How to Accurately Calculate Average PPFD

Average PPFD is the most important factor for most growers either domestic or professional as there are no set benchmarks pertaining to exact spectrums per plant species yet, but we know that regardless of spectrum we can get a satisfactory yield by having the right amount of average PPFD, you can this this evident in the HPS world where plants are given a spectrum that we would consider under nourishment but still provide good yields with differing quality levels:

1000de Spectrum 600e Spectrum

Whether you are growing with led, hps or CFL achieving the correct amount of average PPFD 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 and efficiency of the lamps electrical factors.

 

Best Method

Average PPFD can only be calculated accurately using mathematical equations based upon photometric readings for the lamp from integrated spheres and goniophotometers.

Average PPFD should be calculated using what has become industry standard software AGI32, us and several other leading horticultural led companies have adopted the use of this software and it has become industry standard due to its market leading features for horticulture.

A 25% grid should be used for measurements as this allows understanding of the scale for both small and large growers.

Average PPFD measurement planes should be 100% transparent so that we are measuring PPFD hitting a point with no reflective interaction from the bottom surface as this increases the result exponentially an it differs for different light sources and lamp types.

It is not possible to accurately simulate a plants absorption with a measurement plane as there are many different types of plants whose canopy properties change at different points in their life cycle.

Surface Properties:

  1. Lower floor surface should be dark green no greater than 48%.
  2. Indoor wall surfaces should be no greater than 90% light grey reflectivity to allow for ageing of surfaces.
  3. Measurement planes should be 100% transparent
 

Hand Held method

Many led 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 multi height canopy.

Below is a comparison of the two different measuring types Transparent measurement plane and 90% grey reflective measuring plane:

GOD-9 PPFD the Best Method GOD-9 PPFD the Hand Held Method
GOD-9 PPFD the Hand Held Method 600e PPFD the Hand Held Method

You can see how radically the measurements differ, but now this is one of the primary measuring techniques used due to lack of knowledge and professional expertise within the many led companies that exist.

So as a result, it is best to provide the customer with both measurements, so they can have some idea of how to asses one lamp against another or to provide them with measurements against the HPS counterparts they are replacing.

Hand held meters are intended to take spot / field measurements that do not weigh the whole SPD over the area and does not allow for accurate comparison of different lamp types but rather give a general indication of the PPFD in that area.

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.