F.A.Q.

There are many advantages to choosing Black Dog LED over HID or other LED companies:

• Black Dog LED grow lights run much cooler than HID lights and even fluorescent lights by using less power to produce the same light footprint.
• Black Dog LEDs’ exclusive Phyto-Genesis Spectrum™:
  • Combines multiple LEDs with discrete spectra to help you grow healthier, higher-quality, more disease-resistant plants. Each included color targets specific phytoreceptors / photopigments in plants to maximize growth, robustness and quality.
  • Includes ultraviolet (UV) and far-red / near-infrared (NIR / IR) for optimizing photosynthesis and creation of secondary metabolites (photomorphogenesis) such as pigments, flavonoids, vitamins.
  • Contains less “wasteful” light than any HID and many LED lights, causing leaf surface temperatures to remain lower, meaning you can run your grow warmer and save on cooling and ventilation costs.
  • Is perfect for vegetative and flowering growth, maximizing flexibility and results with a single plant grow light.
• We use only high-power 5W LEDs for efficiently growing plants.
• Our oversized heat sinks and larger housings provide better thermal dissipation to maximize the useful life of the light. Cheaper LED grow lights often skimp on heat management, causing LEDs to degrade and burn out quickly.
• Black Dog LED lights have a large, extremely bright and evenly-covered footprint. Other LED grow lights can be brighter, but only in the center of the footprint by using a focusing lens– if you want to grow plants in the entire footprint of your light, even coverage is necessary.
• Black Dog LEDs come with a five year guarantee.
• Every Black Dog LED light is tested, and if necessary repaired in Veitsbronn, Germany – there is no need to ship lights or get replacement components from halfway around the world.
• Black Dog LED has been selling and standing behind our grow lights for over 5 years, since early 2010. We don’t spend a lot on advertising to keep our lights’ cost down; we rely on word-of-mouth from actual superior results to sell our lights.
• We provide superior customer service– we want you to succeed with our lights, and since we grow with them ourselves, we know what we’re talking about!

At Black Dog LED in USA we see growing as a lifestyle, not just a hobby, and we practice it ourselves. We’re constantly running experimental grows trying to improve our lights; our staff have over 80 years of combined indoor growing experience. For growing all types of plants from food to medicine to ornamentals, Black Dog provides the highest-quality, best-growing lights in the most reliable package.

Black Dog LED lights are designed by growers, for growers. Why settle for anything less?

Our lights appear more expensive than some other LED lights, but that’s because our lights have more actual power than most. When comparing equivalent lights, Black Dog LED lights are not more expensive.

When comparing equal-wattage lights (“true watts” and not the more-often-advertised “LED watts”), our lights are clearly not more expensive than most other LED panels. Even though there are more expensive LED grow lights on the market, at 1015 watts, our PhytoMAX 1000 light has more actual power (wattage) than any other LED grow light on the market– and increases yield 30% or more over a 1000W HPS. Some lights less expensive than ours on a watt-for-actual-watt basis are using low-quality components, poor designs that cannot dissipate heat well, less-expensive LEDs, and/or are omitting important spectra for growing plants for the sake of reducing cost.

Some other LED lights put secondary lenses under their LEDs to make them appear brighter and better on paper, but sacrifice the actual growing footprint of the light in the process– it gives them a better PAR-per-dollar rating than our lights, even though they have an extremely small effective footprint. We’re more interested in maximizing your yield-per-dollar, year after year!

All indoor growers now have a choice to make: whether to skip up-front costs and deal with higher long-term expenses, or to pay more up front for the highest quality, most advanced grow light on the market and save money year after year.

Black Dog LED lights start with the best LEDs available. Most of these cost 2 to 3 dollars each, and some cost up to 10 dollars each. Many companies use LEDs that cost 10 to 20 cents each, and simply leave out LED colors which cost too much– UV LEDs are particularly expensive and most often omitted. Black Dog LED lights use 5-watt diodes with an ideal beam angle to achieve a large, evenly-covered footprint and superior canopy penetration.

