Making Sense of Light Sensors
During the winter, greenhouse growers in Northern regions often become concerned that their plants are not receiving enough light to produce good quality crops. However, many do not know what light levels their crops are receiving because they do not understand the radiometric units their environmental computers display each day. In this article, we discuss light sensors, light units, and how to convert the units your environmental computer displays into more meaningful values.
Plants use and respond to wavelengths of solar radiation ranging from UV-B (as low as 280 nm) to far-red (FR) light (as high as 750 nm or so). However, photosynthesis is mainly stimulated by wavelengths of light between 400 and 700 nm, which is known as photosynthetically active radiation (PAR). Outside of the PAR range, energy can contribute less or not at all to photosynthesis but may influence the concentration of pigments (UV radiation), regulate photoperiodic and elongation responses (FR radiation), and increase plant temperature (infra-red; IR). These wavebands as well as other light terms and units are defined in the sidebar.
There are several units used by scientists and growers to measure light both inside and outside the greenhouse. For plant applications, we should use light units that represent the number of photons within PAR since they are what drive photosynthesis and thus plant growth. Therefore, we need to use quantum units. Since the number of photons is so large, we measure them in moles (mol) of photons, and 1 mol = 6.022 × 1023. One micromole (μmol) equals 1 × 10-6 mol so therefore, 1 μmol = 6.022 × 1017.
Most environmental control computers are connected to a pyranometer mounted to a weather station outside the greenhouse (Figure 1). This sensor measures total short-wave energy from 300 to 2,800 nm from the sun and sky. This includes UV radiation, photosynthetic (and visible) light, near infra-red (NIR), and IR. These measurements can be used to assist in an array of climatic and irrigation decisions and environmental control settings such as heating, cooling, shading, irrigation, and supplemental lighting. Figures 2 and 3 show screenshots as examples of radiation sums reported by environmental control computers.
Pyranometers measure total short-wave energy, but we are interested in photons within PAR. A pyranometer provides radiometric units, which are typically expressed in watts per square meter (W∙m-2) or joules per square meter and second (J∙m-2∙s-1). (Note that 1 W = 1 J∙s-1.) Since we are most interested in PAR, we need quantum units in μmol∙m-2∙s-1. Therefore, we need to do some math to convert from radiometric to quantum units.
As noted above, 1 J∙m-2∙s-1 equals 1 W∙m-2. The approximation that 1 W∙m-2 or J∙m-2∙s-1 ≈ 4.57 μmol∙m-2∙s-1 for sunlight (conversely, 1 μmol∙m-2∙s-1 = 0.22 W∙m-2) can be used IF the W∙m-2 or J∙m-2∙s-1 that your environmental computer displays is only for PAR radiation (from 400 to 700 nm). However, almost all pyranometers measure total short-wave radiation and not just the energy in PAR. Therefore, we need to use a different conversion factor.
Approximately 43% of the energy from the sun is within the PAR range, so we need to use the conversion factor of ≈1.96 (rather than 4.57) μmol∙m-2∙s-1 per watt of energy. Check with your environmental control computer company to find out what units their system reports. Some provide instantaneous units in J∙cm-2∙s-1 or J∙m-2∙s-1 while others provide cumulative units (the radiation sum per day) of J∙cm-2∙d-1 or J∙m-2∙d-1. Table 1 provides an example of light data measured outside a greenhouse located in the Midwest.
CONVERSIONS IN ACTION
Let’s say you want to calculate the daily light integral (DLI) that is reaching your greenhouse (outside) each day, and your environmental computer provides a radiation sum in J∙cm-2∙d-1. Let’s use an example of 1,230 J∙cm-2∙d-1. First, let’s convert cm2 to m2:
1,230 J∙cm-2∙d-1 × 10,000 cm2 per m2 = 12,300,000 J∙m-2∙d-1
Next, let’s convert days into hours and then into seconds:
12,300,000 J∙m-2∙d-1 / 24 h per d / 3,600 s per h = 142.4 J∙m-2∙s-1
Since 1 W = 1 J∙s-1, we can convert this to W∙m-2 so it has a value of 142.4 W∙m-2. As discussed earlier, there is 1.96 μmol∙m-2∙s-1 of PAR for every 1 W∙m-2 of short-wave radiation, so we can then convert this into quantum units:
142.4 W∙m-2 × 1.96 μmol∙W-1 = 279.0 μmol∙m-2∙s-1
This is the average value received during a 24-hour period. We can convert this from an instantaneous quantum unit of μmol∙m-2∙s-1 to a daily light integral (DLI) cumulative unit in mol∙m-2∙d-1:
279 μmol∙m-2∙s-1 × 3,600 s per h × 24 h per day = 24,105,600 μmol∙m-2∙d-1
Since there are 1,000,000 μmol per mol, we can then convert this into mol∙m-2∙d-1:
24,105,600 μmol∙m-2∙d-1 / 1,000,000 μmol per mol = 24.1 mol∙m-2∙d-1
This is the DLI delivered outdoors, since your sensor is located outside. For plant growth, we need to know what the DLI is inside your greenhouse; this depends on light transmission through the greenhouse glazing, the greenhouse structure, and overhead obstructions such as hanging baskets and lighting fixtures. The light transmission inside a greenhouse and to your crop is usually between 60 and 80% when there is no shading (whitewash or shade screen).
The transmission percentage can be determined by taking a light reading outside the greenhouse on a clear day around solar noon, then going inside and measuring light intensity in your greenhouse at plant height. You can then calculate what percentage of light outdoors is transmitted inside. In this case, any kind of light sensor can be used to determine the light transmission percentage.
For example, if you measured 1,400 μmol∙m-2∙s-1 outside and 925 μmol∙m-2∙s-1 inside, then your light transmission percentage is approximately:
925 μmol∙m-2∙s-1 / 1,400 μmol∙m-2∙s-1 = 66%
Using the example above, the indoor DLI would be 24.1 mol∙m-2∙d-1 × 0.66 = 15.9 mol∙m-2∙d-1.
The light transmission percentage of a greenhouse changes over time, especially when whitewash is applied or removed, or when new glazing is installed. Therefore, it should be measured periodically to improve accuracy of your readings.
To conclude, if your environmental control computer measures shortwave radiation and reports it in the unit of J∙cm-2∙d-1, then you can convert that value into DLI using this conversion: 1 mol∙m-2∙d-1 = 51.0 J∙cm-2∙d-1 or conversely, 1 J∙cm-2∙d-1 = 0.0196 mol∙m-2∙d-1.
For growers who do not have an environmental control computer that measures radiation sum, then the daily light integral outside can be estimated at your location by using DLI maps that are on the American Floral Endowment website at endowment.org/dlimaps (for U.S. locations) or SunTracker Technologies’ website at dli.suntrackertech.com (for worldwide locations). Next, calculate the greenhouse light transmission percentage to estimate the DLI delivered to your crops inside.