There’s no doubt the indoor farming industry is growing. Each month, new farms enter the industry, new technology increases production efficiency and food quality, and hundreds of research projects are conducted around the world. A hallmark of this industry growth are the developments in artificial lighting for indoor farms, namely in LED lighting. As technology advances rapidly, our knowledge of how to use it to its maximum effect is trying hard to keep up. But let’s start with the basics:
The Basics of How Plants Use Light
Light is emitted both as waves and particles. To be more precise, light is emitted as waves of photons, which are essentially bundles of energy. The amount of energy in each photon determines the length of the wave from crest to crest. While wavelengths can vary from nanometers to meters, plant pigments can only use specific wavelengths. Most of those useful wavelengths that occur between 400 and 700 nm on the spectrum.
Growers should strive to match the needs of their plant as closely as possible with their light while considering things like cost and efficiency (spreadsheets can be extremely useful to this end). Light efficiency is the amount of light the plants can use for every watt or kilowatt of electricity that is used. PAR, or photosynthetically active radiation, is the most useful light to the plant. Plant pigments absorb light at specific wavelengths and use the energy in photosynthesis. The three main pigments growers deal with are:
- Chlorophyll a – Absorption peaks at wavelengths around 430 and 662 nm
- Chlorophyll b – Absorption peaks at wavelengths around 453 and 642 nm
- Carotenoids – Absorption peaks at wavelengths around 450 to 454 nm
The most absorbed wavelengths occur around 450 and 660 nm.
Light Intensity: Useful Measurements of Light
Light intensity is commonly measured in three ways: luminosity, PAR and PPFD. Luminosity (or lumens) is a measure of how bright a light appears to the human eye. It is not limited to useful light. As a measurement for grow light intensity, luminosity holds little value. If the wavelength is known, however, it is possible to convert luminosity to PAR.
PAR (measured in micromoles/sec-m2) is the measure of the photosynthetically active radiation, or the useful light energy to plants at a given point in space. A PAR measurement by itself is of little use, but knowing where the measurement is being taken relative to the light source will give you an idea of intensity. Some LED companies do a great job of displaying charts with PAR measurements at various points (coverage at different heights) on their labels. This is the best information you can get.
PPFD (measured in micromoles/sec-m2) stands for photosynthetic photon flux density, and is a measure of the truly useful photons within PAR when the exact spectral composition is known. PPFD measures only the usable portions of PAR, but functions like an efficiency rating of PAR. In other words, it keeps PAR honest.
Daily Light Integral
The daily light integral (DLI) is the real-life translation of PAR or PPFD values into actual growing time. It describes the combination of light and time and represents the amount of PAR necessary on a daily interval to effectively grow a specific crop/plant. A grower may know how much light at certain wavelengths is hitting a square meter every second. But how many seconds of that light does the plant need? That’s the question that DLI answers.
DLI is measured with a PAR meter in mol·m-2·d-1. Notice it is measured in similar units as PAR, only in the context of one day. For example, 12-14 mol·m-2·d-1 is the recommended DLI for lettuce production in greenhouses and even higher DLI values (15-20+) are required for fruiting crops.
Different Types of Grow Lights
High intensity discharge lighting provides high intensity light with a good spectrum for crops, but at the cost of high heat output and ultimately low-light production efficiency. Two types of lighting dominate the HID stage: metal halide (MH) lights and high pressure sodium (HPS) lights.
- High pressure sodium lights produce more red and orange light than metal halides and produce a lot of heat, which results in high operating costs. Their overall spectrum is great, which has made them the most popular HID light.
- Metal halide lights produce blue-heavy light. Most MH lights are limited by a short lifespan, high energy use and handling challenges (the bulb cannot contact oil, including the oil from fingertips).
- Fluorescent lights have been used for a long time in indoor operations but they lack the intensity for serious production. Capital expense and operating costs of fluorescents are fairly low compared to HID lights. Replacement and disposal costs, along with fragility and a non-specific spectrum have limited the use of fluorescents in larger indoor operations.
- Induction lighting, similar to fluorescents, uses magnetic fields rather than filaments to produce light. Induction lights have long lifespans and moderate efficiency. They have yet to find significant traction in the farming industry.
- LEDs belong to a rapidly growing industry with continually decreasing costs. Benefits such as ruggedness, efficiency, dropping costs of manufacturing, low operating costs and spectrum specificity tip the scales in favor of LEDs for indoor growers.
A Focus on LEDs
Since more and more growers are choosing LEDs to light their farms, and more manufacturers are entering the LED space, a guide to understanding LEDs will benefit growers.
Anatomy of an LED
A light-emitting diode is a device that emits light as one specific wavelength when energy passes through it. Two types of materials, each a different kind of semiconductor, are joined, creating the diode. Each of the semiconductors in the diode has a different charge—one semiconductor is negative and one is positive.
When energy passes through the combined semiconductors, the electrons in the negative semiconductor and the “holes” (positively charged carriers) in the positive semiconductor are activated. The negatively charged electrons in one semiconductor slam into the “holes” (which are positively charged) in the other semiconductor. Since the positive and negative charges aren’t perfectly equal, they cannot perfectly cancel each other out. The excess energy has no place to go, and it is emitted as photons of light.
Source: Maximum Yield, April 7 2020,https://www.maximumyield.com/modern-farmers-guide-artificial-lighting/2/1484