Black Dog LEDs also distinguish themselves from other lights by using quiet. long-life ball-bearing fans in every unit, large, solid aluminum heat sinks, and larger, full-metal housings designed to dissipate heat better to ensure longevity.

All things considered, it is easy to see why Black Dog LED lights last longer and grow healthier, stronger plants than any other grow lights. The best way to see for yourself is to try one of our lights- once you see how a Black Dog LED grows plants, we’re sure you’ll be happy.

Black Dog LED was founded in early 2010 to provide the best grow lights possible. Based in Boulder, Colorado, our staff have horticulture and electrical engineering degrees, with over 80 years of combined indoor growing experience.

We’re devoted to providing the best artificial grow lighting possible; to this end we are continuously running experimental grows, testing new technology and ideas to ensure our lights are the best you can buy.

Plant Light Science GmbH & Co. KG who is our exclusive distribution partner for Europe also offers excellent customer service, as well as operating our European repair center

We have a lot of experience growing a diverse range of plants under different combinations of LED colors. In our (and others’) experimentation, it is clear some plants such as green leaf lettuce are capable of growing with extremely limited spectrums (i.e. with only red light), although the plants do not grow “normally”– they exhibit significant differences from the same variety of plants grown in natural sunlight. Many plants will not grow well with limited colors of light (even with red and blue included), and without key spectra, normal pigmentation and other secondary metabolites (such as vitamins, compounds related to flavor. etc.) may not be produced by the plant in normal quantities, or at all. However, if all necessary spectra are included in a light, all plants will grow optimally and produce desired secondary metabolites.

Black Dog LED’s Phyto-Genesis Spectrum™ has been painstakingly developed to grow plants of all different types. By going far beyond the “bare minimum” spectra required to just get plants to survive, we successfully grow plants of all species and strains. We have grown over 400 different species of plants under our lights, with representatives from most of the major divisions of the plant kingdom (Plantae), separated by hundreds of millions of years of evolution — and the results are always the same: plants thrive under Black Dog LED.

For the record, we have tested species from the following divisions of plants:

• Chlorophyta (green algae)
• Marchantiophyta (liverworts)
• Bryophyta (mosses)
• Lycopodiophyta (club mosses)
• Pteridophyta (ferns, whisk ferns and horsetails)
• Cycadophyta (cycads)
• Ginkgophyta (Ginkgo)
• Pinophyta (conifers)
• Magnoliophyta (flowering plants)

The 3 divisions we have yet to test are: Charophyta (stoneworts / desmids), Anthocerotophyta (hornworts) and Gnetophyta (gnetophytes).

Choosing the right size light(s) for your setup depends on a number of different things, all explained in our guide to determining the ideal LED grow light setup.

While it is true that green light is mostly reflected by the chlorophyll in plant leaves (this is why they look green), this doesn’t mean plants don’t use any green light. Other pigments in leaves such as carotenes and xanthophylls harvest some green light and transfer it to the photosynthetic process. The small amount of green we include also serves as an aid for viewing the plants, allowing easier diagnosis of issues such as nutrient deficiencies, pest and disease problems.

Here at Black Dog LED, we pride ourselves in providing indoor growers with quality spectrum and a very intense footprint. While things like fans and heatsinks play a crucial role in the longevity of any indoor plant grow light, spectral quality and intensity are determined by the type of light emitting diodes (LEDs) used in a panel. Our proprietary Phyto-genesis Spectrum™ makes use of a very specific and diverse combination of LEDs, each proven through scientific study to stimulate photosynthesis as well as many other desirable metabolic processes in plants. Additionally, we use only 5 watt rated chips to ensure that our lights provide the most uniformly intense footprints possible. In order to include each of the essential spectrums at such high intensities in our lights, we use diodes from several different sources because no one manufacturer provides all of the LED colors required to create our full spectrum. Because they offer the largest selection of wavelengths at higher powers within the visible spectrum, Epistar makes up the bulk of our diodes, while we source specialized high-quality UV and IR chips from other proprietary manufacturers.

