Selecting the best horticultural lighting for indoor grows, greenhouses and plant research can seem daunting, especially when you’re just starting out. While it’s important to choose the most practical horticultural lighting, there are tonnes of options, and several factors you need to consider, including color temperature, lumens, foot-candles, PAR, and PPFD. Although all of these terms are useful for describing lighting, they are not all useful for growers! Here, we will walk you through each of these considerations, to explain which ones are important for your plants – and which ones are NOT! Let’s get started by talking about how humans and plants perceive light.
Did you know that birds are incapable of tasting capsaicin, the chemical that gives hot peppers their heat? Their tongues have no receptors for capsaicin, so no “spicy” signals are transmitted to their brain. In contrast, we humans have special receptors on our tongues called nociceptors, which are plenty capable of sensing spice and sending the appropriate “this is spicy!” signal to our brain. A similar difference exists between the light receptors of human eyes and plant leaves. The human eye has different light receptors than a plant leaf, so we see light differently than a plant. As a result, several of the ways that we describe light are biased towards the type of light that humans are able to see. While plants “see” photosynthetic photon flux, humans see lumens.
Light : A Human Perspective
Lumen (lm) is a unit describing the amount of light (visible to the human eye) emitted by a source per second. Figure 1 shows the wavelength range of light that a human eye can see. Human eyes are most sensitive to light in the yellow and green regions of the spectrum and less sensitive to colors like deep blue and red. Meanwhile, human eyes have a very difficult time seeing infrared and ultraviolet wavelengths of light. Since lumens are a human-centric measurement, we should not use this measure for describing horticultural lighting.
Figure 2: Comparison between foot-candle and lux. Foot-candle is measured over an area of 1 ft^2 while lux is measured over an area of 1 m^2
Color Rendering Index, or CRI, describes the ability of a light source to show an object’s color accurately in comparison with a natural light source (such as the sun on a cloudless day). The highest value a light can achieve is a CRI of 100; lower CRI values result in objects appearing unnatural or discolored. Figure 3 shows an example of how a human eye may see an apple illuminated by lights with varying CRI values. Under a light with a CRI of 100, the apple appears bright red; under a light with a CRI of 70, the apple appears dark and blueish. This measure is dependent on how the human eye sees light, and so is not a useful parameter for choosing horticultural lighting.
Correlated Color Temperature, or CCT, describes the color of a light source and is measured in degrees Kelvin (°K). The higher the CCT of a light source, the cooler the light’s color. For example, a very red light achieves a CCT of about 1000 K while a very blue light can achieve a CCT of about 10,000 K. Warm white lights will have a CCT around 2700 K, neutral white will be around 4000 K, and cool white around 5000 K. Similarly to CRI, this measure is dependent on light perception by the human eye, and once again, is not useful for describing or choosing horticultural lighting.
Figure 1: Wavelengths of light perceived by the human eye. Our eyes are most sensitive to light in the yellow and green regions of the spectrum and less sensitive to colors like deep blue and red
Lux (lx) is a unit that describes the number of lumens visible in a square meter. 100 lumens spread out over an area of 1 m2 will have an illuminance of 100 lx. The same 100 lumens spread out over 10 m2 produces a dimmer illuminance of only 10 lx. For our friends in America, we might use the term foot-candle to talk about light in units of measurement that concern distance in feet and inches. A foot-candle describes the number of lumen per square foot. Thus, one foot-candle is equal to approximately 10.764 lx (Figure 2). As before, this measure is only relevant for how we perceive light, and is irrelevant for plant growth. Like lumens, lux is similarly poor for describing horticultural lighting.
Figure 3: Effect of CRI on how a human eye perceives the color of an apple.