We developed our grow light testing project to provide reliable third-party data about grow light fixtures. We are working in collaboration with Shane Torpey from Migro Lighting who has years of experience with grow light field measurement. Together, we developed this grow light testing protocol based on the sciences of physics and botany. Our grow light field testing is not sponsored by manufacturers. Our interests are in educating growers and being a reliable and credible source for data and information about grow lights. In this article, we describe our testing methodology and the science and theory behind it. We explain what we are measuring, how we measure it, and why it is important.
Grow light testing is a central part of the Coco for Cannabis Grow Light Guide. Brush up on grow light metrics with our Grow Light Metrics Primer. Be sure to see our Grow Light Test Reports in the menu below.
The Need for Grow Light Testing and Standards
The data that are reported about grow lights are often misleading. Manufacturers use metrics like “advertised watts” and “HPS equivalencies”, which are little more than marketing gimmicks. Even when they publish figures for Photosynthetically Active Radiation (PAR) or Photosynthetic Photon Flux (PPF), it can be tough to know if the data is a measurement or a calculation and what it really means. There are no good standards across the industry that allow growers to make accurate comparisons between different fixtures.
We conduct field testing designed to measure the Optimal Usable PPF from grow light fixtures. We argue that this is the most relevant metric to evaluate how a grow light will perform in an indoor cannabis grow. In this article, we explain what Optimal Usable PPF is, why it is the metric that really matters, and the protocol we follow to measure it.
Grow Light PAR Testing: Lab vs Field
As I explain in our Grow Light Metrics Primer, Photosynthetic Photon Flux (PPF) is a measure of the quantity of photons in the Photosynthetically Active Radiation (PAR) wavelengths. Rather than watts or any other measurement, PPF is the single measurement that describes the true “size” of a light fixture. This is the data that we need in order to evaluate the efficiency, coverage area, and harvest potential.
Integrating Spheres vs Field Measurement
There are two very different ways to measure PPF, laboratory testing and field testing. Both styles of testing use sophisticated PAR sensors to count the photons, but the testing itself is set up to count different types of PPF. Lab testing is typically done in a device called an integrating sphere. An integrating sphere captures all the photons that a fixture produces and directs them to a sensor for measurement. The effort in lab testing is to capture and count all the photons that are produced by a light fixture. We call this measurement the “Total PPF” of a fixture.
In contrast, field testing is done in an environment that mimics a grow set-up. Rather than an integrating sphere, we use mylar reflective walls that are similar to grow tents. We do not direct all photons to a single point for measurement, rather we take a series of measurements across an area that represents a grow space. In a field test, our goal is not to gather and measure every single photon that a fixture creates. Rather, our goal is to measure the number of them that arrive to the canopy of our simulated grow space. In an actual grow, these are the photons that would be available to the plants for photosynthesis. We call this measurement the “Usable PPF” of a fixture.
Total vs Usable PPF
The Usable PPF measured in a field test will always be lower than Total PPF as measured in an integrating sphere. This is because, even in an ideal setting, some of the photons that are produced by the fixture do not make it to the canopy of the plants. Photons leave the fixture traveling in many directions and are absorbed or reflected by things other than the plants. In every setting other than an integrating sphere, some photons are invariably lost in radiance and reflection.
Field Testing Counts the Photons That Matter
Lab testing with an integrating sphere is a very good way to measure the Total PPF output from a fixture. Total PPF may be useful to analyze certain aspects of LED fixtures or their component diodes. However, the total number of photons that a fixture produces does not really matter to us as indoor growers. What matters to us, is the number of photons that arrive to the canopy of our plants and can be used for photosynthesis. Our field testing is designed to measure the photons that matter, the Usable PPF.
Defining the Optimal Usable PPF
The Usable PPF from a fixture is not one set value. The actual number of photons that arrive to the canopy depends on the set-up. In particular, Usable PPF is influenced by three factors: the size of the coverage area, the presence or absence of reflective walls and the distance between the fixture and the canopy. In order to get a meaningful and comparable number for Usable PPF, we need to take account for each of these factors.
