In the following we would like to introduce you to the PPF and PPFD measures. They are of great importance and can give you essential information on the performance of a light source, which is used to promote plant growth. Furthermore, the PPF value allows you to compare different light sources and select the appropriate substitute for standard light sources, which are used in horticulture.

PPF is the abbreviation for “photosynthetic photon flux“. It gives the information on the number of photons, which are emitted by a light source. Only photons in the wavelength range from 400nm to 700nm are considered. This is the so-called PAR (photosynthetic active region) and photons within these wavelengths contribute to photosynthesis. The unit of the PPF value is µmol/s (quantity of light particles emitted per second). Knowing the PPF value from different light sources allows an easy and objective comparison on the total light output which can in theory contribute to photosynthesis. Please note that the PPF does not consider the direction in which the light is emitted. It does not give the information on the amount of light which actually arrives at the plants.

Determining the PPF of a light source is complex and costly as it requires specialized measuring instruments (integrating sphere with spectroradiometer and absolute calibration). The photo below shows our sphere in the laboratory.

The table below shows PPF values of standard light sources which are used in horticulture. The values were taken from the datasheets of the manufacturer.

This is a good and easy method to rate and compare different light sources in an objective way. The values given in the datasheets of the manufacturers can easily be compared.

Compared to the influence of different spectra on plant growth – which is difficult to quantify – the PPF value states concrete numbers.

Drawbacks of this method are the availability and reliability of the data. Not all manufacturers publish the PPF values of their products and because they cannot be easily verified one has to trust in the given data.

Example: substituting a traditional HPS lamp with LED luminaires

Assumption: a 600W HPS luminaire should be replaced by an LED system.

From the above table we know that a 600W HPS lamp emits 1100µmol/s and has approx. 640W (incl. ballast losses) electrical power draw on the mains side. Attention: Please note that losses which are introduced by the reflector, glass cover etc. are not considered in this comparison even though they can be very high.

To have comparable growth the substitution system therefore has to also deliver approx. 1100µmol/s PPF. From the datasheet of our S4W LED module we get the information that one module emits 384µmol/s. Hence three S4W units emit 1152µmol/s with a total power draw of 140W * 3 = 420W mains side. This replacement delivers approx. 5% more light within the PAR region and saves approx. 35% energy.

Whilst the PPF value gives information on the total photon flux within the PAR region, the PPFD value determines the photosynthetic photon flux density. It refers to the photon flux in a certain area. The unit is given in µmol/(m²s). In other words, it tells you how many photons which contribute to photosynthesis impinge on the plant per second. Other than the PPF you now know how much light really can be absorbed by the plant.

Attention: Please note that the PPFD value is measured with a detector which has a small measuring area. The measured value is only valid at the exact point of measurement. It is wrong to only do one measurement and deduce the PPFD value for the whole cultivation area based only on this single measurement. Most light sources use reflectors or lenses which steer the light and distribute it across the cultivation area – often extremely inhomogeneous. The intensity variation in the cultivation area can be determined by doing multiple PPFD measurements across the area. Visualizing these measurements results in a so-called PPFD plot. This is a very useful graph which shows the effective light distribution on plant level. It can be used to compare the performance of different illumination systems. PPFD measurements can be carried out on site e.g. in green houses and provide an objective mapping of the current light situation. They furthermore serve as a basis for new illumination system layouts. If compared to an integrating sphere the measurement device is handy and low priced. Attention: PPFD measurements are prone to errors and should be carried out by experts with high quality measurement instruments (preferably with spectrometers not integrating sensors).

further information and ppfd plots

In this section we would like to show you an example of a PPFD plot in order to explain some details. For further plots of common standard setups please refer to the blog entry “PPFD plots”. Knowing the listed conditions is essential in order to compare various plots. Furthermore, the measuring environment as well as the measurement grid has to be the same for comparable plots.

further information and ppfd plots