There are many lights on the market that claim to be the optimal grow light. Since every light cannot be the best, it’s up to you to stumble, trip, and plod through the technical jargon in order to make an informed decision. And the lighting manufacturers do not make this easy! There is plenty of misinformation (even among the so-called experts) when it comes it horticultural lighting. When talking to a lighting manufacturer you might hear terms like lumens, PAR, and PPFD flung around and it can be hard to know which ones are useful and which ones are simply a sales pitch. How do I know the difference, you might ask? Well, I used to sell horticultural lights! In this article, I’m going to cover four critical factors to consider when buying grow lights: intensity, distribution, spectrum, and controls.
1. Light Intensity (PAR, PPF, PPFD)
Light intensity is one of the most important (if not the most important) factor when choosing a grow light. Several parameters are often used to describe light intensity, depending on the application. Some metrics you may come across are PAR, lumen, lux, foot-candles, PPF, and PPFD. Let’s run through these terms to sort out the useful ones from the good-for-nothings.
PAR, or Photosynthetically Active Radiation, is NOT a measurement of light intensity. It simply refers to the wavelengths of light ranging from 400 nm to 700 nm. There are no units associated with PAR because it’s not a measurement term! PAR was actually first defined in the 60s, but today, we know that plants can detect wavelengths outside of this range (such as ultraviolet and infrared light). This means that like lava lamps, bell-bottoms, and mood rings, PAR has become rather outdated.
Lumens describe the amount of light emitted from a bulb every second (Figure 1). Lumens are meaningless to growers because they are specific to our human eyes. Our eyes can see some colors of light better than others and therefore these colors will score more lumens than other colors. But plants “see” light differently than we do. And light that looks popping to our eyes may be crummy for a plant. Thus, lumens are not a good way to measure the intensity of a grow light. Along the same lines, lux and foot-candles are also human-centric measurements of light. Lux and foot-candles describe how much light lands on a surface. Lux is measured in lumens/m2 and foot-candles are measured in lumens/ft2. These terms are garbage for describing lighting for growing plants and should raise some red flags if you hear a manufacturer using them.
PPF (Photosynthetic Photon Flux) is the plant-equivalent of the lumen (Figure 1). It describes the amount of light (in photons) emitted from a bulb every second. PPF is expressed in micromoles per second (µmol/s) and the more PPF a light has, the brighter it is. Photosynthetic Photon Flux Density (PPFD) describes the amount of light actually arriving at the plant. PPFD is measured in micromoles per second per meter squared (µmol/s/m2 or µmol s-1m2). PPFD must be measured at a defined height, because PPFD decreases as you move further away from a source. Both PPF and PPFD are marvelous terms for describing the intensity of a grow light.
Figure 1: Lumens and lux are for describing lighting for humans while PPF and PPFD are used for describing grow lights.
2. Light Distribution
To make sure that all the plants in your facility grow at the same rate and deliver similar yields, the light must be distributed uniformly across the plants. Light distribution describes the pattern of light falling on your plants. The distribution pattern is determined by the angle at which the light beams leave the bulb/fixture and the distance from the fixture to the plants. Manufacturers control distribution using reflectors (typical of HPS, metal halide and other mercury-vapour lamps) and lenses (typical of COB-style LEDs). Since light distribution can actually get quite technical (requiring geometry, physics, and some grey matter), this information is often summarized in lighting footprints/maps. These footprints show how the light is distributed and an example is shown to the right (Figure 2).
For a large facility that requires many lights, a competent lighting manufacturer will provide a lighting plan or simulation that can predict the light distribution over your plants before installation. These lighting simulations take into account facility-specific features like the location of tables, work areas, and walkways, etc. This ensures that no light is wasted on illuminating the wrong places, like the floor, walls, and ceiling. Lighting simulations are commonly free.
Spectrum describes which wavelengths (colours) a grow light emits and the intensity of each of those wavelengths. Typically, a grow light’s spectrum is quantified using the following metrics: CCT, CRI, and spectral graphs. Correlated color temperature (CCT) describes what color a light appears to our eyes. Weirdly enough, the units for CCT are in degrees Kelvin. The explanation behind this unit choice is long and boring and involves black-body radiation, so I won’t go into it here. A warm yellowish light has a CCT of ~2000 K or less, while a cool bluish light will have a CCT of ~5000 K or more. Neutral white light (like sunlight) has a CCT of about 4000 K. Because CCT is a human-centric measurement, it is a poor indicator for how a light will appear to a plant. The color rendering index (CRI) describes how well we can see an object under a light source. CRI is given on a scale from 0 to 100 percent and is often used when choosing lighting for detail-oriented activities (think assembly lines, dental work, and tattoo studios). Since CRI is a human-centric measurement, it really doesn’t tell us much about how a plant will “see” a grow light. However, if you have a grow facility where people need to do detailed work (like diagnosing plant diseases, cutting clones, etc.) they need to be able to see what they’re doing! Lights with a CRI of 80 or greater is enough for human work. CCT and CRI are both poor indicators of a grow light’s performance, since both measurements revolve around how we see light, and not how a plant sees light.
Spectral graphs are the best way to understand the spectrum of a grow light (Figure 3). These graphs show wavelength along the x-axis and light intensity along the y-axis. Each lighting technology (fluorescent, HPS, metal halide, LED, etc.) produces a unique spectral signature. Unfortunately, the effect of spectrum on plant growth is complicated and is only partly understood. As a general rule, a grow light should have high amounts of blue and red light and moderate amounts of other colors of light (green, yellow, orange, purple). Be cautious about gimmicky grow lights that claim to have the perfect spectrum for super-charging plant growth.
When exploring grow light options, be sure to also scope out the controls. Controls are last on this list because they are a “nice-to-have” and may not be essential for hobby growers and small facilities. Look for controls that allow you to fiddle around with light intensity, on/off times, and spectrum. Note that different lighting technologies have restrictions on how finely they can be controlled. For example, most mercury-vapour lamps should not be dimmed below 50% of power because it can degrade the life of the grow light. Furthermore, dimming can result in spectrum changes. On the other hand, LED lights can be dimmed much lower (down to 1%) without negatively impacting the light or changing the spectrum.