How does it work?

Calculate the light a crop is receiving in the greenhouse

When working inside a growing system such as a greenhouse or a plant factory system growers must have enough knowledge about light management. When you understand light you can make smart decisions to promote the best environmental conditions to promote growth, development and yield in your crops.

What is Solar Radiation or Global Radiation?
To understand light, let’s first separate light from solar radiation definition. Solar radiation in Earth covers a range 300 to 1500 nm. Solar radiation includes more than light. Only 45% to 50% of solar radiation represents light, the rest of the wavelengths include UV radiation and heat. To measure radiation energy units like Joules/m2/s or W/m2 are used.

Which is the best way to measure light in order to understand the impact of light in crops?
Light included in solar radiation includes all wavelengths visible to the human eye. In order to understand how plants use light  PAR (Photosynthetic Active Radiation) is used. PAR includes just a portion of solar radiation, from 400 nm to 700 nm. When measuring PAR a specific unit called PPFD (Photosynthetic Photon Flux Density) measured in 𝞵mol/ m2/s is used.

This specific unit will help to really understand how plants are using light. This is why the best way to measure light will be with the help of a PAR sensor, also called Quantum Sensor. However PAR sensors are not as common as radiation sensors, which are usually already included in meteorological weather stations in greenhouses. Meteorological weather stations are located OUTSIDE the greenhouse.

How to calculate PAR inside my greenhouse with typical data from meteorological weather stations?
When getting data from a meteorological weather station outside a greenhouse you have the following problems to approach.

  • Radiation sensors included in meteorological stations measure total global radiation in energy units. A conversion from energy units to PPFD will be required.
  • Data from the meteorological stations provide information about the radiation environment OUTSIDE the greenhouse. We must then calculate a close number for the radiation environment inside the greenhouse.

Calculating PAR from Energy units
Light is about half solar radiation. Assuming this, you can then calculate 50% from the total solar radiation data in order to get a close number for the light present outside the greenhouse.

Now you need to calculate the amount of light inside your greenhouse. Greenhouse structure and coverings reduce light transmission from the outside. You can assume a 70% of light can be transmitted from outside to inside our greenhouse. The next step then will be to obtain the 70% of the total light calculated on the previous step. At this point there is a close number for the light present inside the greenhouse in energy units (Joules/m2/s or W/m2.).

Last step will be to make conversion from energy units to the unit used to understand light in plants: PPFD.

In order to convert energy units to 𝞵mol/ m2/s light source must be considered. The conversion factor will totally depend on the source of light. When working with LED lighting, light quality needs also to be considered in light conversions. Different light colors will have a different conversion factor. Below you will find a useful table sharing conversion factors for different light sources. In order to convert energy units to 𝞵mol/ m2/s we need to multiply energy units by the corresponding conversion factor. Following the example working in a greenhouse environment with natural light. The correct conversion factor to get PPFD from Joules/m2/s will be 4.6.

According to HortAmericas another very used unit to measure light is Luxes. Particularly when working with artificial light can be common to work with a Luxometer.

On the following table you will also found conversion factors to get 𝞵mol/ m2/s from Luxes:

Source: Hort Americas 

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