As a refresher, PAR (Photosynthetically Active Radiation) is a region of the electromagnetic spectrum (400 to 700 nm) that promotes photosynthesis. PPF (Photosynthetic Photon Flux) is a critical metric that tells us how much PAR a light-source emits. PPF does not measure PAR at a specific location (e.g. your crop canopy), but it tells you how many photons within the PAR region are coming out of the light-source every second. PPFD (Photosynthetic Photon Flux Density) measures the amount of photons within the PAR region at a specific location (e.g. the amount of light delivered to your canopy) every second. If you have a PAR meter, it is reporting PPFD (μmol/m2/s) measurements. You must understand the differences in these metrics before you can compare various horticulture lighting systems. Many manufacturers realize this can be a confusing topic, so it is very easy for companies to mislead potential customers with exaggerated marketing claims, misleading information, and by showing a limited set of (or using blatantly wrong) metrics. However, once you understand the differences in these metrics, you will be able to cut through all the ‘marketing’ and ‘hype’ and simply ask manufacturers to provide the data you need to successfully compare lighting fixtures.
In order to explain the correct method for evaluating a horticulture lighting system, let’s first highlight
some of the metrics used today that you need to avoid:
Don’t use electrical watts to compare grow lights
Don’t use lumens to compare grow lights
Don’t be fooled by a company that claims to have a magical growth spectrum
Don’t just look at a single PPFD measurement directly under the fixture
Don’t focus on the wattage of the LEDs used in the fixture (1W, 3W, 5W, etc.)
RULE 1: DON’T USE ELECTRICAL WATTS TO COMPARE GROW LIGHTS
Many people use total electrical watts, dollar/watt or watts/square foot to compare horticulture lighting systems, but these metrics are 100% useless and will most likely lead a consumer to make a poor purchase decision. Why? Simple. Electricity doesn’t grow plants. Furthermore, radiometric efficiency (how much light a fixture emits per watt of electricity) can vary by up to 200% amongst the popular LED fixtures on the market today. Hence, since light (not electricity) grows plants, you need to ask how much light a fixture emits. It sounds simple, but 99.9% of horticulture lighting companies do not advertise this metric. Instead, they focus on electrical watts. Why? Because it is very hard to design an efficient lighting system (measured in μmol/J) that delivers high light levels, but it is very easy to build an inefficient lighting system that consumes a lot of electricity. High efficiency LEDs, power supplies and optics cost more than less efficient components, and many manufacturers use lower quality components to increase profit margins.
Remember…You are not buying watts. You are buying a system that delivers light to grow your plants, so a quantitative measurement of light output and the efficiency in which the system produces that light is the critical metric you should use to compare the performance of horticulture lighting solutions.
RULE 2: DON’T USE LUMENS TO COMPARE GROW LIGHTS
This one’s easy to explain. A lumen is a rating of how bright a light appears to the human eye. However, since human vision is not correlated to photosynthetic grow rates, total lumens is a dead metric. As a rule, if someone is trying to promote lumens for a horticulture lighting system, they should not be selling horticulture lighting systems.
RULE 3: DON’T BE FOOLED BY “MAGICAL GROWTH SPECTRUMS”
Many scientific papers have confirmed that all wavelengths from 400 to 700 nm (the typical PAR range) will grow plants. However, there is a myth that is widely propagated on the Internet that claims plants do not use green light. Many companies promote their magical growth spectrum by publishing the commonly-referenced Chlorophyll A and B absorption spectrum chart. Armed with this chart, they mention that plants are green, so plants reflect green light from the full-spectrum light source. Have you heard this one before? Without going any deeper into this topic, it is important to note that there is no magical spectrum that is going to allow a 50W fixture to replace a 1000W fixture because it only uses the “wavelengths that plants need.” While plants certainly have numerous pigments and photoreceptors across the PAR range, nothing will trump the need for delivering the required levels of light (PPFD) to your plants. Spectrum has a very real effect on plant development, but be cautious of a company that spends too much time talking about their special spectrum (especially if they do not spend equal effort in publishing their delivered PAR measurements). There is a short list of companies who manufacture commercial-grade LED fixtures for the professional horticulture industry, and none of them market the number of LED ‘bands’ in their fixture.
RULE 4: DON’T LOOK AT A SINGLE PPFD MEASUREMENT
Let’s take a quick look at Rule 4. Unless you are growing a small plant directly under your light, a single PPFD measurement doesn’t tell you much. By clustering the LEDs closely together and using narrow beam optics, it is very easy for a manufacturer to show an extremely high PAR measurement directly under the fixture. However, unless you are only growing one plant in this exact location, you need to know how much PAR is being distributed across the entire canopy. Since most LED lighting systems centralize the LEDs into a small fixture footprint, these systems naturally produce very high PPFD levels directly under the fixture. However, these light levels will drop significantly as you move the PAR sensor just a small distance from the main fixture housing. If you are growing over a 4’ x 4’ area, you need to review the PPFD levels over the entire area to calculate the average light level the lighting system is providing. If you only had a center point measurement you may assume a fixture is extremely powerful. However, you would need multiple measurements across the 4×4 grow area to calculate the average amount of PAR that is provided by the fixture. Light uniformity across the grow area varies greatly from fixture to fixture, and unfortunately, most manufacturers do not publish complete PAR maps. It is easy to produce high PPFD numbers directly under the fixture, but it takes a very powerful and well-designed light to deliver high (and uniform) PPFD values across an entire canopy.
RULE 5: DON’T FOCUS ON THE WATTAGE OF THE LED’S
Do you use 1W, 3W, 5W or 10W LEDs? We are asked this question on a frequent basis, but the wattage of the LED does not tell you anything meaningful about the lighting system’s performance. Since LED and fixture efficiency varies widely, the wattage of the LED is not a meaningful metric. Remember, the LED wattage is a system input, and growers care about the system output. Hence, the LED wattage doesn’t tell us anything about the system’s ability to deliver light to your plants.
As a simple analogy, the LED inside a lighting system is equivalent to the engine in a car. By itself, the horsepower rating of the engine doesn’t tell you how fast the car will go. Pair a high-horsepower engine with a poorly designed transmission, and the car will not go very fast. Hence, as far as the driver is concerned, the relevant metrics for a car are related to the overall performance (e.g. 0-60 mph time, top speed, miles per gallon). Any reference to a component inside the car is irrelevant to the driver. It is the same situation with lighting systems. The amount of light delivered to your grow area (PPFD), the electrical watt consumption, and the light distribution pattern are the important metrics to focus on, so ask for more information if a manufacturer wants to focus on the type of LED they use.
Note: Since LED quality varies by a very wide margin, it is important to know the brand of LEDs used in the lighting system. There are a handful of world-class LED manufacturers, so make sure you find out what brand of LEDs are used in the lighting system. Assuming the fixture manufacturer has developed a reliable fixture design, higher quality LEDs should last longer if they are not being over-driven to achieve higher light levels.
Again, you are buying light to grow and develop your plants. In our opinion, you want to buy a lighting system that delivers the required amount of light to your plants for the lowest initial cost, while consuming the fewest electrical watts possible. Ask the fixture manufacturer to provide the following pieces on information: PPF, input watts, and PPFD maps for your intended coverage area. With this information, you can calculate: PPF/$, μmol/J, light distribution patterns, and uniformity levels.