Horticultural lighting measures

With the arrival of LED horticultural lighting, the different spectra available in light are multiplying. Therefore, each type of plant will have different light needs (the usefulness that the plant will have of the different spectral balances is specific to each of them) and it becomes more important than ever to quantify the fraction of light useful to the plant. Moreover, such measurements allow us to compare different horticultural technologies, as well as to keep optimal light parameters throughout the growing cycle.

In short, the various measures

Most of the time we are used to talk about wattage to characterize the light power of a source. This is in fact a misuse of language because this value is directly related to the power consumption and not to the production of light. The unit used to quantify the light emitted by a source is the lumen. The latter represents the total amount of light visible to the human eye emitted by a source, which is already better but still unsuitable for horticulture. Indeed we seek rather to quantify the light received by our plants, which necessarily depends on the distance between them and the lamp. This is where the lux comes in, this unit is used to measure the amount of visible light received by a given surface. But our green friends are, of course, not sensitive to the same wavelengths that we see, which will complicate the situation a little ...

It is for these reasons that ingenious engineers have created the PPF and PPFD! These units of measurement were developed specifically for horticulture and take into account the difference in perception between a plant and the human eye when measuring light. The first is to horticulture what the lumen is to human vision, and the second measures the proportion of useful light absorbed by a given surface (such as the surface of leaves for a plant).

Bottom line:

The differences between technologies

For several years now, we have been looking for ways to grow plants under artificial light, and as you already know, there are many technologies available to light your growing spaces. However, not all are equal, and each one has its advantages and disadvantages. One way to compare them is to look first at the efficiency of these lights, which shows us how much PPF the source is able to generate per watt consumed. But it is also necessary to look at the operating cost, which is sometimes miscalculated since we must not forget for example the purchase price, the wear rate, the power consumption, and the maintenance costs.

 Let's not forget that these values give us an indication of the capacity of the different lamps to provide useful light at equal electrical power.

The adaptation of the spectrum, and the possibilities of positioning the lamp play a very important role for our plants!

The differences between LEDs

The main difference between models is the spectral distribution of energy, indeed all types of spectra appear on the market, each with their own particularity. The typical spectrum of a led last generation consists of two peaks, a blue and a red, which surround "a valley" lower (understand less intense).

Zeus Pro Spectrum

The proportion between peaks and valley and the positioning of the peaks are the main factors that will vary from one spectrum to another. Indeed, we prefer a peak in the red higher to assume a flowering, a blue peak higher to promote rapid growth and shorter inter-nodes. The valley has its importance for the photosensitive reactions of second order (less important for photosynthesis itself but still necessary in some proportion). Here we have to do with a spectrum type FSS which means the presence of a valley, and a peak higher in red than in blue. In other words a balanced spectrum for a whole crop cycle with a preference for flowering (note also the extension of the foot of the red peak to the infrared, also useful for flowering). There are also similar spectra with a blue peak at the same height as the red, a lower valley or even a second peak in the red. Each of these has a particular field of application.

Another difference between LEDs can be their efficiency, which is not to be neglected if you are looking to save power. Indeed, as explained above, a high efficiency means needing less electricity to generate the same light. We will then favor models with an efficiency of at least 2.6 µmol/d.

Recommended PAR values

For fast-growing plants, such as tomatoes for example, the recommended DFP levels depend on the stage of the crop primarily.

For germination, cuttings and/or slowed growth, we aim for 200-400 µmol/s/m² of average PPFD,

For an optimal growth we will aim at 400-600 µmol/s/m² of average PPFD,

For an optimal flowering we will aim at 600-950 µmol/s/m² of average PPFD,

Above 1000 µmol/s/m² of average PPFD, CO2 should be added in proportion (but not exclusively).

Be careful, these values are given as an indication and are calculated for healthy plants in an optimal climate. Always increase the PPFD gradually to ensure that the intensity is not harmful to the canopy.

How to read a PAR table ?