On occasion, we are asked to divulge the names of these manufacturers in order to “lend credibility to our sourcing.” However, we do not share this information with the public for one solid reason: it is not in our best interest. The Black Dog LED brand has become synonymous with quality, intensity, reliability, and performance, and we know that other companies would love to be able to copy our spectrum. We’ve spent too much time and money researching and developing one of the best indoor LED plant grow lights in the industry to simply give this information away. When you grow with Black Dog LED plant grow lights, you can grow with the confidence that you’ve chosen one of the best possible indoor LED plant grow light available.

Yes, many LED companies are offering dimmers and the ability to switch off sections of your light to change how it is supposed to work. We have tried many different spectrums and actively changing these spectrums throughout the growing cycle. We have proven what the research tells us; that any major change in spectrum will cause plants to stall while they adjust to the new light.

We know the best scenario possible is to provide the perfect spectrum based on research and provide as much power as possible without waste. Why would you want to buy a sports car but shut off half the engine? Dimmers mean you are using only part of what you paid for in terms of wavelengths and and power, which is why we don’t use them in our products. You are better off keeping the same spectrum and using 2 smaller lights or backing off a bigger one to avoid any shock to your plants from veg to flower.

Your Black Dog LED grow light arrived in the mail; you unpack it, put on your no. 5 welding glasses (safety first!), plug it in and take a look to check out that beautiful spectrum… But you notice that some of the diodes aren’t lit up. Don’t worry, your panel is working perfectly! Those diodes emit light in the ultraviolet and infrared spectra, which are outside of the 400-700nm range of visible light. We’ve included these spectra of light because they encourage resin production and complete phytochrome response. When we say that our lights are full spectrum, we mean it, and now you can see (or not see) why.

Each model of our PhytoMAX-2 Series lights have different recommended hanging distances to achieve the full flowering or vegetative footprint. These heights are given on the page describing each model light, just under the footprint diagram, so go the page for PhytoMAX lights, and choose your model to see the recommended hanging height.

Please note that these are recommended heights and if you are using a light mover or adjusting the light intensity (for more or less light) the heights should be adjusted accordingly. Hanging the light higher will decrease intensity and give a larger footprint, and hanging the light lower will increase intensity but decrease the footprint. Please call us with any specific height questions for your own setup.

Yes, our lights will run on any 50-60 Hz alternating current voltage between 100V-277V for PhytoMAX-2 lights. All that is needed is the right cord to plug our lights into the wall. Our lights use an IEC C14 (male) power connector– the same kind commonly used by computers and HID ballasts– so all you need is an IEC C13 (female) power cord of the right gauge that will work with your wall outlets. For 120V, we recommend a minimum of 16 gauge cords, and for 240V a minimum 18 gauge cord (smaller gauge numbers are thicker, heavier-duty wires, so 14 gauge will always be “heavy duty” for any voltage). These cords are usually very easy to find- they are sold at office supply companies, home improvement and hardware stores, and of course on the internet.

We include an 2.4 meter (8-foot), heavy-duty 230V cord with every light.

The answer depends entirely on the size of the plants. The number of plants that can be grown under a light is the number that would fit in the appropriate flowering or vegetative footprint we specify for the light model. Most commonly-grown plants can be maintained at different sizes through pruning, and the size of determinate plants (those which die after flowering) is dictated by the size they were when flipped into flower, so there is no standard number of plants that will fit in any given area– it is all about how big you want to grow them!

We use primary lenses because they allow the light produced from an LED to most efficiently reach your plants. To understand why, some background information about the refraction of light is important.

Refraction happens when light traveling through one medium (air, water, glass, etc.) enters a different material and is bent (refracted). This occurs because light travels through different materials at different speeds– the speed of light in a vacuum is constant, but it varies in air, water, glass or any other matter. Refraction is responsible for how prisms “split” white light into its constituent colors. Objects appear to bend when you partially dip them in water due to the different degree of refraction between air and water. Refraction is also responsible for rainbows and liquid crystal (LCD) displays.

