Lighting Metrics and Strategies Explained: Choosing the Right Horticultural Lighting for Your Greenhouse or Indoor Grow

Two numbers on a grow light spec sheet do real work. PPFD tells you how much photosynthetic light reaches the canopy. Efficacy tells you how much of that light you get per unit of electricity. Lumens, lux, footcandles, CRI and colour temperature all describe light as the human eye sees it, and none of them predict plant growth. Definitions for each term are collected in our horticultural lighting glossary. This article covers which metric does what, how to convert between them, where conversion is even possible, what to compare when you are choosing between fixtures, and how to measure whether the light you installed is paying for itself.

Human Metrics Versus Plant Metrics at a Glance

Why Humans and Plants Perceive Light Differently

The human eye has different light receptors than a plant. As a result, how we describe light is biased toward the type of light we can see. While plants see photosynthetic photon flux, humans see lumens. Our eyes are most sensitive to yellow and green light, fall away sharply toward deep blue and deep red, and are nearly incapable of perceiving infrared and ultraviolet light. By contrast, plant photoreceptors are great at perceiving blue and red light and can detect many other spectrums, including ultraviolet and infrared light.

Why Lumens, Lux and Footcandles Mislead You on Grow Lights

Candela is the basic unit of measure of light intensity from any point in a single direction from a light source.

Lumen (lm) is the basic unit of measure for light visible to the human eye. A source with a luminous intensity of 1 candela emits 1 lumen into a solid angle of one steradian. On a sphere of radius 1 ft, one steradian corresponds to 1 ft² of the sphere’s surface. Since lumens are a photometric measurement for humans, we do not use this unit of measure to describe horticultural lighting.

Illuminance refers to the density of light over a given surface area and is expressed in either lux or footcandles:

Lux (lx) is a photometric unit that describes the number of lumens visible in a square metre. 100 lumens spread out over 1 m² will have an illuminance of 100 lx. The same 100 lumens spread over 10 m² produces a dimmer illuminance of only 10 lx.

A footcandle describes the number of lumen per square foot. Therefore, one footcandle is equal to approximately 10.764 lx. This measure is only relevant to how we perceive light and is irrelevant to plant growth. Like lumens, lux and footcandles are not useful for describing horticultural lighting.

Colour Metrics That Do Not Apply to Plants

Color Rendering Index (CRI) describes the ability of a light source to show an object’s colour accurately compared to standardized colour samples under a reference light source. The highest value a light can achieve is a CRI of 100. Lower CRI values result in objects appearing unnatural or discoloured. A lower CRI indicates lower average fidelity across the standardised test colours. It does not tell you how any particular object will shift, since two sources with the same CRI can render an individual colour quite differently. This measure depends on how the human eye sees light, making it an unuseful parameter for choosing horticultural lighting.

Correlated Color Temperature (CCT) describes the temperature of the theoretical black-body radiator whose colour most closely matches that of the light source. The source is not itself heated to that temperature. CCT is measured in kelvin (K). The higher the CCT of a light source, the cooler the light’s colour. The measure only applies meaningfully to white light. Saturated red or blue horticultural channels sit too far from the black-body curve for a CCT value to mean anything. Warm white lights will have a CCT of around 2700 K (since they emit more energy at the red end of the spectrum), neutral white will be around 4000 K, and cool white around 5000 K (emitting more energy at the blue end of the spectrum). Similar to CRI, this measure depends on light perception by the human eye. Once again, it is not useful for describing or choosing horticultural lighting.

PAR, the Wavelengths Plants Actually Use

Plants use the electromagnetic spectrum for photosynthesis, known as Photosynthetically Active Radiation (PAR), which encompasses the wavelengths from 400 nm (nanometres) to 700 nm. In addition to the spectral range of PAR, we also understand plants can detect wavelengths of light outside of this range, including ultraviolet and infrared wavelengths.