When coverage area, reflection, and hanging height are optimized, we can measure an “Optimal Usable PPF”. This is the most meaningful data for us as indoor growers because it describes the number of photons we can capture from the fixture when everything is dialed in properly. It is a reliable measurement that can be repeated by other testers and used to compare and evaluate different grow light fixtures.
The Importance of Coverage Area
The size of the grow or test area has a significant impact on the Usable PPF. If we put a fixture in an area that is too small, the Usable PPF will be reduced because more photons will be lost in reflection. If lights are over an area that is too large, the usable PPF will be higher, but much of the canopy will be under less than optimal densities of light.
We can see how coverage area size affects both the density of photons and the Usable PPF by comparing two tests that Shane did with the Mars Hydro TS 1000. In both tests the light was hung at the same height of 45cm (18in) and reflective walls were set up at the edge of the test area. The only difference was that in one test the area was 60 x 60cm (2’ x 2’) and in the other test the area was 90 x 90cm (3’ x 3’).
In the smaller test area, Shane measured a Usable PPF of 242.1µmol and an average Photosynthetic Photon Flux Density (PPFD) of 673µmol. This average PPFD is close to ideal, which suggests that the smaller test area is the optimal coverage area for the TS 1000. However, in the larger test area, the Usable PPF went up to 262.3µmol. This makes sense, because in the smaller area more of the photons hit the walls and in the larger area, they had a bigger target canopy. However, the slight increase in Usable PPF comes at a steep cost to average photon density (PPFD). In the larger area the average PPFD fell to only 325µmol, which is far from ideal.
The Optimal Coverage Area for Grow Lights
As I explain in our article, “How Much Light (PPF) Do You Need for Indoor Cannabis”, with normal levels of carbon dioxide, the optimal density of photons (PPFD) striking the canopy is between 500 and 1000µmol. To compensate for issues with uneven distribution, we recommend targeting an average density of 700µmol (PPFD). This means that the optimal quantity of photons is 700µmol of Usable PPF per square meter or 65µmol of Usable PPF per square foot of grow space. To determine the appropriate coverage area of a fixture in square feet, we divide the estimated or measured Usable PPF of the fixture by 65.
The Importance of Reflective Walls
Grow light fixtures are designed to point most of their photons towards the canopy. However, the photons spread out as they travel. The reflective walls in a grow tent serve to redirect wayward photons back toward the canopy and thereby minimize “spillover losses”. The mylar material that is used is effective and reflects up to 85% of the PAR photons that strike it. This helps to minimize reflective losses and maximize the Usable PPF. If the walls are less reflective, a larger portion of the photons will be absorbed. If the walls are too far away or are missing, a significant portion of the photons will escape the grow area unused.
Optimal Grow Setups Minimize Spillover and Reflective Losses
Home growers can avoid spillover and reflective losses by using grow tents or lining their grow space with reflective material. It is important to achieve full canopy coverage with plants extending wall to wall. When reflective walls are close to the canopy of the plants, it minimizes spillover losses and maximizes Usable PPF.
Commercial growers avoid spillover losses from individual fixtures in large spaces by running them in an array. When lights are run in an array, there may be no reflective walls near a fixture, but there will be neighboring fixtures and plants. The photons that would have been reflected off the walls are instead absorbed by neighboring plants. Those photons “spillover”, but they are not lost. Furthermore, the “spillover effect” from neighboring fixtures compensates the plants for the lack of photons being reflected from the walls.
To determine the Optimal Usable PPF, we need to minimize spillover and reflective losses in our test set up. Since we are testing individual fixtures, we use reflective mylar walls set at the edge of the testing area. This most closely resembles a grow tent; however, it is also a reasonable proxy for the Usable PPF that would be achieved from the fixture in a commercial scale array.
The Importance of Hanging Height
Hanging height has a considerable impact on Usable PPF. If we put a fixture very close to the canopy, we will capture a larger percentage of the photons produced by the fixture. As we move the fixture away from the plants, we create more opportunities for photons to escape without hitting their target. This means that the Usable PPF will be lower when the fixture is hung further from the plants.
