What is PAR in Grow Lights? What is the Best for Plants?

PAR is one of the several parameters to check when choosing a grow light for your plants. But here is where the confusion starts. Is a high PAR better for plants or not? How much PAR is enough? If you have no idea, let me guide you!

In general, PAR is how much plant-usable light is produced by a grow light. Overall, higher PAR improves photosynthesis and results in greater harvest in plants. However, PAR that is either too high or too low can greatly harm a plant’s development. Each plant has its preferred PAR ranges.

Are purple—also referred to as magenta and “burple”—grow light systems better than white ones? Is green light as useless as they say? Read on as I answer all these questions and bust all the myths surrounding horticultural lamps!

PAR in Gardening (The Answer is in Science!)

Studies define PAR or photosynthetically active radiation spectrum as the “portion” of light that a plant can use to grow and develop. In scientific terms, this goes from 400 to 700 nm or 350 to 750 nm.

In the image above you can see the colors that plants like (within the window). This is what PAR is, the part of the rainbow that plants use to grow [1].

Now, if this sounds too complicated, think of it in this way—the light you see in your everyday life is the combination of different “basic” light colors. The classic rainbow is a breakdown of those colors that nature does for us! All those colors are “inside” the white light we see.

Of such colors (measured in nanometers, nm). Some are more readily useful to plants than others. For instance, blue and red are easily absorbed by plants, while green is a bit less. These plant-absorbable colors of light range from 400–700 nm and they are collectively called PAR. Hence, PAR is a good light that promotes plant development.

Now, this is a very easy explanation that encompasses—simply and comprehensively—some basics of quantum mechanics. I will not dig too deep into it. This is all you need to know to understand what light is! If you want to learn more, you will find a more complex and complete discussion at the end.

Now let’s move to the PAR in grow light!

What Does PAR Mean in Grow Lights? (3 Ways)

A PAR value in a grow light identifies how much of the useful plant light is produced by the device. There are three basic ways the PAR is measured and reported in grow light manuals: PPF, PPFD (in a PAR chart), or DLI. Among the three, PPFD is most commonly used by grow lamp manufacturers.

Photosynthetic photon flux (PPF) refers to PAR emission values, whereas photosynthetic photon flux density (PPFD) refers to PAR concentration values. I will explain these further in the following sections.

Some people use the term intensity to define both PPF and PPFD. Even though these two values are closely related, they measure different things. As such, I decided to use other terms to more accurately describe the two.

Other PAR-related values gardeners will likely encounter are as follows:

• Photosynthetic Photon Efficiency (PPE)
• Yield Photon Flux (YPF)
• Yield photon flux density (YPFD)

PAR #1—PPF (Photosynthetic Photon Flux)

In a grow light, the PPF is the total amount of usable photosynthetic photons (PAR light) emitted by a light source each second that is available for plants.

The PPF is quantified in μmol/s or μmol·s^-1—micromoles per second. The higher the value the more usable light for plants is produced.

Measurement of PPF values equally considers photosynthetic photons, regardless of the specific light colors (wavebands) they are classified under.

To give you a more plain description, the PPF is the PAR output or emission of a grow light in a single second.

PAR #2—PPFD (Photosynthetic Photon Flux Density) and the PAR Chart

PPFD in a grow light is the total amount of PAR light emitted by a light source each second that is available for plants within an area of one square meter. This is typically shown in a square or rectangle grid.

In a grow light manual, this is shown as a square grid with numbers inside. Such numbers are the PFFD. It is quantified in μmol/m²/s or μmol·m^-2·s^-1—micromoles per square meter per second.

Since this takes into consideration the surface area a grow light can cover, you can think of the PPFD as the concentration of the device’s PAR output in a given area.

An example of such a PAR chart is shown below.

This is what you typically see in a grow light manual. It resembles a checkerboard with a number in each square. These are the PFFD values.

The PAR chart changes according to 1) height, 2) surface area, 3) location, and 4) uniformity.

1. PAR Chart is Affected by Height

The PAR chart provided by a grow light manufacturer is based on a specific distance between the bottom-most part (the face of the bulb or panel) of a grow lamp and the uppermost part of the plant canopy.