PPF Versus PPFD, and Why PPFD Is the One That Matters

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

While lux and footcandles describe the amount of human-perceived light per unit area, Photosynthetic Photon Flux Density (PPFD) represents the amount of light (intensity) useful for photosynthesis (PAR) that arrives at the plant. PPFD is given in units of micromoles of photons in the PAR range delivered to one square metre per second (µmol/m²/s). PPFD is the number that describes what the crop actually receives, which is why it is what a light plan is designed against. PPFD must be measured at a defined height since it decreases as you move further away from the light 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.

PPFD is a spot measurement rather than a single fixed property of the room. In practice the average of several readings is taken across the canopy at a defined height, because intensity falls away both with distance from the fixture and with horizontal distance from directly beneath it.

Intensity and spectrum do different jobs. Light intensity is the main driver of yield, while spectrum shapes how the plant grows, which is covered separately in how plants respond to different wavelengths. Colour temperature and CRI are not how spectrum is specified for plants, because both describe appearance to the eye rather than photon distribution.

Daily Light Integral, the Plant’s Daily Light Dose

Daily light integral (DLI) is the total quantity of photosynthetic light delivered to a given area over a full day. If PPFD is the rate at which light arrives, DLI is the accumulated total once the day is finished.

Measuring rainfall is the closest everyday comparison. A rain gauge cannot tell you the rainfall for a day from one instantaneous glance; it accumulates over 24 hours. DLI works the same way, accumulating PAR over the day rather than describing a moment.

DLI is expressed in moles per square metre per day (mol/m²/day). Note the change of prefix from PPFD: because a full day accumulates such a large number of photons, DLI is stated in moles rather than micromoles. One mole is one million micromoles.

Working it through: a crop receiving 100 µmol/m²/s over a 10-hour photoperiod accumulates 100 × 10 × 3,600 = 3,600,000 µmol/m²/day, which is 3.6 mol/m²/day. Extend the same intensity to a 14-hour photoperiod and the figure becomes 5.04 mol/m²/day. Intensity and duration can be traded arithmetically to reach the same total. They are not biologically interchangeable, because day length also drives flowering and other developmental responses in its own right, so managing photoperiod in greenhouse crops has to be decided alongside intensity rather than derived from it.

DLI is the number growers plan against rather than instantaneous intensity alone, because it is the total dose that accumulates. Raising the daily light integral during propagation has been shown to increase root and shoot dry mass in cuttings, one of the clearest demonstrations of that dose relationship (Torres & Lopez, 2011).

Converting Between Lux, Footcandles and PPFD

There is no single conversion factor between lux and PPFD, and any calculator offering one is giving you an approximation at best. The reason is that the two measure different things. A lux meter is weighted to the sensitivity of the human eye, which peaks in the green, while PPFD counts photons across 400 to 700nm regardless of colour. So the ratio between them depends entirely on the spectrum of the light you are measuring.

Apogee Instruments, who manufacture the quantum sensors used to measure PPFD, publish separate conversion factors for each source. To convert PPFD to lux, multiply by:

The practical consequence is worth sitting with. One µmol/m²/s reads as 54 lux under sunlight and 82 lux under a mogul-base HPS lamp, a spread of more than 50 per cent. A lux meter that says two fixtures are equally bright can be describing two very different amounts of usable light. LED spectra vary widely enough that a single published factor for LED is not meaningful either.

Footcandles carry the same problem with an extra step, since one footcandle is roughly 10.764 lux. Converting footcandles to PPFD means inheriting every limitation above.

Measure PPFD directly with a quantum sensor, sold in horticulture as a PAR sensor, rather than converting from a light meter built for human vision. In a greenhouse this is also how you confirm the crop is receiving a consistent level right across the growing area.

How Much PPFD Does a Crop Need

The target light level at the crop canopy should always be the starting point of a light plan. Everything else, the layout, the mounting height, the fixture choice, follows from the number you are trying to hit.