A material’s index of refraction measures the degree to which light is bent when entering or exiting the material. The greater the difference between two different materials’ index of refraction, the more likely the light is to be reflected back into the first material.

The light-emitting portion of an LED (called a die or chip) is primarily made of silicon, with miniscule amounts of various other elements added to affect the color of light produced. Bare, uncoated silicon has a refractive index of 3.4-3.9, while air has a refractive index of 1.0003. The large difference between the refractive index of silicon and air means that light leaving the LED chip exposed directly to air is often just reflected back into the silicon, as if it were a mirror. Light produced in the LEDs is useless if it never manages to hit your plants!

A lens placed directly on the silicon die actually helps to harvest more light from the LED. Glass, silicone and acrylic have a refractive index of about 1.5, intermediate between that of silicon and air. This intermediate step allows more photons out of the silicon and into the air, actually increasing the amount of light the LED emits. “COB” or “integrated” LEDs are often much less expensive because they don’t use primary lenses, but are much less efficient in terms of photons-per-watt.

Glass and acrylic lenses cause some of the light to be lost, which is one reason we don’t use secondary lenses with our lights, but as a primary lens it actually gets more light out of the silicon LED than any losses it incurs, so primary lenses produce a net gain in light and make LEDs more efficient.

Black Dog LED always uses primary lenses but never secondary lenses for our LEDs– we maximize efficiency and evenly cover the entire footprint to keep all your plants happy, rather than just being bright in the center to look good on paper.

LEDs typically have a plastic or glass lens over the actual LED die (the name of the chip that actually produces light); this lens helps the light produced by the LED chip escape from where it is produced. Photons produced by the silicon chip (LED die) tend to be refracted back into the chip if the surface of the die is exposed to air– plastic and glass lenses pressed against the LED die actually help these photons to leave the LED rather than refracting back into the silicon. These “primary” lenses are present on most LEDs to make them more efficient, and can be designed to focus light to different angles.

Secondary lenses for LEDs are designed to refocus the light from the diode and primary lens into a new, usually narrower beam. Many LED grow light companies are using secondary lenses and claim to “amplify”, “magnify”, or “boost the output of” the light. The secondary lenses are magnifying the light in exactly the same way a magnifying glass does in the sun- but no additional light is being produced or “harvested” from the LED, it is just being focused to a narrower beam or even a point. In fact, about 10% of the light is reflected or refracted by the secondary lens and is lost- but the remaining 90% gets focused into a more-intense beam.

If you’re trying to market your product based on a single measurement of intensity, using secondary lenses will make the light really bright immediately under the center of the light so that any lumen, PAR, YPF or other intensity measurements taken there are impressive.

But just like a spotlight or laser, just off to the side of the narrow beam of light, there is almost no light. Any plants trying to grow in this region are only getting light reflected off the plants immediately under the light fixture– but the grow light certainly looks impressive in an ad with its extremely high PAR value or “569% more light” due to “powerful optics”!

Our PhytoMAX-2 Series lights use a primary LED lens designed to spread the light from each LED diode 120 degrees wide, to maximize the even coverage of our lights over their entire intended footprint to maximize your yield. We don’t want the brightest grow light as measured at a single point; our lights grow plants well over their entire footprint by spreading the light out.

Secondary lenses don’t “amplify” light, they lose about 10% of it and refocus the rest to make a single measurement look better on paper, to the detriment of your plants, and that’s why we don’t use them.

Our vegetative and flowering footprints are based on the light intensity requirements for some of the most commonly-grown high-light plants such as tomatoes, peppers and Basil.

There are hundreds of thousands of species of plants, covering a huge range of light intensity and duration requirements. We simply cannot provide recommendations for every kind of plant, so we had to base our recommended footprint sizes on the plants most commonly grown under artificial light.

For plants requiring less light intensity, such as lettuce, the footprint coverage can be larger than what we recommend. Plants requiring more light intensity such as most Cacti would require a smaller lighting footprint to get enough light.

To make the footprint larger, you just need to hang the light higher over the plants, and to make it smaller, move it closer to the plants.

