Horticultural Lighting 101

krunchy bubble high res cannabis plant stem

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.

difference between lux and footcandles
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.

Wavelengths of light perceived by the human eye
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.

change in colour rendering demonstrated on apple
Figure 3: Effect of CRI on how a human eye perceives the color of an apple.

While plants “see” photosynthetic photon flux, humans see lumens

Light: A plant perspective

Human eyes are great at seeing yellow and green light, so-so at seeing red and blue light, and nearly incapable of seeing infrared and ultraviolet light.  By contrast, plants “see” a completely different spectrum than us.  Plant photoreceptors are great at perceiving blue and red light, and are capable of detecting many other spectrums of light – everything from ultraviolet to infrared light!  Plants are able to use a spectral range for photosynthesis, referred to as Photosynthetically Active Radiation, or PAR that encompasses the wavelengths from 400 nm (nanometers) to 700 nm (Figure 4).  The spectral range of PAR was first defined in the 60s, but today, we understand plants to be capable of detecting wavelengths of light outside of this range (such as ultraviolet and infrared wavelengths).  Despite this advancement in our knowledge, lighting manufacturers continue to define PAR to be the wavelengths between 400 – 700 nm.  For this reason, when we compare horticultural lighting, it’s important to pay attention to both the PAR value as well as the spectrum outside of the traditional PAR range.  Click here to learn more about how we tailor URSA light spectrums to plant photoreceptors.

We mentioned earlier that lumens measure the amount of light emitted by a source per second, which is visible to the human eye.  For plants, we use something called Photosynthetic Photon Flux, or PPF, which describes the quantity of PAR that is produced by a source per second.  This measurement is expressed in micromoles per second (µmol/s).  PAR is an important parameter when considering horticultural lighting, but even more importantly is PPFD.

While lux and foot-candles describe the amount of human-perceived light per unit area, Photosynthetic Photon Flux Density (PPFD) describes the amount of light useful for photosynthesis (PAR) that arrives at the plant.  PPFD is measured in micromoles per second per meter squared (µmol/s/m2 or µmol s-1m2) and is perhaps the most important value for growers to pay attention to while comparing lights.  PPFD must be measured at a defined height, because PPFD decreases as you move further away from a source.  Ideally, PPFD is also measured over a defined area, as most lights are brightest directly below the source and have reduced intensity as you move further away.  Figure 5 shows an example of typical PPFD measurements for URSA Lighting’s Optilux luminaire.

PPFD and PPFDi demonstrated on a basil plant
Figure 6: On the left is the Intercepted/intracanopy PPFD of a basil plant; On the right is how to calculate leaf area index.

To ensure that all plants thrive and have similar yields, it is important that the light distribution is as uniform as possible throughout the grow area.  Light distribution describes the direction and intensity that a luminaire emits light.  The distribution pattern is determined by the height of the light and the angle of the light beams.  As the height of the light above the canopy increases, light intensity decreases.  When comparing horticultural lighting, note the recommended hang height because some luminaires (like high pressure sodium, HPS) emit large amounts of infrared light that increase air temperature.  A HPS light placed too close to a plant can increase leaf temperature and negatively impact photosynthesis. The light distribution pattern is also dependent on the direction that light is emitted from the luminaire.  Let’s suppose we have a light shining above the plant canopy (Figure 7).  If a beam of light falls straight down onto the canopy, the light is quite bright!  However, if a beam of light falls at a slanting direction, the light falling on the canopy is not as intense.  This effect is known as Lambert’s Cosine Law, and is important to consider when choosing how you will position your horticultural lighting within your growing space.  Figure 5 shows an example of the light distribution from the Optilux over a 4’ x 4’ area.  If we wish to expand our growing area to a 4’ x 16’ area, we could overlap the edges of the Optilux distribution pattern to achieve a uniform light distribution (Figure 8).  Before you purchase your horticultural lighting, it is essential that you consider the best distribution for your grow space.

crosshatching with four optilux lights
Figure 8: Overlapping (“cross-hatching”) the light distribution from four Optilux lights projected in a 4’ x 16’ area.
photosynthetically active radiation graph
Figure 4: PAR is an outdated measure of the wavelengths of light perceived by plants that spans from 400 to 700 nm. Today we understand that plants can perceive light outside of this range.
Optilux light distribution
Figure 5: PPFD measurements performed over a 4’ x 4’ grid for an Optilux light measured at a height of 24”.

We will expand our conversation about PPFD to include a second important parameter: PPFDi or Intercepted Photosynthetic Photon Flux Density.  Depending who you’re talking to, PPFDi is sometimes referred to as intracanopy PPFD or simply local leaf irradiance. PPFDi tells us the amount of irradiance within the plant canopy (Figure 6)1.  We’re going to get into a bit of math here to show you how to calculate PPFDi for your plants.  The equation for PPFDi is:

PPFDi = PPFD exp(-kL)

Where: PPFD is the amount of light that arrives at the top of the plant; k is a coefficient that describes how easily the leaf can be penetrated by a beam of light; and L is the leaf area index.

The value for k is related to leaf angle: canopies with more horizontal leaves (like sunflower, tomato, and cannabis) have large k values (0.7 – 1.0); canopies with more vertical leaves (like wheat and bamboo) have small k values (0.3 – 0.6)2.  Leaf area index can be determined by dividing the total leaf area by the ground area covered by the canopy (Figure 6).  A horticultural lighting manufacturer will not list PPFDi because it is dependent on the species-specific k value and plant-specific L value.

Lambert’s Cosine law light angle
Figure 7: Lambert’s Cosine law states that the intensity of the light is proportional to the cosine of the angle of the light beam.

We will end with a mention of Daily Light Integral (DLI), a parameter describing the cumulative PPFD over an entire day3.  A grower can think of DLI as the plant’s daily dose of light.  DLI is strongly and positively related to photosynthesis, growth rate, and overall productivity.  The units for DLI are similar to that of PPFD, but extended over an entire day (µmol/d/m2 or µmol d-1m2).  The value for DLI will depend on both the intensity and duration of light the plants receive.  For example, a plant that receives 100 µmol s-1m-2 over a 12 hour photoperiod will have a DLI of 4.32 mol/d/m2.  If we extend the time our light is on to 16 hours, we are now delivering a DLI of 5.76 mol/d/m2.  As a side note: sometimes DLI is referred to as PPFDD.  Horticultural lighting manufacturers will never list DLI because it is dependent on how long you choose to illuminate your plants for.  With that in mind, DLI remains an important parameter for a grower to consider when purchasing horticultural lighting.

The most important metrics to consider when selecting horticultural lighting for your indoor grow, greenhouse or plant research are described above.  Next time you are comparing horticultural lighting options for your facility, keep these parameters in mind.  URSA Lighting will always publish these metrics in the product literature to help you make the best decision.  Click here to see some of our most popular options for horticultural lighting.


  1. Ruppert, H., Kappas, M. & Ibendorf, J. Sustainable bioenergy production– an integrated approach. (Springer, 2013).
  1. Brian J. Atwell, Paul E. Kriedemann & Colin G.N. Turnbull. Leaf area index and canopy light climate. Plants in Action (2010).
  2. Torres, A. P. & Lopez, R. G. Photosynthetic daily light integral during propagation of Tecoma stans influences seedling rooting and growth. HortScience 46, 282–286 (2011).
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