There is no universal target, because the right level depends on the crop, its growth stage, the daylength you intend to run, and how much natural light the site already receives. Two things make the question answerable in practice.

First, PPFD and photoperiod together determine the daily dose, so a lower intensity over a longer day can deliver the same daily light integral as a higher intensity over a shorter one. That trade-off is usually where the economics sit.

Second, supplemental lighting is sized against what the facility is short of, not against the crop total. In a warehouse or vertical farm where artificial light is the only source, the fixtures supply the entire daily light integral. In a greenhouse they close a seasonal gap, so the same crop in February and in June needs very different amounts of supplemental light, and running hours are correspondingly lower.

A light target only pays off if the rest of the environment keeps pace. Temperature, CO2, humidity and nutrition all set limits on what a crop can do with the photons it receives, and the measured crop yield response to extra light is itself larger at higher CO2 and higher temperature. Adding light to a facility limited by something else spends electricity without buying growth.

How intensity translates into crop output is covered separately in light intensity and its effect on yield. Setting the right canopy light level for a specific site is the calculation a light plan exists to do, because it needs the facility geometry, the glazing, the latitude and the target crop before it produces a number. You can request a light plan and our team will build one for your facility.

Daily Light Integral Targets by Crop Type

Published targets are conditional, and quoting one without its conditions is how facilities end up over-lit.

Lettuce is a lower-light crop, and the figure most often quoted for it is a ceiling rather than a floor. Cornell University’s hydroponic lettuce programme brought butterhead lettuce (cultivar Ostinata) to a target shoot fresh mass of about 150 g in 35 days from seed at a daily light integral of 17 mol/m²/day, held at an ambient CO2 concentration of 350 to 400 ppm. Tipburn was observed whenever more than 17 mol/m²/day was supplied, so the research describes 17 as the level not to exceed rather than a target to aim past. Tipburn is a calcium deficiency in the rapidly growing leaf tips, and overhead fans that raised plant transpiration delayed its onset, which is why that light figure cannot be read apart from the airflow that goes with it.

Fruiting and high-light crops are grown at substantially higher integrals. In one controlled cannabis trial, dry inflorescence yield increased linearly with canopy-level PPFD all the way to 1,800 µmol/m²/s, the highest intensity tested, indicating photons were still limiting whole-canopy photosynthesis at that level. That is a research maximum rather than a commercial target.

Higher light is also not unconditionally better. As intensity rises the other environmental parameters have to keep pace, and cultivars within the same crop can differ in how they respond to temperature and intensity, so some perform poorly under high light.

What More Light Is Actually Worth

Growers have long worked to a rule of thumb that a 1 per cent increase in light gives a 1 per cent increase in crop yield. Researchers at Wageningen University tested that assumption across a range of greenhouse crops and found the response is crop-specific and usually below 1 per cent. They measured roughly 0.7 to 1 per cent for fruit vegetables such as tomato, cucumber and sweet pepper, 0.8 to 1 per cent for soil-grown vegetables such as lettuce and radish, 0.6 to 1 per cent for cut flowers, and 0.25 to 1.25 per cent for bulb flowers. The relative gain is largest at low light levels, at higher CO2 concentrations and at higher temperatures, which is why supplemental light returns most in winter.

Efficacy, and Why µmol/J Is the Number to Compare Fixtures On

Two fixtures can deliver identical PPFD to the same canopy and cost very different amounts to run. Photosynthetic photon efficacy is what separates them. It is the fixture’s photosynthetic photon flux divided by the electrical power it draws, expressed in micromoles per joule (µmol/J).

A fixture rated at 2.0 µmol/J produces twice the usable photons per unit of electricity as one rated at 1.0, so over a season the difference lands on the energy bill rather than on the crop.