Our flowering footprints are based on the light intensity needed to grow, flower and fruit high-light plants when the light is only on for 12 hours per day.

For plants sensitive to the length of the day (photoperiod sensitive), it isn’t possible to provide them with more light by just running the lights more hours per day– but for non-photoperiod-sensitive plants, running the light more hours per day will give them more light. For example, increasing from 12 hours to 18 or 20 hours per day will provide 50-67% more light to the plants, so it is possible to cover 50-67% more area with the same wattage light. Additionally, purely vegetative growth generally requires less energy than flowering or fruiting, so plants can get by with a little less light intensity.

No, LED lights do not cause any kind of nutrient deficiency.

A rumor was started that LED lights cause magnesium deficiency when people noticed that some plants develop purple petioles (leaf stalks) or streaks on the stems under LED lights, but not under HPS lights. While purple coloration on stems and petioles can be one of the signs of magnesium deficiency, it is also a sign that the plant is producing natural purple pigments (anthocyanin) in response to ultraviolet (UV) light. Many artificial lights (including HPS and most LEDs) don’t give off UV light, so plants grown under these lights don’t produce this natural pigmentation. Under these UV-lacking lights, purple coloration is often a sign of magnesium deficiency. However, when grown under UV-containing Black Dog LED lights or natural sunlight, plants will produce their full range of natural pigmentation– it is not necessarily a sign of a nutrient deficiency.

The major symptom of magnesium deficiency is usually yellowing, blotchy-looking (chlorotic) leaves, accompanied by purple stems and petioles. When growing under Black Dog LED grow lights, unless the leaves are chlorotic, purple stems and petioles are not a sign of a magnesium deficiency– they are a sign of a happy, healthy plant.

Yes, every light produces heat.  It does not matter if the light comes from a bulb, diode, or a star like our sun; they all produce heat.  LEDs provide a more efficient means for converting energy to light than other methods and therefore produce less heat, but they can not break the laws of physics. Physics dictates that anything that consumes electrical power will emit heat; claims that LED lights don’t produce heat are entirely false– just ask any physics teacher.

While LED lights still generate heat, there are important differences between traditional lighting technology and LEDs:

• HID lights (metal halide, high pressure sodium and ceramic metal halide) require heat to produce light by arcing electricity through selected gasses, making them extremely hot, to the point the gasses glow.  This means HID bulbs themselves are extremely hot– hot enough to start a fire, and many gardens have gone up in flames because of this danger.  LEDs’ electroluminescence technology is entirely different and does not require heat to produce light; LEDs themselves will not get hot enough to start a fire.
• Much of the energy used by HID lights is emitted as infrared light (above 800 nanometers). This “light” is not usable by plants and only works as a “heater”, warming up the plants — and everything else under the light.  This is why HID light feels warm on your skin, while LED light does not.  Our LED grow lights don’t waste energy creating unusable and detrimental infrared light; all the energy goes toward growing your plants.
• Because LEDs aren’t wasting energy producing light plants can’t use, we can use less energy overall to get the same (or better!) growth from plants.  Less energy consumed means less heat; for a given growing area, LED lights will put off less heat than any equivalent artificial light.

LED diodes are rated based on the amount of power (wattage) they can theoretically handle, if they are perfectly cooled. Excessive heat causes LEDs to degrade (“burn out”) and makes them shift the color of light they are giving off, so in the real world LEDs are never run at their full rated wattage. This means that “LED Watts” is a completely useless number for comparing the light output from two different lights– for example, you can have two “500 LED Watt” lights, with one running 100 watts of actual power through the LEDs, and the other running 300 watts of actual power.

Many companies selling LED grow lights only use the LED watts to advertise their lights, as the number is always larger and more impressive than the actual power draw, but it really tells you nothing about how much wattage is actually being used to produce light. The only reason we include LED watts on our website is because so many people request it as a means of comparison (for which it is completely useless– actual wattage is the most accurate means of comparing relative power of any plant grow light).