The smaller the above distance, the higher is the PPFD.

Increasing the height will reduce the number of plant light a plant can efficiently receive. Contrarily, decreasing the height will raise the photosynthetic photon flux density. So if you have plants that demand less light (e.g., herbs) and a grow lamp with high PPFD, position the light higher.

Pro Tip: Avoid having a tight hot spot right below the grow light by hanging it at least 30 cm (1 ft) from the plant canopy. Lighting with considerably low PPFD can be positioned closer (15 cm or 0.5 ft). Installing a horticultural lamp in a space that already receives a lot of natural light (east or west-facing windows), requires appropriate adjustments.

A reliable manufacturer will likely suggest 2 to 3 different hanging heights to indicate their product’s practicality, being suitable for virtually any kind of plant.

2. PAR Chart is Affected by the Surface Area

The PAR chart of a grow light is often referred to a total surface area it can serve. Often the coverage area of simple and readily available grow lights is 1 m².

Newer, bigger, and more complicated models that move and are made for professional and industrial use will have a much wider scope.

The surface area is important as you do not want your plants placed well outside the border where they will receive little to no photosynthetic photons.

3. PAR Chart is Affected by the Location of Light Source(s)

The PAR chart of a grow light always assumes that the grow light is positioned at the center. This explains why the PFFD values are higher at the center and lower at the border.

Plants at the border can absorb about 10 times fewer photons than those at the center.

Pro Tip: Set your light-loving cacti right under the light source, your herbs that require only partial lighting around them, then low-light plants such as pothos at the edges.

Bulbs could produce a tight hot spot at the very center or emit a more spread-out beam of light with moderate to high intensity.

The same can be said for panels—multiple hot spots or more even light distribution and intensity.

4. PAR Chart Should be Considerably Uniform

It’s perfectly normal for the area right underneath a light source to get more PAR compared to the surface around it.

Lighting systems that produce an incredibly uneven distribution of photosynthetic photons will also cause irregular growth for plants.

Pro Tip: Bought a cheap grow light that emits a good amount of PPFD around the center but then almost nothing within just a few inches? Get some aluminum foil, wrap the inside of a large box with it, and place your plants inside. This can improve the uniformity and intensity by 20–35% without the need for an expensive grow tent like this one on Amazon.

A grow light with a uniform distribution over the coverage area, across multiple hanging heights is recommended. Especially so if you are planning to cultivate plants that have similar needs—such as rosemary, thyme, and oregano.

A grow light has an average PPFD. Some manufacturers declare only the average or the highest value of PFFD. If this is the case, then look for a chart similar to the one above. Good quality grow lights always provide the full PAR chart.

So be sure to check if a grow light’s PPFD rating, for your plants and total growing area, in particular, is adequate—if not the perfect amount!

PAR #3—DLI (Daily Light Integral)

The DLI is defined as the intake value of PAR for 24 hours. It is quantified in μmol/m²/d or μmol·m^-2·d^-1—micromoles per square meter per day.

In other words, it is the accumulation of PPFD for a whole day. This is where the recommended length of exposure to the sun—or artificial lighting—for plants (photoperiod) comes in.

Photoperiod vs Intensity: PPFD / DLI - LED Lights Lettuce Experiment (What is DLI?)

Plants, much like us, can tell day from night. They have their biological clock. Light’s presence and absence in a plant’s environment are sensed by its photoreceptors.

However, not all plants will reap the most benefits under a lone daily light integral rating.

Below is a guide for the conventional light requirement of plants and their ideal DLI ranges:

• Low-light plants (shade or indirect lighting): 4–15 μmol/m²/d
• Moderate-light plants (partial lighting): 15–35 μmol/m²/d
• High-light plants (full lighting): 35–70 μmol/m²/d
• Very high-light plants (full intense lighting): 70–130 μmol/m²/d

What PAR is the Best For Plants?

As a rule, the photosynthetic productivity of plants increases proportionally to photosynthetic photon flux density. That is until the upper limit of the suitable PPFD range is surpassed. On the other hand, insufficient PPFD results in stunted plant growth and abnormal development.