For a measured baseline, Nelson and Bugbee tested commercial fixtures in 2014 and found mogul-base HPS at 1.02 µmol/J, the best double-ended HPS at 1.66 to 1.70 and the best fluorescent at 0.95. LED efficacy has moved substantially since that work was published, and current figures for listed fixtures are published on the DesignLights Consortium Horticultural Qualified Products List. Published efficacy figures for each fixture are on the LED fixture pages, and efficacy is the metric that makes the ROI of an LED retrofit arithmetic rather than opinion.

Two cautions when comparing published figures. Efficacy should be quoted for the whole fixture including its driver, not for the diodes alone, and it should be stated over the same waveband, since a figure that counts far-red photons is not comparable with one that counts only 400 to 700nm.

Light Distribution and Uniformity

To ensure all plants have similar yields, the light distribution must be as uniform as possible throughout the growing area. Light distribution describes the direction and intensity of light emitting through a luminaire. The distribution pattern itself is a property of the reflector or optic. Mounting height and aiming then determine the pattern that lands on the canopy. As the height of the grow lamp above the canopy increases, intensity at the canopy falls and the lit footprint spreads wider. The distribution angle itself is set by the reflector or optic and does not change with height. Lighting manufacturers publish photometric distribution diagrams for their luminaires, which provide a visual guide to the type of distribution expected.

The curve above is the light distribution of the Beta reflector fitted to our double-ended HPS luminaires. The light source sits at the centre, the radiating lines mark the angle either side of straight down, and the distance from the centre out to the curve is the luminous intensity in that direction rather than a distance from the lamp. The Beta reflector concentrates its output between 0 and 45 degrees in the lower hemisphere, which is what carries light down into a tall crop, and it redirects 93.5 per cent of the lamp’s output onto the crop surface.

Reflector efficiency is the percentage of light emitted by the HPS lamp that the reflector redirects onto the crop surface. It is a different measurement from fixture efficacy in µmol/J, which is described in the section above.

How to Measure Yield

Yield is the measurement that tells you whether the light is paying for itself. Supplemental lighting can increase plant density while obtaining greater fruit production or yields, but that only shows up if you are measuring the space consistently.

The most common way to measure a crop like cannabis is in grams or kilograms per square foot. Measuring yield allows the grower to understand how well the space is being utilized, whether grow conditions need to be adjusted, or whether some varieties produce better than others.

To calculate this, you need to look at your cultivation space and area of plants. Using cannabis as an example, and measuring yield in grams per square foot of growing space:

If you have a 5,000 square foot cultivation area with 100 tables of 4 ft by 8 ft, your grow space would be about 3,200 sq ft. If you reported a yield of 195,000 grams from that space, you can now do your calculation. The final product weight should be reported dry and destemmed, as it should be in a condition that will qualify as a final product.

Total yield in grams ÷ total cultivation area = grams per square foot

195,000 ÷ 3,200 = 60.94 grams per square foot

That space averages about 61 grams. To make this even more accurate, you should measure the yield in grams per square foot per year to get an overall average and a better understanding of where you can improve. From this number you can assess whether you are reaching your goals and whether the parameters should be adjusted. Are there areas to improve, such as increasing CO2, temperature, light levels or plant spacing?

Grams per light is another way to calculate yield, but it is not the most telling way to determine whether your space is efficient, since it only tells you the yield under a single light. You also need to consider the wattage and the spacing of the lights to see efficiency properly. Because it lacks specifics such as the size of the space and how many plants sit under one light, it is not a recommended way of measuring yield or efficiency.

Production Efficiency

While crop yield is an important way to measure profitability, the amount of product a grower can produce measured against the resources used to produce it is a more holistic view of efficiency. It is worth maximizing productivity per square foot the way other industries do, without sacrificing efficiency. The size and setup of your facility account for a good deal of it, and environmental control systems that adjust parameters automatically help streamline growth. Poor production management and planning lead to inefficiencies.

Numerous other factors can affect a crop’s yield, including facility design, genetics, CO2, temperature and nutrients. Keeping these in balance is what raises efficiency across the operation. If temperature, light and CO2 levels are not balanced, the plant will not grow optimally and energy is wasted. An optimized environment will help you get the most out of your lighting system and achieve the highest yields from your crop without sacrificing plant health or quality.