First, it is important to know what “white” light really is. White is not a spectral color, but rather a combination of different light colors. Human eyes only have 3 kinds of color-sensitive cells called cones– red, green and blue– and any light which stimulates all three of these at similar levels will appear white. There are many different ways that humans perceive white light from the combination of different component colors. Equal amounts of red, green and blue light, even without any other colors, will appear white to the human eye. Yellow light stimulates both the red and green cones, so blue and yellow light combined will also appear white. This is just one example; there are many, many other combinations which appear white to the human eye, even though the light is not a complete spectrum.

LEDs’ electroluminescence technology (how LEDs make light) is not capable of producing white light directly from the diodes; individual LEDs can only produce one color of light. The first “white” LEDs were actually red, green and blue (RGB) LEDs combined, and indeed the light appears white to the human eye. However, if you view something that only reflects orange light under the RGB light, it will appear black, as there is no true orange from the light source that the object can reflect back. This means that RGB LEDs have a poor Color Rendition Index (CRI).

Almost all “white” LEDs on the market today are actually just a blue LED with a phosphor coating which converts much of the blue light into different colors. The most commonly available “white” LEDs use a phosphor called Yttrium Aluminium Garnet (YAG) which predominantly creates yellow light; the combination looks white to the human eye and has a much better Color Rendition Index (CRI) than RGB LEDs due to the wider spectrum created by the phosphor. However, 20%-40% of the light produced by the blue LED is lost in this process, so these “white” LEDs cannot be as efficient at creating light as a pure-color LED (and LED’s cannot be made to produce a “pure-color” white). White LEDs are good if you’re looking to illuminate your home or office as this efficiency loss is easily justified by being able to see comfortably, but for growing plants they are wasteful.

Plants preferentially absorb red and blue light. Much of the light produced by “white” LEDs are in spectra (colors) that plants do not use. This unused light is just converted to heat within the leaves, requiring lower environmental temperatures to maintain optimal leaf surface temperatures. When combined with the 20%-40% efficiency loss, white LEDs are less than half as efficient for growing plants than the correct mix of pure-color LEDs– white LED grow lights force you to cool your growing environment more, just like HPS and MH, losing a lot of the other advantages LEDs offer.

We’ve investigated the newly-available 10-watt “white” LEDs for growing plants, but our 5-watt pure-color LEDs produce more usable light, require less cooling, and use half the electricity.

It’s easy to be confused by the idea of beam angle and how it can affect plant growth. Each individual diode (LED stands for Light-Emitting Diode) has a cone-shaped lens that can be designed to focus the light coming from the emitter anywhere from 30° to 180°. In LEDs, beam angle refers to this angle of the light cone the primary lens creates. It is important because it determines the intensity of light reaching the plant as well as the total effective footprint of the light.

HID (MH / HPS) bulbs have a 360° beam angle- half the light produced is aimed up and away from your plants, which is why a reflector is needed to try and reflect as much of this light as possible back down to your garden. LEDs in general are more efficient at growing plants than HIDs because LEDs only produce light aiming toward your plants. Properly-designed LED lights that use an optimal beam angle in the primary lens have no need for a reflector.

Each diode in every Black Dog LED grow light uses a 120° lens, which is the best angle to achieve a large footprint with intense light covering all of the growing area. Many other companies sacrifice the footprint in order to achieve better canopy penetration by using a 60° or 90° lens, or even use secondary lenses to further focus the light into a narrow cone; this is often the only option with weaker LEDs. Black Dog LED’s PhytoMAX-2 Series lights use only the very powerful 5-watt chips, so we can use a more oblique angle to create a generous, evenly-covered footprint while still maintaining superior canopy penetration.

Our lights have the a very large, bright, and evenly-covered footprint so you can grow healthier, high-quality plants everywhere in the footprint. We maximize your yield rather than just the reading from your PAR meter directly under the light!

When looking at LED grow lights there are many specifications thrown around about different types of LEDs. For the uninitiated this can be very confusing. If you want to learn what these different terms mean, check out our blog post that explains what these are and how they affect your indoor gardening.