There are general suggested PPFD ranges for plants based on their specific characteristics. Providing horticultural lighting with values that are either substantially higher or lower than these is not advisable.

PPFD Range (μmol/m2/s)	Plant Type	Examples
50–150	Microgreens	Wheatgrass Red beet microgreens Pea shoots
150–450	Herbs Green leafy vegetables	Basil Mint Lettuce Spinach Kale Cabbage
450–1000	Flowering ornamentals Fruiting crops	Rose Chrysanthemum Zinnia Pepper Tomato Cucumber

You can think of all plants as having a preset total capacity for PAR. This holding capacity could also increase as plants mature.

To better illustrate this we can think of the capacity as different cup sizes and PAR as water. A seedling could have the capacity of an espresso cup, while the capacity of a mature flowering plant could be as big as a tumbler.

Despite having a considerable difference in capacity, if both are filled to the brim but you continue to pour water into these cups, all the excess water will simply overflow. Conversely, if the cups only have a few drops of water in them, it won’t take much effort to knock them off a table.

Best Light Color For Plants (Fruiting vs Vegetative Growth)

On average, plants of different species greatly utilize the blue and red light color at greater proportions compared to the rest.

A grow light producing warm light with a color temperature of around 2100K has more red light, ideal for flowering and fruiting. On the other hand, cool light with a color temperature of 6500K has more blue light, ideal for vegetative growth.

Plants require all photons from every single PAR waveband (colors) throughout their life. At certain stages, they may need a specific color than others but they still utilize all photons from the sun and grow lamps.

Depending on the type of plant—fruiting plants, ornamentals, herbs, leafy greens, etc.—a higher percentage of a particular color may also be essential.

Moreover, all wavebands can either affect plant growth and development positively or negatively depending on its concentration (percentage) in the spectral output of horticultural lights. Refer to the table below for more details.

PAR Wavebands (Colors)	Range of Wavelength (nm)	Positive Effect on Plants	Negative Effect on Plants
Violet	380–430	Stimulates photosynthesis	Inhibits photosynthesis Overheats tissues
Blue	430–500	Stimulates photosynthesis Boosts chlorophyll production Produces healthier leaves Improves biomass Reduces height Prevents stretching	Burns leaf tips Decreases leaf area
Cyan	500–520	Stimulates photosynthesis	Facilitates stretching
Green	520–565	Stimulates photosynthesis Penetrates canopy deeply Prompts shade avoidance Prevents stretching	Facilitates stretching
Yellow	565–580	Stimulates photosynthesis	Facilitates stretching
Orange	580–625	Stimulates photosynthesis Initiates carotenoid creation	Facilitates stretching
Red	625–740	Stimulates photosynthesis Promotes growth Induces flowering Triggers fruiting Manages seed germination Encourages root propagation	Bleaches leaves Facilitates stretching
Far-Red	700-800	Stimulates photosynthesis Promotes growth Induces flowering Improves total biomass Penetrates canopy deeply Increases leaf area Prompts shade avoidance	Overheats tissues Facilitates stretching Bleaches leaves Causes wilting

How Can You Measure the PAR Values of Grow Lights?

Values of photosynthetically active radiation can be measured using a multitude of devices. The accuracy of their readings can vary greatly.

Some manufacturers clearly state the PAR values of their grow lamps, others don’t. Hence, a device that can confirm whether such figures are truthful and provide real measurements will be incredibly handy.

Devices that can be used to measure PAR values include

1. Spectroradiometer
2. Energy or thermoelectric sensor
3. Quantum or photovoltaic sensor
4. Ceptometer
5. Spectrometer
6. Light meter
7. Integrating sphere

Unfortunately, these PAR devices are quite expensive for most home gardeners.

Now, you may think that these sensors and meters will give you a straightforward reading of photosynthetically active radiation and a complete breakdown of a light source’s spectral quality and composition. But this is not the case.