Common Questions About Grow Light Metrics

What Is the Difference Between PPF and PPFD?

PPF is the total quantity of photosynthetic photons a fixture emits per second, measured at the fixture and expressed in µmol/s. PPFD is how many of those photons actually land on a square metre of canopy each second, expressed in µmol/m²/s. PPF describes the light source; PPFD describes what the plant receives, which is why PPFD is the number to specify against.

Can You Convert Lux to PPFD?

Only approximately, and only if you know the spectrum of the light source. Lux is weighted to human eye sensitivity while PPFD counts photons across 400 to 700nm, so the ratio changes with the light. One µmol/m²/s equals about 54 lux under sunlight but about 82 lux under a mogul-base HPS lamp. Any calculator offering a single universal factor is producing an estimate, not a measurement.

What Units Is DLI Measured In?

Moles per square metre per day, written mol/m²/day. PPFD uses micromoles because it measures an instantaneous rate, while DLI accumulates over an entire day and reaches numbers large enough that moles are the practical unit. One mole is one million micromoles.

What Daily Light Integral Does Lettuce Need?

Cornell University’s hydroponic lettuce research put it at 17 mol/m²/day, reaching a 150 g butterhead head 35 days from seed at an ambient CO2 concentration of 350 to 400 ppm. Treat that as a ceiling rather than a target, because tipburn was observed whenever more than 17 mol/m²/day was supplied. Tipburn is a calcium deficiency in the fast-growing leaf tips, and overhead fans that increased transpiration delayed its onset, so the light figure only holds alongside the airflow that supports it.

Why Are Lumens Not Used for Grow Lights?

A lumen measures light as the human eye perceives it, and human vision is most sensitive to yellow and green. Photosynthesis draws on photons right across the 400 to 700nm range, which is why PPFD counts them evenly, whereas the lumen weights green and yellow most heavily and discounts the rest. Optimising a fixture for luminous output therefore optimises how bright it looks to a person, which is a different target from how many photosynthetic photons it delivers.

What Is a Good Efficacy for a Grow Light?

Efficacy is measured in micromoles per joule, and higher is better because it means more photosynthetic light for the same electricity. As a measured baseline, Nelson and Bugbee found mogul-base HPS at 1.02 µmol/J and the best double-ended HPS at 1.66 to 1.70 µmol/J in 2014. Compare figures for the complete fixture including its driver, and over the same waveband.

References

Apogee Instruments. Conversion, PPFD to lux. Source-specific conversion factors for sunlight, fluorescent, metal halide and high pressure sodium lamps.

Both, A.J. Ten years of hydroponic lettuce research. Review of the Cornell University Controlled Environment Agriculture programme, Rutgers University.

Marcelis, L.F.M., A. Elings, M.J. Bakker, E. Brajeul, J.A. Dieleman, P.H.B. de Visser & E. Heuvelink. 2006. Quantification of the growth response to light quantity of greenhouse grown crops. Acta Horticulturae 711: 97-104. doi:10.17660/ActaHortic.2006.711.9.

Nelson, J.A. & B. Bugbee. 2014. Economic analysis of greenhouse lighting: light emitting diodes vs. high intensity discharge fixtures. PLoS ONE 9(6): e99010. doi:10.1371/journal.pone.0099010.

Rodriguez-Morrison, V., D. Llewellyn & Y. Zheng. 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12: 646020. doi:10.3389/fpls.2021.646020.

Torres, A.P. & R.G. Lopez. 2011. Photosynthetic daily light integral during propagation influences rooting and growth. HortScience 46(2): 282-286. doi:10.21273/hortsci.46.2.282.

Written by the P.L. Light Systems team. This article was originally written by Steve Szewczyk and has since been updated and expanded. Get in touch with our team about lighting for your facility.