In our previous article on PAR, PPFD, and PPF, we talked in short about PPFD (sometimes the term "PAR" is used incorrectly when referring to PPFD) and defined it as quantity of light (or light intensity) in a specific spot. Light consists of photons so PPFD could also be defined as photon density in a specific spot, which is pretty close to the
PPFD abbreviation's expansion:
Photosynthetic Photon Flux Density
which is measured in unit µmol/m2/s.
Just like we measure speed in miles/hour (or km/h) or any other unit, we can measure light intensity as well. Light intensity, or PPFD, is measured with a quantum sensor tool. This sensor reads how many photons hit it every second and presents a result.
Low intensity light PPFD: 100 µmol/m2/s
Medium intensity light PPFD: 300 µmol/m2/s
High intensity light PPFD: 600 µmol/m2/s
Direct sunlight during peak hours can reach around 1500 µmol/m2/s in mild-tempered countries and over 2000 µmol/m2/s in deserts like in Arizona. A cloudy sky brings down the PPFD levels by up to 95% though.
Why do we want to measure PPFD and why is it important for grow lights?
Since it’s possible to measure both the PPFD output from grow lights and also determine the ideal PPFD levels for plants, the grow light’s output should match the plant’s needs.
Let's say you’re growing tomatoes in a 2x2’ (60x60 cm) tent and you know that tomato is a high light intensity crop that likes PPFD 600 µmol/m2/s and above. Naturally, a grow light matching this plant’s needs should be selected.
Or, if you’re growing microgreens, which are considered low light intensity plants, they will do well with PPFD 100-200 µmol/m2/s. These two scenarios would need very different grow lights.
General PPFD recommendations in µmol/m2/s:
Clones, cuttlings, and propagation:
Young high-light plants, early veg:
Understand your plants’ or grow area’s PPFD needs then find a grow light with adequate performance.
Although plants don’t need maximum PPFD levels over their entire surface, averaging somewhere between 300-600 µmol/m2/s on most of the plant during flowering should be aimed for.
Here’s a tomato plant receiving 600 µmol/m2/s in the center and around 80 µmol/m2/s on its outer leaves.
Tomato plant grown indoors under a generic 5.7W LED lamp
This has to do with light saturation point and light compensation point.
Rate of photosynthesis (0% to 100%) varies with light intensity (PPFD)
These two terms describe photosynthesis activity in relation to light intensity (PPFD) exposure. Doubling the PPFD at low levels can result in doubled photosynthetic activity, while at higher levels, above PPFD 400 µmol/m2/s, doubling the intensity no longer translates to doubling in photosynthetic activity.
Light footprint maps or PPFD chart: how a lamp’s PPFD output should be presented and read.
A light footprint map, also called PPFD chart, shows a grow lamps PPFD results in various measured spots.
E.g.: 600 µmol/m2/s one foot (30 cm) in the center spot underneath the lamp.
Here we see three different lamps with three different 1-dimensional PPFD charts that we found online. For some reason, they all perform perfectly at 24” even though their PPFD levels at this height vary greatly. This is an example of a confusing or misleading PPFD chart. Be critical!
Here’s another footprint map, a 2-dimensional, that doesn’t make sense. The values should decrease somewhat linearly from the center, not jump back and forth. Same thing in the top left corner. Suspicious.
The more measurements a footprint map shows, the more detailed it is. The easier it is to understand how a lamp performs.
PPFD measurements in the center spot at various distances underneath the lamp are useful, but a proper light footprint map should also show PPFD readings not just vertically, but also horizontally; sideways, up, and down from the lamp’s center spot. 3-dimensional.
Afterall, how can we determine how large an area the lamp covers with high enough PPFD if we only get a center spot measurement? The PPFD levels drastically decrease in intensity the further away from the lamp’s center they are measured.
Here’s how we create our light footprint maps. We created a second version of our first map as we received feedback it wasn’t clear enough. Which one do you prefer?
Original design. Shows the PPFD values in a 3x3’ tent at three different hanging heights, 12", 18", 24" and in all directions from the center.
Calculating the average PPFD over two different areas also shows why a tight grow space is important.
Lenses, reflector cups, and diffusers
Generic household LED bulbs usually have a milky diffuser to spread the light somewhat evenly. These bulbs’ goal is to light up a room or a large area.
With grow lights, it’s the other way around. Some models come with lenses that focus light into a narrow area. The goal is to focus the light so that as many photons as possible hit a specific area underneath the lamp; a plant for instance.
These two lamps could even draw the same amount of power. As we test in our video, a 10W LED bulb has a vastly different light footprint map than a 10W Ikea LED grow bulb.
Same goes for grow lights. Focusing light gives a smaller footprint than not focusing it, but focusing intensifies the light in the given area.
If my goal would be to grow one high light intensity plant, I would likely pick a grow light with lenses that focus the light into an area big enough for my plant (let’s say 1x1’ or 30x30 cm). However, if I would be growing herbs, microgreens, salads in a 3x3’ (90x90 cm) area, I would rather pick a lamp that gives low light intensity output but over a large area.
Lamps with really narrow focusing lenses, 30 degrees in this case, can create very high intensities but only in a small area.
We talk more about this as well as other topics in our video. If you find LED and light related information interesting, check out our full video. We elaborate on some specifics and also do more measurements and testing.