Remember: Though wavelengths can be used to determine specific wavebands, PAR is not a distinct unit of measurement. Rather, multiple values and units of measurement (as discussed in earlier sections) are used in connection with PAR.

The PAR (Photosynthetically Active Radiation) Spectrum In More Details

Many experts point out that the PAR spectrum goes well beyond the limits of the visible light region—into the ultraviolet (UV-A) and (IR-A) infrared ranges.

At the most basic level, the electromagnetic spectrum is composed of varying radiant energies that can be broadly divided into 7 major bands. Each band has particular properties such as wavelength range that differentiate them from each other.

Of all the bands in the electromagnetic spectrum, many mistakenly believe that the PAR spectrum is only restricted within the region for visible light. This is largely due to the wide use of the 400–700 nm range as a descriptor of PAR in several studies.

In the 1970 research done by Dr. Keith J. McCree, it is shown that plants exhibit photosynthetic activity upon exposure to radiation within the 350–750 nm range of the electromagnetic spectrum [1]. This goes over the visible light range of 380–740nm.

Dr. Keith was the one who established the McCree Curve, otherwise known as the McCree PAR Chart or Action Spectrum.

Moreover, some studies have found that some amounts of UV-A and IR-A (both of which can be found in natural and artificial light sources) produce stimulatory effects on plant growth and development [2].

What Happens When PAR is Available for Plants?

When photosynthetic photons are available from a light source, plants also take in carbon dioxide from the air and water from the soil. All three of these components are vital for the plants to release oxygen, as well as create their food—sugars.

Once a living plant is exposed to full-spectrum light—be it from the sun or a grow light—it can absorb the individual packets of light (photons) using various plant pigments involved in photosynthesis [3].

Light-absorbing pigments are abundantly found in the leaves of most plants. Among these pigments, you are probably most familiar with chlorophyll. But not many are aware of the fact that there are many types of chlorophylls. Just to name a few, there are chlorophylls a, b, c, d, and f.

Besides chlorophylls, the following accessory pigments also absorb PAR photons:

• Carotenes
• Xanthophylls
• Anthocyans

Each of these photosynthetic pigments absorbs PAR photons with a characteristic efficiency that has its highs and lows across the spectrum.

Are Only Certain PAR Wavebands Useful? Which are Useless?

A few wavebands bring about significant effects on plant morphology, but the photosynthetically active radiation spectrum as a whole is necessary for the conversion of water and carbon dioxide into organic matter such as sugar and starch.

It is notable that a wide range of plants—and plant pigment—display high levels of photosynthetic productivity upon absorption of photons within the blue and red wavebands of the PAR spectrum.

So does this mean that only blue and red horticultural lights—either separately or in conjunction—are the only useful wavebands in grow lights for plants?

The answer is: No—not a single PAR waveband can effectively drive the proper growth and development of plants on its own.

Blue and/or red lights are not better than a “complete” white light. Narrow-band or monochromatic grow lights are inadequate for efficient photosynthesis. More importantly, contrary to popular belief, green light isn’t useless at all.

Keep in mind that the PAR region is a mostly-visible spectrum that also has a subset of wavebands—from violet to red. Together, these different colors (wavebands) will appear as a bright white light to the human eyes.

Having said that, I strongly recommend the use of full-spectrum white grow lamps over “burple” and other targeted spectrum lighting options.

Trivia: The term full spectrum can be used interchangeably with wide-band, wideband, hybrid, and broad-spectrum.

Another reason why I recommend broad-spectrum white horticultural lamps over single and double-colored systems is that it is easier to spot abnormalities and diseases under the former than the latter.

Pro Tip: If you find that the spectral quality of your grow light is lacking in certain aspects (e.g., blue), you can simply add supplemental lighting to resolve the issue.

Does correlated color temperature (CCT) have anything to do with PAR?

A grow light’s color temperature (Kelvin or K) can provide a rough estimate of its spectral PAR composition. Warm light (2100K) has more red photons for flowering and fruiting. Cool light (6500K) has more blue photons for vegetative growth. Surprisingly, “cooler” blueish white lights have higher temperatures than “warmer” red-orange white lights.

