics ideally block out any light below
this range.
400 – 520 nm This range includes violet, blue, and green bands. Peak
absorption by chlorophyll occurs, and a strong influence on photosynthesis.
(promotes vegetative growth)
520 – 610 nm This range includes the green, yellow, and orange bands and
has less absorption by pigments.
610 – 720 nm This is the red band. Large amount of absorption by
chlorophyll occurs, and most significant influence on photosynthesis.
(promotes flowering and budding)
720 – 1000 nm There is little absorption by Chlorophyll here, but
Phytochrome uses a nice portion. Flowering and germination is influenced.
Near and above the higher end of the band is the Infrared spectrum, which
can also be heat and could cause elongation or affect water
absorption/transpiration.
Our solution. I did not want to solder hundreds of the 5mm LED’s on to
light boards. Instead I chose HPLED’s and found that 3-10watt LED’s fit
nicely into an inexpensive light fixture that was gutted for this purpose.
We already have a 12 VDC solar system in
our greenhouse, so the circuit was
designed for 12 VDC. I calculated the
resisters needed and soldered the array
with the resisters together. Next I epoxied
the LED’s directly to the back of the
aluminum fixture with special heat sink
epoxy. The back of the fixture was already
cast with heat fins so this works great! I
made new reflectors out of roof flashing
because the existing fixture reflector had
two large holes in it for the halogen lamp. I
wired them to a timer and they have
worked well for the last year.