Possibly the most important factor to consider when buying and setting up a LED grow light: The lamp’s light output.
Regardless if you’re growing weed, herbs, spices, or vegetables, all plants have certain light requirements. Some houseplants do well in a shady corner while other plants, especially fruit-bearing plants like cannabis, peppers, and tomatoes need a lot of light to grow to their full potential. Just like humans count their energy needs in calories, PPFD or PAR is a similar measurement for plants.
An average adult human needs to consume around 2000-2500 kilocalories (kcal) a day to gain enough energy to maintain a good lifestyle. Similarly, plants need a certain amount of light to grow well. Light consists of photons and we measure the photons with the unit umol/m2/s (micromoles per meter squared per second).
PPFD, or photosynthetic photon flux density, is a spot measurement of light. In other words, “how many photons hit a specific area every second”.
PPFD is sometimes referred to as “PAR”. PAR stands for “photosynthetically active radiation” and can loosely be translated to “emitted light”. While PPFD refers to a quantity of light in a certain spot, PAR refers to a general amount of light emitted by a lamp.
Natural sunlight during peak hours of a summer day can deliver a PPFD of around 2000 umol/m2/s. A cloudy day on the hand we might only see around 100-200 umol/m2/s on the same location. In a window without direct sunlight, the levels would be even lower, around 20-50 umol/m2/s.
A generic 5W LED bulb in E27/E28 socket might put out around 300 umol/m2/s at a 12” distance. In a dim room with only a small light source, the PPFD levels would be only a few umol/m2/s, possibly 5-10. Hopefully, this gives you an idea of light conditions in various places.
Marijuana plants require different light quantity based on their cycle. A clone or a seedling only needs a little light, around 100 umol/m2/s. Once it reaches early veg it can take more, 200-300 umol/m2/s. During late veg it can take even more, around 300-400 umol/m2/s. Once the plant reaches late flowering, 600-700 umol/m2/s is a good number to aim for. With an additional CO2 boost and ideal grow conditions, cannabis plants can take up to 900 umol/m2/s over the course of 12-14 hours per day.
Once again, measuring and delivering the right amount of light to plants is similar to feeding humans with the right amount of food. If we give plants too little light, they will grow slower, smaller, and produce less fruit or buds. Giving too much light can, however, cause tip burn and also hinder growth. Finding the sweet spot is learned by experience.
LED grow light sellers should publish data of their lamps where PPFD is measured so that potential customers can clearly see if the lamp delivers enough light for plants to thrive.
It’s important to note that the closer a lamp is to a certain area, a canopy or a leaf, for instance, the more intense its light will be (higher PPFD).
I.e. a lamp that hangs at 12” from a plant will deliver more light (higher PPFD) than if it’s hung 18” from the plant. The theory behind this is that photons emitted from the lamp fall in an unorganized manner and scatter with distance. The further a lamp is from plant or measuring point, the fewer photons will hit that area.
Finally, let’s take a look at light footprint maps, or PPFD/PAR charts, that manufacturers should show with their LED grow lights. It’s worth mentioning that the values manufacturers present aren’t always truthful. It’s easy to make up or round up numbers as end consumers can rarely verify the PPFD values as the tools to measure PPFD are quite expensive. The more information a company gives you (the more comprehensive a chart is), usually, the more transparent and trustworthy that company is. Also, try to verify how the PPFD tests were made. If the company also has Youtube videos that show carried out tests, then it’s even more likely that the values are correct and legitimate. We’ll take a look at maps from three different brands:
Here we see a 1D PPFD map. This is the most common map we see, at least on Amazon, and it’s rather vague. We only see the PPFD/PAR values in the centre spot underneath the lamp without getting a good understanding of how the light from the lamp spreads and what area it covers. On the 24” distance, we see a couple of values on the x-axis but only in one direction. As the lamp is rectangular, the spread will not be the same on each side of the lamp. We’re also not being told how the tests were carried out or what instrument was used to measure the PPFD values.
LEDTonic’s light footprint map is a lot more detailed. They show the PPFD values on three different distances, 12”, 18”, and 24” on a 3×3’ (90×90 cm) map. We see here that the centre 1×1’ square is getting good intensity at 12” but even a 2×2’ area is illuminated well.
The outer parts of the 3×3’ area see fairly low PPFD levels, around 100-150 umol/m2/s. This would be a good place to put clones and seedlings, for instance.
The map is clear, easily readable, and we see that the values are measured with an Apogee MQ-500, which is a good brand of quantum sensors.
HLG has also created a 2D light footprint map where we see plenty of PPFD values over a 2×2’ area. This particular map is showing the values for when the lamp is hanging 12” from the ground. HLG has also published a similar map for a height of 18” but as the layout is identical, we’ll focus on this one.
We see that the centre value reaches a PPFD level of 640 umol/m2/s and the other values close to the centre are also around 400-600 umol/m2/s, which is within range of what a cannabis plant needs during flowering.
The map is clear, easy to read and gives us a good idea of what we can expect from the lamp. HLG doesn’t mention how they measured the values but as they are an industry leader, the values they provided should be accurate.