We come across many growers who are still skeptical of LED lighting for horticulture. Some of them can’t justify the initial upfront investment, some of them don’t want to sacrifice their HPS fixtures which also provide heat for their growing space, and some of them had a really bad experience with the LEDs before.
As with majority of problems in life and in business, most of them can be solved by obtaining intelligence through education. Therefore, we decided to write this series of articles explaining how lighting works in the horticultural world, what to look for when specifying your lighting and what to straight up avoid.
Let’s first start with the basics.
Let’s start with the basics.
Terminology
Humans use photons to see
Plants use photons to live
Photon
Particles of light which contain energy that is inversely proportional to the wavelength. The shorter wavelength colors such as violet and blue have photons which contain more energy than the longer wavelength colors such as orange and red.
Plants use the energy of the photon to produce carbohydrates such as glucose from water and carbon dioxide. They feed themselves with these carbohydrates.
PAR
Photosynthetically Active Radiation
Defined as the electromagnetic spectrum of radiation from 400 to 700 nanometers that photosynthetic organisms are able to use in the process of photosynthesis. 400 to 700 nm roughly corresponds to the visible colors of light from violet to red.
It is currently the most widely accepted standard.
PBAR
Plant Biologically Active Radiation
Is a new emerging standard from ASABE, the American Society of Agricultural and Biological Engineers. More recent research shows wavelengths from the UV-B range to Infrared can also have impacts on things like plant morphology, seed germination and leaf expansion. PBAR covers the electromagnetic spectrum from 280nm to 800nm, significantly more than what is covered by PAR.
PPF
Photosynthetic Photon Flux
PPF is simply a count of how many photons per second are emitted from the light fixture. This is typically a very large number on the order of hundreds of quadrillions of photons per second. To simplify, we group them into units called micromoles, abbreviated as μmol. The definition of a mole, ff you remember Avogadro’s number from Chemistry class, is 6.022 x 1023. A micromole is simply this number divided by one million, or 6.022 x 1017.
PPFD
Photosynthetic Photon Flux Density
This is arguably the most important metric for growers when dealing with horticultural lighting. PPFD measures how many photons per second are hitting a given area, which is typically a square meter.
So PPF/m2 = PPFD
The units are μmol/s per square meter, commonly written as μmol/m2s
DLI
Daily Light Integral
DLI is a metric that can be specified instead of PPFD for the given growing space.
It is measured in mol/m2 per day
You can also work backwards to determine needed PPFD from DLI.
For example:
Suppose a grower tells you only that their crop requires a DLI of 30 mol/m2d
The other variable you need to know is how many hours of light per day they have the lights on for. Let’s say it’s 12 hours per day of light. The steps are as follows:
Convert that to seconds, since PPFD is measured in μmol/m2
12 hours = 43200 seconds
Divide DLI by number of seconds for number of mol/m2s
30/43200 = 0.000694444 mol/m2s
Then multiply by 1 million (106) to get μmol/m2s
Target PPFD is 694.44 μmol/m2s
Analogous Terms
For those of you coming from the human-centric lighting world, an easy way to keep PPF and PPFD separate is to think of them as analogous to lumens and lux, respectively.
Lux is lumens/m2 and PPFD is PPF/m2.
Fixture Efficacy
Just because the fixture has high wattage, it doesn’t mean it’s good!
You need to consider how efficient the fixture is in converting electrical energy into photon energy. A lot of inefficient fixtures will turn energy into heat, or they will use it to spin the cooling fans and what’s left is used to power the LEDs.
Fixture efficacy is measured in μmol/J, pronounced “micromole per Joule.”
We know that a light fixture’s PPF is measured in μmol/s. The measure of electrical power, the watt, is equivalent to 1 Joule per second (J/s)
Therefore, when measuring the fixture’s efficacy, we measure PPF output per watt of electrical input power
Typical fixture efficacy
| Ceramic Metal Halide lamp: | 1.3-1.8 μmol/J |
| Double-ended High Pressure Sodium lamp: | 1.7-1.8 μmol/J |
| Low efficacy LED fixture: | 1.3-1.9 μmol/J |
| High efficacy LED fixture: | 2.3+ μmol/J |
This number will likely increase for LEDs as the years go by as LED manufacturers squeeze more efficiency out of their products.
As a word of caution – be aware that some LED lighting manufacturers publish “theoretical” output and efficacy numbers. When comparing lighting fixtures, always make sure that you are comparing delivered numbers which include power supply losses and lens losses.
Also, their marketing teams are smart enough to know that “red & blue” spectrum is more efficient than full spectrum (which appears white) so they only post the highest number. Always ask questions and make sure you are comparing apples to apples.
PAR Map
A map of PPFD coverage
PAR map is also sometimes referred to as a lighting plan. It shows exactly how the light is distributed and how many photons fall on the surface of a specific area.
In order to have an accurate PAR map, you need to know the following variables:
- Size of area to be mapped
- Size of discrete measurement areas
- Hanging height of the fixture
Our simulation software can produce PAR maps with pseudo-color intensity representation that show uniformity. Uniformity is important because it ensures that each plant receives the same amount of light and therefore can perform at the same level.
Lighting manufacturers have their fixtures tested and the output files are then used to create the most suitable layouts for each project. Keep in mind, each crop has different light requirements, while lettuce can use as low as 80 PPFD, cannabis with supplemental CO2 can take advantage of 1000+ PPFD.
If you only take away one thing from this article, it should be the need for PAR Maps. PAR maps tell us whether the specific lighting fixture is going to work or not. A lot of discount LED manufacturers from overseas will promise you the moon in the form of a 300W fixture that would replace a 1000W HPS but the science is lacking. A 300W LED fixture will NOT have enough PPF output to compete with the 1000W High Pressure Sodium lamps. At the current state of technology you need at least 600 watts worth of LED lighting with high efficacy to be able to do that.
In the next article, we will dive into the importance of the spectrum, what does each color do to the plant and you can use that information to optimize your grow.
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