Radiation reflected from surfaces (e.g., plant canopies, soil) provides information about the state of the surface. Reflectance is the ratio of radiation reflected by the surface to radiation incident on the surface.
Two common indices calculated from reflectance measurements are normalized difference vegetation index (NDVI) and photochemical reflectance index (PRI). NDVI is calculated from red and near infrared (NIR) reflectance and provides a measure of surface greenness. PRI is calculated from green and yellow reflectance and provides an indication of canopy photosynthetic activity.
Apogee Instruments S2 series two-band sensors consist of a cast acrylic diffuser (upward-looking sensor) or acrylic window (downward-looking sensor), pair of photodiodes that measure specific wavelength ranges, signal processing circuitry mounted in an anodized aluminum housing, and a cable to connect the sensor to a measurement device. S2 series sensors are designed for continuous irradiance (upward-looking sensor) or radiance (downward-looking sensor) measurements in indoor and outdoor environments. Reflectance derived from paired upward-looking and downward-looking sensors can be used to calculate NDVI or PRI.
S2-121-SS | S2-122-SS | S2-421-SS | S2-422-SS | |
---|---|---|---|---|
Power Supply | Self-powered | Self-powered | 5.5 to 24 V DC | 5.5 to 24 V DC |
Output (sensitivity) | 2 mV per W m-2 (Green) 2 mV per W m-2 (Yellow) |
10 mV per W m-2 (Green) 10 mV per W m-2 (Yellow) |
- | - |
Calibration Factor (recipricol of sensitivity) | 0.5 W m-2 per mV (Green) 0.5 W m-2 per mV (Yellow) |
0.1 W m-2 per mV (Green) 0.1 W m-2 per mV (Yellow) |
Custom for each sensor and stored in firmware | Custom for each sensor and stored in firmware |
Calibration Uncertainty | ± 5 % | ± 5 % | ± 5 % | ± 5 % |
Output Range | 6 mV (Green) 6 mV (Yellow) |
10 mV (Green) 10 mV (Yellow) |
SDI-12 | SDI-12 |
Wavelength Ranges | Green detector = 532 nm with 10 nm FWHM* Yellow detector = 570 nm with 10 nm FWHM* |
Green detector = 532 nm with 10 nm FWHM* Yellow detector = 570 nm with 10 nm FWHM* |
Green detector = 532 nm with 10 nm FWHM* Yellow detector = 570 nm with 10 nm FWHM* |
Green detector = 532 nm with 10 nm FWHM* Yellow detector = 570 nm with 10 nm FWHM* |
Measurement Range | 2x full sunlight | 2x full sunlight | 2x full sunlight | 2x full sunlight |
Measurement Repeatability | Less than 1 % | 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 | Less than 2 % per year |
Response Time | Less than 1 ms | Less than 1 ms | Less than 0.6 s | Less than 0.6 s |
Field of View | 180° | 35° | 180° | 35° |
Directional (Cosine) Response | ± 2 % at 45°; ± 5 % at 75° | ± 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 | Less than 0.1 % per C |
Housing | Anodized aluminum body with acrylic diffuser | Anodized aluminum body with acrylic window | Anodized aluminum body with acrylic diffuser | Anodized aluminum body with acrylic window |
IP Rating | IP68 | 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 | -40 to 70 C; 0 to 100 % relative humidity |
Dimensions | 30.5 mm diameter, 37 mm height | 23.5 mm diameter, 43 mm height | 30.5 mm diameter, 37 mm height | 23.5 mm diameter, 43 mm height |
Mass (with 5 m of cable) | 140 g | 110 g | 140 g | 110 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 | 5 m of shielded, twisted-pair wire with TPR jacket and stainless steel connector |
Warranty | 4 years against defects in materials and workmanship | 4 years against defects in materials and workmanship | 4 years against defects in materials and workmanship | 4 years against defects in materials and workmanship |
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