We have data to show this from a series of tests that Shane did with a 600w HPS grow light in a 4’ x 4’ grow tent. Data from Integrated Sphere testing suggests that the Total PPF (with reflector losses) of the fixture that Shane tested is about 900µmol (Nelson and Bugbee). At 40cm above canopy, Shane recorded a Usable PPF of 871µmol. When the fixture was raised to 50cm the Usable PPF fell to 812.5µmol. At 60cm he recorded a Usable PPF of 777.6µmol. When the fixture was raised to 70cm the Usable PPF fell to 730.1µmol.
600w HPS Hanging Height Test
The Optimal Hanging Height for Grow Lights
If we were only to consider the Usable PPF then we would want to hang the fixture closer to the plants and capture more of the photons. However, there is a limit to how close the lights can be. That limit is determined by the density of the photons (PPFD) and the concentration of carbon dioxide. I explain in our article, “How Much Light (PPF) Do You Need for Indoor Cannabis”, that at ambient levels of carbon dioxide, plants are not able to process more than 1000µmol of PPFD. PPFD is a measurement of the density of the photons striking any part of the plant. When the density of photons (PPFD) striking the plant exceeds 1000µmol it will provoke photoinhibition and potentially damage the plant.
For the 600w HPS fixture that Shane tested, the optimal hanging height is 60cm above the canopy. In the PAR map above you can see that, at 60cm, the highest PPFD reading is 990µmol. If the fixture were hung lower, some regions of the canopy would be exposed to photon densities that are too high. If the fixture were hung higher, it would reduce the Usable PPF. When the fixture is hung at the optimal height it will produce the highest Usable PPF without damaging the plant. This is the Optimal Usable PPF.
The Optimal Usable PPF
We define the Optimal Usable PPF of a fixture to be the amount of PAR photons that arrive to the canopy of the plants in a grow space with reflective walls when the fixture is hung at the height where the maximum PPFD value is between 950-1000µmol. Our field measurement PAR tests are designed to measure the Optimal Usable PPF of grow light fixtures.
The Coco for Cannabis & Migro Grow Light Testing Protocol
Our grow light testing protocol is informed by the sciences of physics and botany along with Shane’s considerable experience testing different grow light fixtures. We have established this protocol as our standard for grow light field measurement of Optimal Usable PPF. It is a protocol that can be duplicated by other testers and produce reliable results.
Test Equipment and Setting
As I explain above, the testing conditions can have a significant impact on the Usable PPF that is measured from a light fixture. Because we are testing grow lights, we designed our testing conditions to mimic actual indoor growing conditions.
Mylar Reflective Walls
We use mylar panels as reflective walls on all four sides of the test area during PAR testing. The mylar panels reflect photons back toward our simulated canopy. This gives us the most accurate measurement of the Usable PPF that the fixture can deliver in a grow tent or in a large array.
Black Non-Reflective Simulated Canopy
We use a black leatherette mat on the bottom of our testing areas. This mat is the simulated canopy where we take PPFD measurements. The black leatherette absorbs nearly 100% of the PAR photons that strike it. This simulates a full canopy of leaves where we assume all the PAR photons would be absorbed by the plant. If photons were reflected from the floor of our testing areas, then our readings would be much higher. However, we would no longer be measuring the Usable PPF because photons would have more than one chance to be counted.
We use the Apogee SQ-500 Quantum sensor to take PPFD measurements. The SQ-500 is the same sensor as the Apogee MQ-500. They are analytical grade analog PAR sensors. As Shane explains in the video below, there are several reasons why the Apogee SQ-500 is the ideal sensor for our grow light tests.
The SQ-500 is not powered by external current, rather the PAR photons that hit the sensor generate an electrical output measured in millivolts. This produces quick measurement response and stable measurements.
The SQ-500 is designed to measure the density of photons in the PAR wavelengths (400-700nm). With any sensor, the measurement error (spectral error) will depend on the light source. The SQ-500 does an excellent job measuring all horticultural grow light technologies. The Spectral error for High Intensity Discharge lights (HPS, MH, CMH) is less than 1%. For LEDs the error is only about 3% (Blonquist and Johns).
