Apogee NDVI Sensors
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.
The range of Apogee NDVI (Normalized Difference Vegetation Index) sensors include
- S2-411 (Faces Up | Has SDI-12 Output)
- S2-412 (Faces Down | Has SDI-12 Output)
- S2-111 (Faces Up | Has Output of: 18.5 mV (Red), 26 mV (NIR))
- S2-112 (Faces Down | Has Output of: 20 mV (Red), 20 mV (NIR))
Introduction to Reflectance Measurements
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.
S2-411 NDVI Sensor & S2-412 NDVI Sensor (SDI-12 Outputs)
Apogee two-band radiometers combine two photodetectors that measure specific wavelength ranges. They are designed to continuously measure reflectance for calculation of the normalized difference vegetation index (NDVI). Digital SDI-12 output is standard with analog options available by request. Domed diffuser promotes self-cleaning to minimize errors from dust and debris.
Wavelength Ranges
NDVI (S2-411, S2-412)
Red detector = 650 nm with 10 nm full width half-max
NIR detector = 810 nm with 10 nm full width half-max
The spectral responses can be seen in the graph below.
Utilising the S2-411 & S2-412 Sensors in a custom IoT (Internet of Things)
The SNiP-NDVI is a ‘Sensor Node Integrated Package’ for LoRaWAN or CAT-M1 communication of real-time plant light reflection measurements, providing an accurate indication of the presence of chlorophyll, which correlates to plant health, stress and resilience to disease. This SNiP for continuous plant, light and environmental monitoring.
The base SNiP-NDVI integrates 1x S-NODE and 2 sensors to a site’s unique network, communication and power requirements: 1x S2-411-NDVI-Sensor (Faces Up; SDI-12), and 1x S2-412-NDVI-Sensor (Faces Down; SDI-12). The S-NODE can support an additional 2 compatible sensors.
| S2-111-SS | S2-112-SS | S2-411-SS | S2-412-SS | |
|---|---|---|---|---|
| Power Supply | Self-powered | Self-powered | 5.5 to 24 V DC | 5.5 to 24 V DC |
| Output (sensitivity) | 5 mV per W m-2 (Red) 6.67 mV per W m-2 (NIR) |
10 mV per W m-2 (Red) 6.67 mV per W m-2 (NIR) |
– | – |
| Calibration Factor (recipricol of sensitivity) | 0.2 W m-2 (Red) 0.15 W m-2 (NIR) |
0.1 W m-2 (Red) 0.15 W m-2 (NIR) |
Custom for each sensor and stored in firmware | Custom for each sensor and stored in firmware |
| Calibration Uncertainty | ± 5 % | |||
| Output Range | 18.5 mV (Red) 26 mV (NIR) |
20 mV (Red) 20 mV (NIR) |
SDI-12 | SDI-12 |
| Wavelength Ranges | Red detector = 650 nm with 10 nm FWHM* NIR detector = 810 nm with 10 nm FWHM* |
|||
| Measurement Range | 2x full sunlight (1000 W m-2) | |||
| Measurement Repeatability | Less than 1 % | |||
| Long-term Drift | 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° | 40° | 180° | 40° |
| Directional (Cosine) Response | ± 2 % at 45°; ± 5 % at 75° zenith angle | |||
| Temperature Response | 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 | |||
| Operating Environment | -40 to 70 C; 0 to 100 % relative humidity | |||
| Dimensions | 30.5 mm diameter, 36.6 mm height | 23.5 mm diameter, 40 mm height | 30.5 mm diameter, 36.6 mm height | 23.5 mm diameter, 40 mm height |
| Mass | 51 g | 110 g (with 5 m of cable) | 51 g | 110 g (with 5 m of cable) |
| Cable | 5 m of shielded, twisted-pair wire with TPR jacket and stainless steel connector | |||
| Warranty | 4 years against defects in materials and workmanship | |||