Light is a term that usually refers to the part of the solar radiation that is visible to the human eye. The human eye only perceives a small part of the full solar spectrum. Even several frequencies of the so-called electromagnetic spectrum do not reach the surface of the earth. In addition to the observable light, the electromagnetic spectrum contains gamma, X and UV rays, infrared and radio waves.

The light that is also observable by humans is the smallest segment of the electromagnetic radiation and contains the 7 colors of the rainbow within the visible spectrum, which for us is between 380 and 780 nanometers. When we put all these colors together, this combined light appears white to the human eye. Sunlight can be split into different colors by means of a glass prism or, for example, raindrops. think of the most famous example, the rainbow.

There are different ways to quantify or measure light. The most well-known units of measurement are the lux and the lumens and have their origin in the color intensity and light sensitivity perceptible to the human eye. Plants, on the other hand, have a different perception of light compared to the human eye. The “Photosynthetically Active Radiation” is known today as the most important industry standard and better known by its abbreviation “PAR”.

LED grow lighting requires a new way of thinking about how we measure light for use in plant cultivation. Lumen is the light unit familiar to most growers. The definition of lumen is the total light energy produced in the human-observable spectrum, but tells us nothing about the distribution of this light energy within this spectrum. As a result, no information is known about the distribution of the useful light frequencies for the plant within the spectrum.

The lumen problem is especially pronounced when measuring light at the ends of the human visual response curve. Take, for example, three lamps: red, green, and blue, each of which emits the same number of watts of optical energy. The red and blue lamps will hereby have a much lower lumen value compared to the green lamp, because the human visual response is very low in the red and blue region, and highest in the green region. Let the red and blue light frequencies be the most important for plants and the green ones the least important. Therefore, a high lumen value does not make a grow light better suited for growing plants.

comparison-human-response-par

In the same way, the lux value will tell us little about the specific light frequencies that are important for the plant. The light sensors in lux meters have their own response curves that can over- or under-react to light of different frequencies and are also intended to measure the amount of light energy within the human-observable spectrum. In addition, lux meters usually have different settings for types of “sunlight”, “fluorescent” and “incandescent” lamps, making a comparison of the lux value just a little less relevant.

Light is emitted as waves and particles. More precisely, as waves of photons, which are essentially bundles of energy. The energy level of each photon determines the distance between 2 waves. Wavelengths can vary from nanometers to several meters. But plant pigments can only absorb specific wavelengths. Namely these between 380 nm and 730 nm. PAR, or photosynthetically active radiation, is therefore the most useful part of the light spectrum.

Growers strive for a perfect spectrum in relation to cost and efficiency. Light efficiency is the amount of light a plant absorbs per watt or kilowatt of electricity consumed. Plant pigments absorb light at specific wavelengths and use this energy to carry out photosynthesis. The 3 main pigments in plants are:

Chlorophyll a: Absorption peaks at wavelengths around 430 and 662 nm

Chlorophyll b: Absorption peaks at wavelengths around 453 and 642 nm

Carotenoids: Absorption peaks at wavelengths around 450 and 454 nm

The most absorbed wavelengths are around 450 and 660 nm. They call these the absorption peaks.

Light Intensity: Useful units of measurement of light

Light intensity is usually measured and expressed in 3 different units of measurement: lumen, PAR and PPFD. Lumen is used to express the brightness of light to the human eye. What we as humans see and what the plant uses from the light is different. For example, a bright yellow light will give high lumen values. But the plant absorbs very little energy from this yellow part of the spectrum. The lumen unit of measurement therefore has little value when it comes to plant lighting. When the emitted spectrum of the lamp is known, it is possible to convert lumen values ​​to PAR.

PAR is measured in micromol/m²s. It is a measurement of the photosynthetically active radiation. Or in other words: measure the light within the useful spectrum for plants at a certain point in the room. The measurement does not take into account the amount of high-energy useful wavelengths for plants (eg blue 440-460nm and red 640-660nm) compared to the low-energy wavelengths (eg green). When you know where the measurement was taken in relation to the light source, this gives a good idea of ​​the intensity of the light. PPFD is therefore the most relevant unit of measurement.

PPFD (also measured in micromol/m²s) stands for photosynthetic photon flux density. This is the unit of measurement for useful photons within the PAR spectrum when the exact emitted spectrum is known. PPFD thus measures the useful wavelengths of the PAR and is thus an efficiency value of PAR.

Ledgrowers.be measures almost all its LED lamps with an Apogee Quantum meter. For example, an average is taken of different measuring points on a certain surface under the lamp. This at different lamp heights. Only in this way can objective advice be given about the recommended illuminated surface and the lamp height.

The Dagelikse Licht integral is the translation of PPFD values ​​into current growth time. She describes the combination of the amount of light and time. So the amount of PPFD needed per day to effectively grow and flower a particular plant. A grower may know how much light is radiating to a certain area per second, but how many seconds of this light does a plant need? This is the question the DLI answers.

DLI is measured with a PAR meter in mol/m² day. Note that this is the same unit of measurement as PPFD and PAR measurement, but in the context of a day length (rather than per second).

An average of 26 mol/m² day is suitable for most taller plants to thrive. For example, tomato plants already need 20 mol/m² day, flowering plants often need more than 32 mol/m² day.

Translated to PPFD, this average of 26 mol/m²day is:

300 µmol/m²s during a 24h exposure.

600 µmol/m²s during a 12h exposure.

800 µmol/m²s during a 10h exposure.

Since the PPFD decreases squared with height, it is best to hang the lamp 10 to 20 cm closer to your plants during flowering. The PPFD increases considerably and the area to be exposed decreases slightly. You can easily solve this by illuminating the edges with, for example, a simple Led Flower Strip at a short distance from the plant.