Apogee PRI 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 PRI (Photochemical Reflectance Index) sensors include

• S2-121 (Faces Up | Has Output of: 1.43 mV per W m⁻² (Green); 1.43 mV per W m⁻² (Yellow))
• S2-122 (Faces Down | Has Output of: 14.29 mV per W m⁻² (Green); 12.5 mV per W m⁻² (Yellow))
• S2-421 (Faces Up | Has SDI-12 Output)
• S2-422 (Faces Down | Has SDI-12 Output)
• S2-411 (Faces Up | Has Output of: 5 mV (Green), 10 mV (Yellow))
• S2-412 (Faces Down | Has Output of: 5 mV (Green), 10 mV (Yellow))

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.

S2-421 PRI Sensor & S2-422 PRI Sensor (SDI-12 Output)

Apogee two-band radiometers combine two photodetectors that measure specific wavelength ranges. They are designed to continuously measure reflectance for calculation of the photochemical reflectance index (PRI) of plant canopies. Digital SDI-12 output is standard with analog options available by request. Domed diffuser promotes self-cleaning to minimize errors from dust and debris. For best results, we recommend pairing upward-looking sensor(s) with downward-looking sensors.

Wavelength Ranges

PRI (S2-421, S2-422)

• Green detector = 532 nm with 10 nm FWHM
• Yellow detector = 570 nm with 10 nm FWHM

*FWHM = full-width half-max

PRI = (ρ_Green – ρ_Yellow) / (ρ_Green + ρ_Yellow)

The spectral responses can be seen in the graph below.

The S2-421 PRI Sensor (Faces Up), and the S2-422 PRI Sensor (Faces Down) are available as a pair in a preconfigured IoT (Internet of Things) setup for continuous plant reflectance monitoring.