PPE – Photosynthetic Photon Efficiency

In a grow light, the PPE is the efficiency of a grow light in producing PAR light (400–700 nm only) and it is measured in μmol/W. This determines how much good plant light is produced for the electricity consumed.

The higher the PPF the less the grow light will consume to produce the same amount of useful plant light without transforming much of the electrical energy into unwanted heat.

Find out the PPE by simply dividing the PPF (μmol/s) by the power rating (Watts or W – this affects your bills) of your light source. The resulting value is ordinarily expressed as μmol/W—and no, you don’t have to worry about the “s” which stands for seconds.

Real example: A grow light with a PPF rating of 1000 μmol/s and a power rating of 450W has a PPE value of 2.22 μmol/W. Another lamp, with the same wattage (450W) but with a PPF of 500 μmol/s will produce only half of the usable light, despite costing you the same to run!

FAQs

Are grow lights waterproof?

Not all grow lights are waterproof—especially the cheapest ones. But waterproof models are highly recommended. Plants that are placed very close to the light are going to be watered regularly and accidental contact of water with the grow light might damage it easily if it is not waterproof or water-resistant. Look for an IP rating of at least IPX5.

Is the light emitted by grow lights dangerous for human eyes?

No, the type of light produced by the average LED grow light system is not more dangerous than natural light as they are strictly designed to meet several safety standards. But If you stare for a long time at a LED light bulb it could, of course, cause damage, in the same way that staring at the sun can.

Will all white LED grow lights produce full-spectrum light?

Yes, all white LED grow lights produce full-spectrum light. However, the difference among thousands of different designs lies in the spectral quality or recipe of its light output. For instance, some may have more green components to produce a “whiter” light. As a direct result, blue and red components are substantially lowered.

Do LED lights use less electricity?

Yes, among the available light technology LEDs are the most efficient producing the higher light intensity for the least of the energy provided. In fact, LED grow lights are considered the most energy-efficient lighting system by experts in the field of horticulture.

Can you leave the grow lights ON all the time?

Yes, you can leave your grow lights to operate 24/7. Many plants will not suffer from a 24-hour light cycle provided that other factors (e.g., PPFD, DLI) are adjusted. But winter-blooming plants might not thrive as they normally would. Additionally, some plants require a time of complete darkness each day for optimal growth and development.

How is yield photon flux (YPF) related to photosynthetic photon flux (PPF)?

In contrast to PPF, YPF (yield photon flux) is measured using the weightage of different wavebands of PAR. It takes into account the level of photosynthetic response observed for each light color. Simply put, red and green grow lights can have the same PPF. But the green one will have a lower YPF since red light gets a higher response in plants.

How is yield photon flux density (YPFD) related to photosynthetic photon flux density (PPFD)?

Similar to photosynthetic photon flux density, yield photon flux density is the concentration value of the spectral output of the light source. The only difference is that YPFD is weighted in proportion to the level of photosynthetic response observed for each PAR waveband (colors), from 350–750 nm.

Can you guarantee optimal photosynthesis with a perfect PAR output?

Unfortunately, no—PAR alone does not guarantee optimal photosynthesis. Favorable levels of temperature, water, nutrients, are also required. If these factors are otherwise limited, photosynthesis will be substantially inhibited.

Summary of PAR in Grow Lights

1. PAR—or photosynthetically active radiation—is the single most reliable factor that determines a grow light’s effectiveness in driving photosynthesis and inducing the growth and development of plants.
2. Some strictly specify the PAR spectrum at 400–700 nm, while others give a more extensive range at 350–750 nm.
3. Even though higher PAR values (e.g., PPF, PPFD, DLI) generally mean better plant growth and development, each plant responds more readily and positively to particular ranges. Providing too much or too little results in detrimental effects on the plant.

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Sources

• “The action spectrum, absorptance and quantum yield of photosynthesis in crop plants” by K. J. McCree in ScienceDirect
• “From physics to fixtures to food: current and potential LED efficacy” by Kusuma et al in Nature
• “Photosynthetically Active Radiation: Measurement and Modeling” by Mõttus et al in ResearchGate