No, LEDs don’t require a ballast. HID lights require a ballast in order to generate the extremely high voltage required to initiate the electric arc inside an HID bulb. LED lights operate in a completely different way and don’t need a ballast; they do require direct current (DC) but this power converter is built in to our LED fixtures.

Comparing different grow lights is actually incredibly difficult since you have to look at power, spectrum, cooling, and a host of other factors. You may not have all the gear handy to do your own tests on lights but at least you can learn how to compare them and what to look for in your next indoor gardening grow light by reading our blog post where we cover all of this in detail.

Lumens are a measure of luminous flux, or the total amount of visible light radiating from a source, weighted by the human eye’s sensitivity to the particular wavelength of the light. Lumens are the best measurement to use when evaluating how well a light will illuminate an area for human eyes. The human eye is most sensitive to light in the yellow range of the spectrum, so 100 photons of yellow light have a higher lumen rating than 100 photons of blue light or 100 photons of red light.

Plants preferentially absorb red and blue light. Lumens preferentially weight yellow light and de-weight red and blue light, making lumens just about the worst light intensity measurement possible for evaluating how well a light will grow plants.

Lumen Weighting (yellow) versus Photosynthetic Efficiency (green)

Lumens’ measurement of human-visible luminous flux differs from PAR, which measures radiant flux — the total number of photons in the visible spectrum without weighting for human visibility. Yield Photon Flux (YPF) is like lumens in that photons are weighted based on their wavelength, but YPF weights them based on their usefulness to a plant rather than to the human eye, and YPF considers photons outside of the human visual range. For this reason, YPF is the best measurement of light intensity for growing plants, although it still has significant drawbacks as we explain here.

Photosynthetic Active Radiation (PAR) designates the spectrum or range of colors of light from 400 to 700 nanometers that plants are able to use for photosynthesis. PAR measurements are usually expressed as photosynthetic photon flux density (PPFD) in units of μmol m-2s-1 — how many micromoles of photons (602,214,150,000,000,000 photons) within the PAR wavelengths of 400nm-700nm that go through 1-square-meter each second, though most devices that measure PAR do it only at a single point, rather than over a whole square meter.

There are important caveats, but in general, the higher the PAR measurement a light has, the better it will grow plants. Too much PAR (too much light) is wasteful and can even damage plants, although this is almost never a problem with artificial grow lighting. More fundamentally, PAR measurements do not account for the relative usefulness of particular wavelengths to the plant– leaves’ preferential absorption of different spectra (colors) mean some photons are more useful to the plant than others, even within the PAR range. For example, plant leaves reflect much of the green light that hits them– most of it is not used by the plant, but this is right in the middle of the PAR spectrum at 495nm-570nm. In addition, plants require many spectra to perform well, although a light with a single color can have the same PAR measurement as a multi-spectrum light. Just having a high PAR value doesn’t indicate that a light will grow plants well; the spectrum must also be considered.

PAR also assumes that all photons outside of the 400nm-700nm range have no use in photosynthesis or for overall plant health. However, plants do use light outside of PAR, such as far-red light above 700nm which increases photosynthetic efficiency (due to the Emerson effect) and for hormone regulation. UV light (below 400nm) plays an important role in triggering plants to create pigmentation and other substances, such as vitamins.

PAR measurements can vary significantly within the lighting footprint of a light, so any single PAR measurement is uninformative as to how the light will grow plants. For example, a laser can focus all of its light onto a PAR-measuring instrument and have an incredible PAR value, but this doesn’t mean it will grow plants well. Many LED grow lights are sold with secondary lenses which focus the light into a narrow cone. These lenses give the light to achieve a very high PAR measurement at the center of its footprint, but just off to the sides the PAR falls to a point it can no longer sustain a plant.

