Correctly Measuring PPFD
Accurately Calculating Average PPFD
Light uniformity is the most important factor for most growers either domestic or professional as there are no set benchmarks pertaining to exact spectrum's per plant species yet.
Whether you are growing with led, hps or CFL achieving the correct amount of average light for your plants in their current environment will allow you to achieve the results you require.
Total PPF output and efficacy are not good general indicators of how the lamp will perform in a grow room or glass house environment but are indicators as to the power output and efficiency of the lamps electrical factors.
Best Method = Use Maths
Average PPFD can only be calculated accurately using mathematical equations based upon photo-metric readings for the lamp from integrated spheres and goni-photometers (.ies files that any lamp manufacturer should be willing to make available).
Plants spectral requirements can of course vary but for most plants the requirements are approximately within the 400-700nm bandwidth.
If the SPD (spectral power distribution) of a light source is known then the photosynthetic energy available to plants can be calculated and a conversion factor determined to be used in calculating photosynthetic photon flux density in μmol/s-m2.
Average PPFD should be calculated using what has become industry standard software AGI32 , all leading horticultural lighting companies have adopted the use of this software and it has become industry standard due to its market leading features for horticulture.
Measurement planes should a suitable colour, reflectivity and transparency to simulate your plants canopy, we use:
- Dark green 5% reflective measurement plane to simulate the maximum light absorption from a plants canopy.
- 94% reflective walls.
This method will of course produce a lower results vs using a handheld meter to measure light in a grow tent due to the additional reflection of light back to the meter from the rooms surfaces vs the absorption of the light in a real life environment.
The exact conditions for your measurements planes and surfaces may vary from one environment and species to another and you may choose to set your own measurement conditions.
It is important when comparing led lamps output that you have information about the measurement method and environment so you can best assess what you consider to be the actual output vs other lamps.
Measurement Plane Colour and Reflectivity
Many times you will see grids that have been measured in a reflective grow tent and these readings can be very misleading and will differ radically form real life readings.
Below are calculations for the Cropmaster PRO 1000 on a 5% dark green measurement plane and also on a 90% reflective light grey measurement plane and you can see the different reading you can achieve is vast.
Holding a handheld par meter 60cm under our Cropmaster PRO 1000 at the central point shows a reading in excess of 1000μmol/s-m2 which can be misleading.
PPFD Grid Using AGI32 Generated Values
Its important that we display the data in a relevant manner based upon the grow room so we can see how the lamp is working for grower. We advise showing average PPFD either:
- Large room show grid per lamp coverage area as below 10 x 10 grid for 100 lamps.
- Small rooms show per number of plants ie 1.2 x 1.2m space = 4 plants so a 2 x 2 - 4 x 4 grid.
We write our own software for comparing efficacy of spectrum and generating PPFD grids, this allows us to display the usable PPFD as well as the total PPFD which is extremely useful when considering your growing needs.
If you would like us to compare any spectrum's then please email the excel data to email@example.com.
Hand Held method
Many lighting companies measure a PPFD grid in a grow tent using a hand held PPFD meter and although this is one way of measuring it is not the correct method as it does not allow for any light loss due to absorption by the plants canopy.
The measurements differ radically by as much as 150%, but now this is one of the primary measuring techniques used due to lack of knowledge.
Hand held meters are intended to take spot / field measurements that do not weigh the whole SPD over the area but rather give a general indication of the general PPFD in that area and are subject to different readings due to many factors:
- Correct Calibration
- Regular Re-Calibration
- Human Error
- Not a full representation of the SPD
- Measuring in an empty space gives higher readings due to reflection.
It is even more important to the commercial grower as the many different variables that can come into play every minute in glass house grow environment would give a huge difference in readings when taken by hand from one minute to the next over a large area.