Specific wavelengths of radiation elicit distinct plant responses. Radiation that drives photosynthesis is called photosynthetically active radiation (PAR) and is typically defined as total radiation across a range of 400 to 700 nm. PAR is often expressed as photosynthetic photon flux density (PPFD): photon flux in units of micromoles per square meter per second (µmol m-2 s-1, equal to microEinsteins per square meter per second) summed from 400 to 700 nm (total number of photons from 400 to 700 nm).
In addition to PPFD, red and far-red wavelengths are of particular interest because they influence photosynthetic and morphogenic activity. Phytochrome pigments sensitive to varying ratios of red and far-red light provide information to the plant about its light environment and, therefore, its optimal growth patterns. Increasing the fraction of red light indicates less shading and generally results in more conservative vertical growth patterns; increasing the far-red fraction indicates more shading and results in more aggressive vertical growth patterns. Sensors that measure PAR and red/far-red radiation are often called PAR-FAR sensors. These sensors quantify both PPFD and far-red radiation incident on the sensor’s detecting surface.
Apogee Instruments S2 series PAR-FAR sensors consist of a cast acrylic diffuser, pair of photodetectors that measure specific wavelength ranges, signal processing circuitry mounted in an anodized aluminum housing, and a cable connected to the radiometer measurement device. Sensors are potted solid with no internal airspace and are designed for continuous measurement in indoor and outdoor environments. S2 sensors output an analog voltage for each photodetector that is directly proportional to red/far-red radiation incident on a planar surface (does not have to be horizontal), where the radiation emanates from all angles of a hemisphere.
| S2-141-SS* | S2-441-SS | S2-442-SS | |
|---|---|---|---|
| Power Supply | Self-powered | 5.5 to 24 V DC | 5.5 to 24 V DC |
| Current Draw | - | 1.4 mA (quiescent), 1.8 mA (active) | RS-232 37 mA; RS-485 quiescent 37 mA, active 42 mA |
| Output (Sensitivity) | 0.01 mV per µmol m-2 s-1 (PAR) 0.03 mV per µmol m-2 s-1(Far-red) |
- | - |
| Calibration Factor (reciprocal of sensitivity) | 100 µmol m-2 s-1 per mv (PAR) 38 µmol m-2 s-1 per mv (Far-red) |
Custom for each sensor and stored in firmware | Custom for each sensor and stored in firmware |
| Calibration Uncertainty | ± 5 % | ± 5 % | ± 5 % |
| Output Range | 0 to 40 mV (PAR) 0 to 20 mV (Far-red) |
SDI-12 | Modbus |
| Measurement Repeatability | Less than 1 % | Less than 1 % | Less than 1 % |
| Long-term Drift | Less than 2 % per year | Less than 2 % per year | Less than 2 % per year |
| Non-linearity | Less than 1 % (up to 4000 µmol m-2 s-1) (PAR) Less than 1 % (up to 1000 µmol m-2 s-1) (Far-red) |
Less than 1 % (up to 4000 µmol m-2 s-1) (PAR) Less than 1 % (up to 1000 µmol m-2 s-1) (Far-red) |
Less than 1 % (up to 4000 µmol m-2 s-1) (PAR) Less than 1 % (up to 1000 µmol m-2 s-1) (Far-red) |
| Response Time | Less than 1 ms | Less than 0.6 s | - |
| Field of View | 180° | 180° | 180° |
| Spectral Ranges | 389 to 692 nm ± 5 nm (PAR) 700 to 750 nm ± 5 nm (Far-red) |
389 to 692 nm ± 5 nm (PAR) 700 to 750 nm ± 5 nm (Far-red) |
389 to 692 nm ± 5 nm (PAR) 700 to 750 nm ± 5 nm (Far-red) |
| Directional (Cosine) Response | ± 2 % at 45°; ± 5 % at 75° | ± 2 % at 45°; ± 5 % at 75° | ± 2 % at 45°; ± 5 % at 75° |
| Temperature Response | Less than 0.1 % per C | Less than 0.1 % per C | Less than 0.1 % per C |
| Housing | Anodized aluminum body with acrylic diffuser | Anodized aluminum body with acrylic diffuser | Anodized aluminum body with acrylic diffuser |
| IP Rating | IP68 | IP68 | IP68 |
| Operating Environment | -40 to 70 C; 0 to 100 % relative humidity | -40 to 70 C; 0 to 100 % relative humidity | -40 to 70 C; 0 to 100 % relative humidity |
| Dimensions | 30.5 mm diameter, 37 mm height | 30.5 mm diameter, 37 mm height | 30.5 mm diameter, 37 mm height |
| Mass (with 5 m of cable) | 140 g | 140 g | 140 g |
| Cable | 5 m of shielded, twisted-pair wire with TPR jacket and stainless steel connector | 5 m of shielded, twisted-pair wire with TPR jacket and stainless steel connector | 5 m of shielded, twisted-pair wire with TPR jacket and stainless steel connector |
*The Output, Calibration Factor, and Output Range are variable from sensor to sensor, and the specifications listed are typical values.
Predicting Stem Elongation and Leaf Expansion: Percent far-red is a better predictor than PPE
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Far-red: The Forgotten Photons
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Turning Photons Into Food
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Toward an Optimal Spectral Quality for Plant Growth and Development
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PAR, PPF, PPFD, and PFD Explained
Photobiology Simplified with Dr Bruce Bugbee
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