Finally, the SQ500 offers excellent cosine correction. The sensor itself is shaped as a dome and receives light from a wide angle (over 150 degrees). The cosine correction allows the sensor to accurately measure those photons no matter which angle they hit the sensor. This is a critical feature for measuring light in a reflective space like a grow tent because light comes in from many angles after bouncing off the reflective walls.
Preparing the Fixture for Testing
In order to get accurate and reliable readings, it is important to properly prepare the fixture. LED diodes degrade through time, losing as much as 4-7% efficiency per year. As a result, we conduct our tests on brand new fixtures.
We assemble the fixture following manufacturer’s instructions. We setup the fixture in the testing area and turn it on for at least 30 minutes prior to testing. LED diodes are more efficient when they are cold, so it is critical to get the temperature up to the normal operating range before taking measurements.
Determining Test Area Size
We test each fixture in an area that corresponds to the expected coverage area of the fixture based on estimated Usable PPF. Mylar panels are set at all four sides of the test area, and the fixture is hung in the center. We use 5 standard test area sizes that correspond to common grow light and grow tent sizes and shapes.
Grow Light Testing Areas
Determining Hanging Height
Once the fixture is hung in the test area, we determine optimal hanging height by measuring PPFD. We will raise or lower the fixture until the maximum PPFD reading anywhere in the test area is between 950 and 1000µmol. We record this height as the “Optimal Hanging Height” in our test reports.
Measuring Average PPFD
Our simulated canopy is divided into a grid with 15 x 15cm (6 x 6in) cells. We place the PAR sensor in the middle of each cell in the grid and ensure all four walls are in place before taking a reading. PAR sensors measure the density of photons (PPFD) that strike the sensor. We record the PPFD reading for each cell in our PAR maps. We take the sum of the PPFD readings and divide by the number of cells to establish the Average PPFD.
Converting Average PPFD to Usable PPF
The Average PPFD that we measure is the average density of photons that strike the canopy. We can convert from a density measurement like PPFD (µmol/m2/s) into a quantity measurement like PPF (µmol/s) by multiplying the density by the area. PPFD is factored in square meters, so we multiply the average PPFD by the square meters in our test area. The result is the Optimal Usable PPF of the fixture.
Evaluating Grow Lights with Usable PPF
Learn how we use Optimal Usable PPF measurements in our article, "Calculating Grow Light Efficiency, Coverage and Harvest Potential"!
Be sure to see our Grow Light Calculator!
See our Grow Light Test Reports and Reviews in the menu below.
A Coco for Cannabis and Migro Collaboration
Our Grow Light Guide is a collaboration among Dr Coco and Dr Photon from Coco for Cannabis and Shane Torpey from Migro Lighting. At Coco for Cannabis, our mission is to help growers maximize the success of their cannabis crops by providing scientifically accurate information and sharing proven growing practices. We are excited to collaborate with Shane because he has earned a well-deserved reputation for doing fair and accurate grow light testing. We created this guide to share reliable grow light data and empower home growers with the best information about grow lights.
Grow Light Guide Articles
- Grow Light Metrics Primer
Learn about the science of horticultural lighting. We explain the key grow light metrics and terminology: PAR, PPF, PPFD, and more. Start here to make the most of our Grow Light Guide!
- Grow Light Calculator
Our grow light calculator provides an accurate way to analyze and compare different grow light fixtures. You can enter your own grow light data or select preloaded fixtures.
- Calculating Grow Light Efficiency, Coverage & Harvest Potential
We explain how to use the Grow Light Calculator and the science behind it. Learn how we use Usable PPF to calculate our efficiency ratings, coverage estimates, and benchmark harvest targets.
- How Much Light (PPF) Do You Need for Indoor Cannabis?
We review the science of cannabis photosynthesis and explain the optimal quantity (PPF) and density (PPFD) of light for indoor cannabis. We include a Grow Space Calculator which shows the optimal light and harvest potential for any grow space.
Read All Our Articles!
Blonquist, Mark and Josiah Johns “Accurate PAR Measurement: Comparison of Eight Quantum Sensor Models” Manufacturer Publication: Apogee Instruments, Inc., Logan, Utah
Nelson JA, Bugbee B (2014) Economic Analysis of Greenhouse Lighting: Light Emitting Diodes vs. High Intensity Discharge Fixtures.