PAR measurements are also higly dependent on the distance they are taken from the light source. The inverse-square law of light means that the photon flux density (what PAR is measuring) will decrease by the square of the distance from the source– so if a PAR measurement was 100 at 1 inch from the fixture, it will be 25 at 2 inches, and 11.1 at 3 inches. It’s easy to claim a high PAR reading for a light fixture if the measurement is taken close to and directly below it. It is therefore critical to know how far from the light PAR measurements were taken, in addition to where in the footprint the measurement was taken.

For these reasons, single PAR measurements and PAR alone should not be used as a measure of how good a grow light is, and can be very misleading when comparing lights. Even multiple PAR measurements over the entire footprint at the recommended height will not indicate how well a light can grow, as the right spectral distribution of light is critical and is not considered by PAR. Only by measuring PAR over the entire footprint of the light, at the recommended hanging distance above the plants, and considering the entire spectrum (including outside of PAR!) can useful comparisons be made.

Yield Photon Flux (YPF) is a measure of light intensity, weighted based on the light’s usefulness to plants. Unlike PAR, which considers only photons in the 400-700nm visible light spectrum and weights each photon equally, YPF considers photons from 360-760nm (ultraviolet through near-infrared) and weights each photon based on the plant’s photosynthetic response to the particular wavelength of light.

The weighting employed by YPF measurements eliminates some of the shortcomings associated with PAR measurements. For example, 100 photons of pure-green light have the same PAR value as 100 photons of red light, even though most of the green photons will be reflected by the plant while most of the red photons will be absorbed. YPF accounts for this and 100 photons of green light have a lower YPF than 100 photons of red light.

Unfortunately, YPF still has shortcomings when used as a measure of how well a particular light will grow plants. YPF does not account for the fact that different wavelengths of light are used by plants to initiate different biochemical reactions and that a wide spectrum is necessary to grow plants to their maximum potential. All-red light will have a higher YPF score than a broader spectrum containing all the wavelengths of light plants require for photomorphogenesis (creation of secondary metabolites such as pigmentation and flavonoids).

Like PAR, YPF is also measured at a single point, which does not indicate how well plants will grow over the entire footprint of an artificial light source. If light is being focused into a narrow beam by secondary lenses, the YPF score in the center will increase, even though plants can only be grown directly under the light. The inverse square law of light means that YPF measurements will decrease by the square of the distance from the source– so if a YPF measurement is 100 at 1 inch from the fixture, it will be 25 at 2 inches, and 11.1 at 3 inches. As with PAR, it’s easy to claim a high YPF reading for a light fixture if the measurement is taken close to and directly below it.

YPF is a better measurement of how useful light is to plants than PAR, but still shares most of the shortcomings of PAR. A red laser pointer has an amazingly high YPF score but can’t be used to grow plants.

Only by considering multiple YPF measurements taken over the entire footprint of the light, at the recommended hanging distance above the plants, and considering the entire spectrum, can useful comparisons be made between grow lights.

When you get anything warm enough it will start to give off light, and the hotter it gets, the more energetic the light gets, shifting from the red end of the spectrum at about 1350 °F to blue when temperatures get closer to 17500 °F.

Correlated color temperature is a measurement of the average hue of light as it appears to the human eye, expressed as the temperature (in Kelvins) something would need to be heated to glow at approximately the same color. We have a more in-depth discussion of what color temperature is (and isn’t) here.

Color temperature is useful whenever you’re comparing different lights that look white to the human eye, and for things such as adjusting “white balance” in photographs, to make them look right to humans.

When comparing “white” grow lights (multi-spectral lights that appear white to the human eye), the color temperature can be useful as an indication of the relative balance of red light to blue light in the spectrum- the higher the color temperature, the more blue there is in the spectrum (compared to red). But if the light doesn’t look roughly blue, white, yellow, orange or red to human eyes, color temperature doesn’t apply– no matter how hot you get something, it will never glow purple or green, so color temperatures for these colors don’t exist.

So while color temperature can be a useful comparison among grow lights that look white to humans, it isn’t useful overall in comparing grow lights. The correlated color temperature definition is based on how humans perceive the light, not on how plants perceive it.

Studies have shown that plants respond to both UVA and UVB light, but UVB light damages cells and can cause cancer.