Firefly Growlight

Say Watt

Full Spectrum Sunlight

When looking for the latest information on grow lights, whether you like HPS or the newest LED grow lights, you are bombarded with technical specs and difficult terms such as photons, PAR, PPFD, PFD and many more. You are almost supposed to be a scientist to be able to understand what all these things mean and how they influence your grow. In this article, we will try to shed some light (pun intended) on things and hopefully answer some questions about what is out there. After you have read this information, you should be able to categorize all information out there that will help you to figure out 3 things: 1) what is the input going into the light, 2) what is the quantitative output and 3) what is the qualitative output coming out of it?

Input: Energy

Firstly, there is electricity consumption. Growlights love energy. One 1000W HPS grow light, running for 12 hours a day will consume as much energy as 4 laundry machines running at the same time for one full cycle. Imagine having a bunch of them switched on at the same time and you will understand why laundromats have coin-slots in their machines. Growlights are just very powerful devices that transform electrical energy into pure light and heat. The first one is intended, the second one is a by-product and, in some cases, not wanted (as much). One of the main arguments to look for newer generation growlights is that they promise to be more light, less heat. In fact, the most popular replacement of a 1000W HPS growlight, is currently a 600W LED growlight. They promise the same amount of light output, but at a 40% lower energy consumption. Manufacturers are correct about the lower consumption, but the output needs to be further explained.

Output: Quantity of light

“Up until 50 years ago, you all thought that the atom was the smallest thing, until you split it open and this whole mess of crap came out…” -Phoebe, Friends

Let’s begin with the capital P that appears in some abbreviations that come up when looking for grow lights, namely Photons, or as Pheebs puts it: the mess of crap inside Atoms. Just like Electrons, Neutrons and Protons, Photons are elementary particles that can be emitted in light and radio-waves. They carry energy and can thus be considered a way to transfer energy from one point to another. In the field of horticulture, hydroponics and/or artificial grow lights, photons can help explain why plants grow. The (artificial) light source emits photons when it is switched on. The photons are then caught and absorbed by plant tissue, mostly leaves. By means of photosynthesis, this “light-energy” is then transformed into usable energy for the plants to grow and reproduce. See it as someone pitching you a Mentos, and you catch it with your mouth and eating it. Except the person pitching it, is a giant burning gas-ball in space. Photons are emitted in different frequencies which we perceive as colours. The lower the frequency, the lower the number of photons that are emitted, but the more energy they carry.


This brings us to the first abbreviation, namely PAR or Photosynthetic Active Radiation. PAR is also sometimes referred to as “the visible light spectrum”. It contains all the frequencies of emitted photons that we humans can visibly detect. The frequencies range from 400-700 nanometres (nm) and can be defined as follows:

Colour Wavelength (nm)


Photon energy is often shown as electron Volts (eV), but another common measure for photon energy is Joule. As the table shows, the higher the wavelength, the less energy per particle can be transferred. Furthermore, the table goes a bit further than just the “visible light spectrum” which helps explain the next bit: Photosynthetic Photon Flux.


Photosynthetic Photon Flux or PPF is a real strong measure of light output. It’s measured in μmol/second. It adds up the number of photons that are emitted from a light bulb in any direction per second. The traditional way to calculate the number of photons that are emitted from a light source is by means of an Ulbricht sphere in which the light source is placed and switched on. The inside of the perfect sphere is covered with reflective materials that bounce around all light which can then be caught and measured. This is quite easy to do with a 600W SE lightbulb, but how can we measure it with a 1.2m2 – 8 lightbar – LED fixture? The answer is: It’s possible, but it’s quite difficult (and you need a large sphere). Manufacturers of the actual LED-chips (Osram, Samsung, Cree, SEOUL Semiconductors etc.) will measure the PPF of 1 chip and then many manufacturers of growlight systems will just multiply and average this number with the mix of chips they are using to build a growlight to get a “potential PPF”. It’s important to note this in order to correctly explain “efficiency” which is another term used by LED-growlight manufacturers as a quality characteristic.

Decreasing energy consumption is important as said before, but growers also need a high output. So how can we get the best of both worlds? Well, by dividing the output by the input we get a measurement of efficiency or “how much output per input do I get?”. We divide the number of photons (PPF) by the power consumption (Watt) and there we get a number like 2.7 μmol/Watt (or Joule). The higher this number, the more efficient the light. Sounds simple, however as explained before, the PPF is not so easy to measure and an important factor that is omitted in these calculations is the transfer of power between the socket and the LED; namely the ballast/driver, the wiring, and the soldering. So, by using top-quality (expensive) intermediaries for this power transfer, more of the light potential can actually be used. Especially the driver/ballast make a real difference in the efficiency of the grow light, and obviously the cost.


