Specific wavelengths of radiation elicit distinct responses in plants. 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.
Red - far-red sensors are designed to approximate phytochrome spectral sensitivity. Quantifying the amount of red and far-red light incident on the sensor is important in determining the red / far-red ratio (red / far-red) and the far-red fraction (far-red / red + far-red).
Apogee Instruments S2 series red - far-red sensors consist of a cast acrylic diffuser, pair of photodiodes that measure specific wavelength ranges, and signal processing circuitry mounted in an anodized aluminum housing, and a cable to connect the sensor to a measurement device. Sensors are potted solid with no internal airspace and are designed for continuous measurement in indoor and outdoor environments.
| S2-131-SS* | S2-431-SS | S2-432-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) | 0.08 mV per µmol m-2 s-1 (typical value, variable from sensor to sensor) | - | - |
| Calibration Factor (reciprocal of sensitivity) | 12 µmol m-2 s-1 per mv (typical value, variable from sensor to sensor) | 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 33 mV | SDI-12 | Modbus |
| Wavelength Ranges | 645 to 665 nm ± 5 nm (Red) 720 to 740 nm ± 5 nm (Far-Red) |
645 to 665 nm ± 5 nm (Red) 720 to 740 nm ± 5 nm (Far-Red) |
645 to 665 nm ± 5 nm (Red) 720 to 740 nm ± 5 nm (Far-Red) |
| Measurement Range | 0 to 400 µmol m-2 s-1 | 0 to 400 µmol m-2 s-1 | 0 to 400 µmol m-2 s-1 |
| 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 |
| Response Time | Less than 1 ms | Less than 0.6 s | - |
| Non-linearity | Less than 1 % (up to 400 µmol m-2 s-1) | Less than 1 % (up to 400 µmol m-2 s-1) | Less than 1 % (up to 400 µmol m-2 s-1) |
| Field of View | 180° | 180° | 180° |
| Directional (Cosine) Response | ± 2 % at 45°; ± 5 % at 75° zenith angle | ± 2 % at 45°; ± 5 % at 75° zenith angle | ± 2 % at 45°; ± 5 % at 75° zenith angle |
| 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.
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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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