PPF & PPFD Guide to compare GROW-LEDS

Essential information to compare horticultural lights

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

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.

integrated sphere

Integrating sphere

Overview of PPF values

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.

light sourcePPFdetails
250W high pressure sodium lamp SE450µmol/s230V SE
400W high pressure sodium lamp SE725µmol/sPhillips MASTER GreenPower CG 400W 230V SE
600W high pressure sodium lamp SE1100µmol/sPhillips MASTER GreenPower CG 600W 230V SE
600W high pressure sodium lamp SE1150µmol/sPhillips MASTER GreenPower 600W/400V SE
1000W high pressure sodium lamp DE1850µmol/sMASTER GreenPower 1000W 400V DE
fluorescent tube 36W47µmol/sMASTER TL-D Reflex/830 36W
fluorescent tube 58W73µmol/sMASTER TL-D Reflex/830 58W
315W CDM lamp590µmol/sPhillips Master Colour

Comparison of illumination systems using PPF values

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.

PPFD

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).

PPFD measurement devices

Quantum Meter - TS256

BTS256-PAR

spectral lightmeter to measure PAR values

DLI, Qsi, R/FR, …

http://www.gigahertz-optik.de/en-us/product/BTS256-PAR

MSC15

MSC15

Low cost spectral light meter

for measurement of illuminance, spectrum, color, and color rendering index

http://www.gigahertz-optik.de/en-us/product/BTS256-PAR

Comparing 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.

The picture below shows the PPFD plot of a 600W HPS lamp. Following parameters and conditions applied:

• light source: Phillips Master Greenpower 600W SE 230V
• reflector: Hortiline Northstar
• ballast: Gavita Digi Star 600
• power draw mains side: 642W
• distance between light source and detector: 50cm
• measuring environment: closed cultivation box with reflective side walls; dimensions: 1,2m x 1,2m x 2m (HOMEbox Ambient Q120)
• measurement grid: 10cm

PPFD plot -ppf

Interpretation

In the above PPFD plot the light distribution across the cultivation area is depicted as a color gradient. The lamp focuses the light to the center of the measured area. PPFD values drop towards the border area. When cultivating e.g. fruit-bearing short-day plants indoors, PPFD values of approx. 600µmol/(m2s) are desirable. It is obvious that this value is exceeded in the center but in the border area the PPFD values are dramatically lower.

In addition to the visualization with the help of a PPFD plot the calculated average PPFD value across the entire cultivation area has high significance. With the 600W HPS setup this average value is 510µmol/(m2s), which is a very good amount.

As a comparison, we would like to show you a PPFD plot of a setup with 3 pcs. SANlight S4W LED modules. Measurements were carried out identical to the 600W HPS system afore mentioned. Further details:

• light source: 3 x SANlight S4W LED module
• power draw mains side: 421W
• distance between luminaire and detector: 30cm
• measuring environment: closed cultivation box with reflective side walls; dimensions: 1,2m x 1,2m x 2m (HOMEbox Ambient Q120)
• measurement grid: 10cm

PPFD_3x SANlight S4W Grow LED _3x400@300mm_zelt_1,2m2 ppf

Interpretation

The above PPFD plot with the 3 pcs. S4W LED luminaires shows a more homogeneous light distribution within the cultivation area. The average PPFD value is 535 µmol/(m2s). Plant development as well as fruit production will be more homogeneous and the overall yield will be higher.