Another often seen term is Photosynthetic Photon Flux Density or PPFD. It measures the PPF on for example one square meter (or for our American friends; 3 square feet) at a certain distance from the light source. This number is a lot easier to measure with a spectrometer. Often a grid is made on the floor, a light source is hung above it at various distances, and in every square of the grid, the PPF is measured. Not only does this give a more accurate measurement of the real light output, it also mimics what the plants below it will actually get. Finally, it allows manufacturers of growlights to manufacture some nice light-maps that justify 1.2m2 LED-light constructions because the light in the corners of your growroom are so important. However, plants grew quite well in the 90’s of the last decade with a lightbulb and a reflector, so do we really need light above every corner of each growbox? The answer is: not really, if you have good reflective material surrounding the plants and light-beams are dispersed under different angles into your growroom, you should be ok.

So here we have discussed some ways to look at the quantity of light that comes out of modern grow-lights. Let’s have a look at the quality.

Output: Quality of light

We already discussed PAR, which is the visible element of light that humans can see. However which light is useful for plants? Research in numerous renowned institutions has focused on proving the efficiency of red and blue light on plants, by measuring photosynthesis under different wavelengths. Not completely by chance, this research coincided with the developments of Red and Blue LED-chips in the early 2000’s and 2010’s. However, LED-growlights did not become dominant in professional greenhouses as the main light source until powerful enough white-light LED-chips were created and became more affordable. The first-generation LED grow lights were used as support lights – in between the canopy or placed in interval combined with HPS – for different crops. The reason that white light LED-chips have really pushed the development of LED as a growlight is because white light actually consists of a wide spectrum that includes violet, turquoise, green and yellow light in different strengths.


Sunlight consists of all the wavelengths between 100nm up to 1000nm (1mm). Depending on the hour of the day, there will be less UV-violet-blue in the spectrum. We can see this in the so-called “golden hour” of sunrise and sunset, when the sky will show more red and yellow colours. However, at midday, the number of photons emitted will be very high for each wavelength and the sky will look blue to us. This effect will also happen (although to a lesser extent) after the longest day of the year, when days start becoming shorter, and autumn starts. The further we are from the equator, the stronger this light effect becomes. Some plants species are more adapted to blue light for this reason, especially if they also originate from higher altitudes.


So what about UV? UV or Ultra Violet can be divided in 3 types: UV-A, UV-B and UV-C. They are all shorter wavelengths (lower frequencies) then visible light. UV-C is the first category above X-rays and has wavelength 100-280. Luckily for us it is 100% filtered out by the Ozone-layer, because it will burn our skin off. UV-B is the wavelength between 280-315. It’s partly filtered out by the Ozone- layer, but in places where this layer is thinner, your skin will burn much faster than elsewhere, unless you wear enough sunscreen. For plants, there is not a lot of benefit of UV-B or C. There have been various trials from grow-light manufacturers among others and none of these trials found conclusive data on improved crop quality. Then finally, UV-A is the wavelength between 315 and roughly 400. For growing plants, UV-A together with blue spectrum appear to signal blue skies and lots of light everywhere to plants. This in turn triggers that the plant will stop trying to find more light and thus reduce stretching and leaf making. Plants will stay smaller and more compact, while still producing plenty of energy by photosynthesis. In flowering plants, UV-light is perceived as an outside threat (a-biotic) to the plant by for example larger animals. The plant will thus activate one of its defence mechanisms: namely to push the production of metabolites, such as terpenes and cannabinoids.


Back to growlights; in case of High Intensity Discharge or HID lights such as HPS or (Ceramic) Metal Halide, each type of bulb can produce a different spectrum. Generally Metal Halide bulbs will give more blue light, whereas HPS is known to give high power Reds. It is therefore that many growers switch the bulb when plants go from vegetative to generative stage, or keep mother- plants under MH. The downside of HID lamps is that they often have quite narrow or peaky spectrums. In case of HPS, they just have lots of photon-energy in them. This results in high power output and very good results in terms of harvest weight. This has to do with stretching of the plants – the plant under red/yellow light will stretch a lot more which increases dry matter and thus bigger size and more weight. However, for growers that like to improve the flavour profiles and other metabolites (such as CBD/THC in legal or medicinal projects), it is better to use a wider spectrum. LED’s can prove their worth here, especially the latest generations with lots of other colours and plenty of energy in each wavelength. Be aware that even in the newest LED’s there are some cheaper products on the market that will claim “Full spectrum” or “wide spectrum” however on the fixture you will see 4-5 different colour LED-chips. These often include cheaper whites that are not really suitable for growing plants. They are quite good and energy saving when used as floodlights in gardens etc, but they lack power. Look out for LED’s that offer a real sunlight spectrum with 1 LED-chip. This will ensure that the white LED is powerful enough to provide plenty of all colours to your plants and will give a better and more homogeneous light distribution as each LED will actually give the full spectrum. Not done reading, yet? Check for more info about the full sunlight spectrum here!