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Correctly Measuring PPFD and Accurately Calculating Average PPFD

Correctly Measuring PPFD and 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.

Professional Method
Professional Method

The best way of measuring PPFD is of course with high quality lab equipment in a professional environment. We send all our lamps for testing to the Lighting Industry Association in the UK who are currently the leading authority in the UK.

Lab grade equipment is used to test the lamps output and produce accurate reports for all the relevant and required horticultural metrics along with general electrical metrics.

Integrated spheres and goniophotometers are used to measure the output of the lamps, the speheres measure the ppf and spectrum whilst the goniophotometers measure the ppfd.Using professional services to measure output has lots of advantages:

  1. Many more metrics available.
  2. Equipment is regularly re-calibrated to high standard.
  3. Precise results and reports.
  4. A full representation of the SPD.
  5. Data files available for simulations and for re-distribution to customers

Hand Held method
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:

  1. Correct Calibration
  2. Regular Re-Calibration
  3. Human Error
  4. Not a full representation of the SPD
  5. Measuring in an empty space gives higher readings due to reflection.

Best Method = Use Maths
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.

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.

Measurement Plane Colour and Reflectivity
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

Industry Standard Software
Industry Standard Software

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:

  1. Dark green 5% reflective measurement plane to simulate the maximum light absorption from a plants canopy.
  2. 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.

PPFD Grid Using AGI32 Generated Values
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:

  1. Large room show grid per lamp coverage area as below 10 x 10 grid for 100 lamps.
  2. 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 paul@budmaster.co.uk.

Contact Us

Budmaster LEDs
Llanfairpwch, Anglesey, North Wales
tel: +44 1492 573195
sales@budmaster.co.uk
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