What Makes a Good Grow Light?

There are many horticultural lights that claim to be the optimal grow light. This leaves growers and facility managers with the task of navigating a lot of technical jargon while sorting through the various options. To make matters worse, misinformation is rampant, even among seasoned growers. Some lighting manufacturers capitalize on this lack of information in order to sell their products, and buying need to be wary. One approach is to differentiate between useful and uninformative metrics in the lighting industry.

When researching lighting technology for your facility, you might hear terms like lumens, lux, CRI, CCT, PAR, PPFD, and more. In this article, I’m going to show you which of these metrics (and more) are informative for deciding whether a grow light is good quality. My approach is to cover four defining features of a horticultural light: intensity, colour, distribution, and controls.

1. Light Intensity

Light intensity is the most important factor when choosing a grow light. Several parameters are often used to describe light intensity, depending on the application. Lights intended for human use (industries, offices) use a different set of metrics than those intended for horticultural use (greenhouses, green walls). Some companies attempt to re-purpose industrial lights as grow lights, and you may want to stay away from these products.

Unsuitable Metrics: Lumens, Lux, Foot-candle

Lumens describe the amount of light emitted from a bulb every second (Figure 1). Lumens are useful for describing lights for human eyes, but not plants. Our eyes detect light differently than a plant. Lights that appear bright to our eyes may appear dark to a plant. Along similar lines, lux and foot-candles are also human-centric measurements of light. Lux and foot-candles describe how much light lands on a surface (a desk or workspace, for example). Like lumens, this term unsuitable for understanding horticultural light intensity.

Good Metrics: PPF and PPFD

Photosynthetic Photon Flux (PPF; µmol/s) is the horticultural equivalent of the lumen (Figure 1). It describes the amount of light emitted from a bulb every second and the more PPF a light has, the brighter it is.

Photosynthetic Photon Flux Density (PPFDµmol/s/m2 or µmol s-1m2) describes the amount of light actually arriving at the plant. It is the plant equivalent of lux or foot-candle. What makes both of these measures plant-specific is that they measure light only in the 400 – 700 nm range (the range of light visible to a plant). This range of wavelengths is known as Photosynthetically Active Radiation (PAR). PAR is not a measure of light intensity and has no units. Even this metric is becoming a bit outdated. It was first defined in the 60s, but today we know that plants can detect wavelengths outside of this range, such as ultraviolet and infrared light.

grow light par lux ppfd

Figure 1: Lumens and lux are for describing lighting for humans while PPF and PPFD are used for describing grow lights.

2. Light Colour

Different lighting technology produces different colours of light and this impacts plant growth and productivity. In understanding light colour, you may come across several metrics, but not all of them are useful.

Unsuitable Metrics: CCT and CRI

Correlated Colour Temperature (CCT) describes what colour a light appears to our eyes. A warm yellowish light has a CCT of about 3000 K, or less, while a cool bluish light  will have a CCT of about 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.

Colour 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 (assembly lines, dental work, etc). CRI doesn’t tell us how good a light is for plant growth. However, you may consider it if you have a facility where people need to do detailed work, like diagnosing plant diseases. 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.

Good Metric: Spectral Graph

Spectral graphs are the best way to understand the spectrum of a grow light. These graphs show wavelength along the x-axis and light intensity along the y-axis. Spectral graphs are useful for understanding what colours of light are being produced. Our understanding of the interaction between spectrum and plant growth is still early. 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, and UV). Don’t be fooled by companies that claim to have the optimal lighting spectrum!

3. Light Distribution

Good Metric: Lighting Footprint

To ensure consistent product, a facility should have uniform light distribution. The distribution of light across a surface is determined by the angle of the light and the distance from the fixture to the plants. Light angle is controlled using reflectors or lenses, depending on the lighting technology. Since light distribution can actually get quite technical (requiring geometry, facility layout, and some grey matter), this information is often summarized in a lighting footprint (Figure 2).

For a large facility that requires many lights, a good lighting manufacturer can provide a simulation that predicts light distribution 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.

grow light distribution

Figure 2: Light is brightest directly below the grow light and dimmer at the edges and corners.

4. Controls

When exploring grow light options, be sure to also scope out the controls. It’s well known that the on-off schedule of lights has a large impact on plants. Look for controls that allow you to change 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 shorten the lifetime. Dimming can also result in unwanted 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.

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