How to calculate the light a crop is receiving in the greenhouse

Light Conversions in the Greenhouse

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

This article has the purpose to guide you on the approach you can take when working inside of greenhouses to correctly calculate the amount of light your crop is receiving.

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 we use energy units like Joules/m2/s or W/m2.

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 we use PAR (Photosynthetic Active Radiation). PAR includes just a portion of solar radiation, from 400 nm to 700 nm. When measuring PAR we use a specific unit called PPFD (Photosynthetic Photon Flux Density) measured in 𝞵mol/ m2/s. This specific unit will help us 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 we 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

  1. We learned light is about half solar radiation. If we assume this, we can then calculate 50% from the total solar radiation data in order to get a close number for the light present outside the greenhouse.
  2. Now we need to calculate the amount of light inside our greenhouse. Greenhouse structure and coverings reduce light transmission from the outside. We 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 we have a close number for the light present inside the greenhouse in energy units (Joules/m2/s or W/m2.). 
  3. 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 our example working in a greenhouse environment with natural light. The correct conversion factor to get PPFD from Joules/m2/s will be 4.6.

Source of LightConversion Factor
Sunlight1 joule = 4.6 𝞵mol/ m2/s 
HPS lamps1 joule = 4.95 𝞵mol/ m2/s 
Red LED1 joule = 5.5 𝞵mol/ m2/s 
Blue LED1 joule = 3.9 𝞵mol/ m2/s 
90% Red + 10 % Blue1 joule = 5.4 𝞵mol/ m2/s 

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 of LightConversion Factor
Sunlight1 Lux = 0.018 𝞵mol/ m2/s
Metal Halide1 Lux = 0.014 𝞵mol/ m2/s
HPS1 Lux = 0.012 𝞵mol/ m2/s
Red LED1 Lux = 0.08 𝞵mol/ m2/s
Blue LED1 Lux = 0.12 𝞵mol/ m2/s
Red + Blue1 Lux = 0.09 𝞵mol/ m2/s

Light intensity and Sum of light

When working with PAR units we can speak about light intensity and sum of light in our plants. Light intensity is the amount of light per second. On the other hand, sum of light is intensity of light for a given time. In order to know light requirements in plants we usually want to know how much light the plant is getting per day. We call this DLI or Daily Light Integral (Measured as mol/ m2/ d).

We can calculate DLI using the following formula:

When you know DLI you have a lot of power to manage growing systems. Use the following table in order to know minimum and optimum light levels for your crops. By using the provided information you will be able to make smart decisions inside your growing systems. For example, if DLI is lower than required you can evaluate the use of artificial lighting. On the other hand, if DLI is above optimum levels and you have a situation where heat is accumulating inside your greenhouse, you can evaluate the necessity of a shade cloth or the use of products to remove light and reduce the heat from radiation inside your system. 

In case of artificial lighting the use of DLI is crucial to manage a plant factory system correctly. This important data can help us to implement the exact number of lamps, proper distance and photoperiod to provide the best lighting conditions to your crop.

The management of light can be key in the success of your production.

At Hort Americas we know how important your crops are. This is why we work hard to provide the best solutions for light management. We have the products and knowledge to help you succeed.