Reading the PAR tables provided by the manufacturers is the key to determining the optimal canopy lamp distance without having to use a PAR-meter. But these diagrams can sometimes be a little difficult to understand, so we will see together what are the parameters to watch when observing such information.

The diagram represents an illuminated area, so the first point is to look at what area it is, then this area is divided into smaller areas, with each of them an average PPFD value. When we talk about average PPFD for a specific lamp, we are generally talking about the average of these average values. In most cases, the manufacturer provides tables with different values, it is different heights of lamp, indeed the closer a lamp is, the more intensely the surface will be lit. But beware, getting closer can also have the effect of a less good light dispersion, with in some cases, the appearance of hot spots, hence the interest of having several measurements on the entire surface.

Other details can be taken into consideration such as the presence or not of reflective walls on the walls, their reflection rate, the quality, but also the shape and size of the reflector if there is one, etc.

With the help of these diagrams we will look for the configuration that provides us with PPFD values within the appropriate tolerances, and as homogeneously as possible on our surface.

Here is an example of the tables provided by Lumatek for their Zeus Compact Pro model:

We can see that these measurements were made without reflective walls, so it will always be necessary to be a little higher than what is announced when we are in a culture tent. For example in a 1.2x1.2 mylar tent, at 30 cm there is still a risk of having some spots above 1000 µmol/s/m² which can be detrimental. For a flowering we will position ourselves rather at 33-37 cm since at 50cm the high values start to fall already very low. Here, considering the walls, we can imagine that the distribution will be even more homogeneous on the sides of our growing area. The average PPFD values for the whole surface will probably only rise by 5% but more significantly on all the squares at the ends.

It should also be noted that doubling the distance does not halve the PPFD values (comparison between 15 and 30 cm), while halving the power with the dimmer (keeping the height of the lamp) will result in a halving of the PPFD values.

Moreover, as expected, the light dispersion at 1.5m height is clearly superior to the one when approaching the lamp which also makes this lamp a very good competitor for propagation.

It only remains to find the compromise between height and power used to be within the tolerances as homogeneously as possible.

To go further

It has been explained above that luxmeters are calibrated to light visible to humans and therefore are not always suitable for measuring light useful to a plant. Here are the different sensitivity curves:

We see very clearly that at 550nm for example we are very sensitive while it will be very little useful to the plant, or exactly the opposite around 680 nm. We also see that the plant uses two different chlorophylls to achieve its photosynthesis, so it is important to use several wavelengths of light, so as not to solicit only one of the two metabolic pathways. It is at the values of the four peaks that it is interesting to align the peaks of the spectrum of a lamp to maximize the power assimilated by the plant. Sometimes we will aim between the two left and between the two right in order to distribute the energy on the two chlorophylls equally but this is not an obligation in itself.

So one might think that the full spectrum feature (the valley between the peaks of the spectrum) is of no interest when looking at this graph but let's be mistaken. On the one hand chlorophyll B can be weakly stimulated by this light, but it also stimulates chlorophyll A by its lower sensitivity in the red. So the plant "feels" like it is exposed to natural light, and the impact that these wavelengths could have on other less important metabolic pathways is not suppressed. And we understand then the interest of not concentrating the majority of the energy emitted in this range but rather on the peaks.

Sources

https://www.sanlight.com/en/about-ppf-and-ppfd/

https://www.horticulture.red/fr/expertise/lumiere/mesurer-lumiere-horticulture-par-ppf-ppfd/

https://fr.wikipedia.org/wiki/Photosynth%C3%A8se

https://fr.wikipedia.org/wiki/Vision_humaine

https://lumatek-lighting.com/lumatek-cmh-lamp-315w-240v/

https://lumatek-lighting.com/lumatek-mh-lamp-600w-240v/

https://www.hortinews.net/dossiers/99-guide-lumiere-et-vegetaux.html

https://growace.com/blog/why-is-par-rating-a-big-deal-for-indoor-grow-light-systems/