NDVI Sensor

Apogee NDVI sensors combine an ultra-rugged form with sophisticated cloud data delivery for an unbeatable price-to-performance ratio.

Continuously monitor NDVI remotely
Built for long-term exposure to the elements
Easy to use and low cost

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Overview / Features: Continuously monitor NDVI

At METER, we think you should spend less time on complexity and more time doing what matters. Apogee NDVI sensors are easy-to-use. And the best part? The ZENTRA system is ready to calculate NDVI from one upward-facing sensor paired with any number of downward-facing sensors for a complete measurement from a single ZL6.

Apogee Instruments S2 series two-badn sensors consist of a pair of photodiodes in upward- and doward-facing sensors that measure specific wavelength ranges. S2 series sensors are designed for continuous irradiance (upward-facing sensor) or radiance (downward-facing sensor) measurements in indoor and outdoor environments. Reflectance derived from paired upward-facing and downward-facing sensors can be used to calculate NDVI. Deploy the downward-facing sensor for all NDVI measurements, and add an upward-facing sensor for best results.

Low cost—low maintenance

By focusing on just two relevant bands, it’s possible to make an accurate multiband radiometer for a fraction of what a full-spectrum spectroradiometer costs. Apogee NDVI sensors are multiband radiometers that combine an ultra-rugged form with sophisticated cloud data delivery for an unbeatable price-to-performance ratio. They’re inexpensive enough to deploy multiple sensors simultaneously, so you can maximize spatial coverage and explore spatial and temporal variability in canopy structure and function.

Deploy for months or even years

Most spectrometers are delicate and expensive, making long-term field deployment risky. Apogee NDVI sensors are built for long-term exposure to the elements. Encased in a durable housing with an epoxy fill, they are watertight, weatherproof, and have fully sealed optics. Mount them on a fence post, tripod, or a meteorological tower. They’re rugged enough to leave in the field for an entire growing season or longer.

Field ready and radiometrically calibrated

Each sensor is radiometrically calibrated to a NIST-traceable standard. Readings are output in units of radiant flux density. Calibration information is stored on board the sensor, so you never have to worry about keeping track of calibration coefficients.

Collect data and monitor remotely

To simplify your workflow, Apogee NDVI sensors are plug-and-play with ZL6 data loggers, which means automatic sensor recognition and zero programming. ZENTRA Cloud lets you monitor remote sites in near-real time from any internet-connected device. Observe field conditions, check and configure sensor and system function, or download data from the comfort of your home or office.

Work less. Measure more.

Apogee NDVI sensors are part of a complete system of affordable, easy-to-use sensors, loggers, and software that require little maintenance and put near-real-time data at your fingertips, so you can publish more and work less.

Specifications: Measurement Specifications

Calibration factor (reciprocal of sensitivity)

Custom for each sensor and stored in firmware

Calibration uncertainty

± 5 %

Wavelength ranges

Red detector = 650 nm ± 5 nm with 65 nm FWHM

NIR detector = 810 nm ± 5 nm with 65 nm FWHM

Measurement range

2x full sunlight

Measurement repeatability

Less than 1 %

Long-term drift

Less than 2 % per year

Response time

Less than 0.6 s

Field of view

180°

Directional (cosine) response

± 2 % at 45°; ± 5 % at 75° zenith angle

Temperature response

Less than 0.1 % per °C

Physical Specifications

Housing

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, 34.5 mm height

Mass (with 5 m of cable)

140 g

Cable

5 m of shielded, twisted-pair wire with TPR jacket and stainless steel connector

Communication Specifications

Data logger compatibility (not exclusive)

METER EM60 series, ZL6 series, ZSC, ProCheck, Campbell Scientific

Electrical and Timing Specifications

Power supply

5.5 to 24 V DC

Output range

SDI-12

Other

Warranty

4 years against defects in materials and workmanship

Compliance

Prop 65 warning

Support / FAQ

Using Apogee NDVI sensors with ZENTRA system

METER Legacy SRS Spectral Reflectance Sensor Manual

METER Legacy Spectral Reflectance Sensor (SRS)(NDVI) Integrator Guide

METER Legacy Spectral Reflectance Sensor (SRS)(PRI) Integrator Guide

METER Splice Kit Repair Instruction Video

Sensor wire splicing guide (complete method)

Sensor wire splicing guide (quick method)

VIDEO: ZL6 + ZENTRA Cloud Troubleshooting

NDVI FAQs

Can I check downward-facing NDVI sensors output using a white panel test?: Yes, you could check downward-facing sensor response in the field on a sunny day using a white panel. Ideally, a Spectralon panel should be used. The SRS field stop calibration check should be done with the field stop sensors pointing down at an angle toward a white Teflon panel or Spectralon panel, so that the field of view is totally encompassed by the white panel. Then, with the hemispherical sensors pointing up to collect irradiance data at the same location and time (with no obstructions in the field of view), radiance values * pi (3.14) should be approximately equal to irradiance values.

Radiance = Irradiance/pi
Irradiance = Radiance * pi

What is the calibration equation of my NDVI sensor?: Because the response of the NDVI sensor is linear, the calibration is a simple multiplier that converts the raw readings to a calibrated value. There is an offset that is set to account for the noise readings that occur when the sensor is in complete darkness (dark offset). Both of these values are sensor specific and vary to some degree. A firmware update protects the calibration coefficients because they are stored in an area of the flash memory that is not touched during an update.

Why does the NDVI calculation show no results if the sensor wavelength output is zero? Is the sensor bad?: The sensor is not bad in this case. The NDVI calculation returns no results when data are zero in the denominator.

Can I use NDVI sensors at the plant level?: Yes, you can use NDVI sensors at the plant level for field application where there is natural sunlight. Make sure there are no shadows in the measurement area, or remove the data during shady or dark conditions. The problem with shady conditions is that the data are not reliable. You may need more sensor replications at the plant level if you see more variation.

If you plan on using a non-natural light source, this could be a big unknown about the effects of light intensity over time (intensity and distance changing over time). You would need to control these variables or measure them. For this reason, we recommend that the NDVI sensors be used in the field with natural sunlight (field instruments only).

What are some applications for NDVI sensors?

Intercepted photosynthetically active radiation
Leaf area index
Phenological stages of plant development
Biomass accumulation
Ground-based monitoring of vegetation density
Drought indication
Altered precipitation studies for drought tolerance
Canopy health/senescence
Spring canopy green-up
Ground truthing remotely sensed NDVI

What is the area of measurement?: It depends on the angle and the height the sensor is installed. Below is an example. The Apogee calculator is a good resource for determining the area of measurement. Use the 36 degree (18 degree half angle) sensor.

Field of view:

The field of view (FOV) is reported as the half-angle of the apex of the cone formed by the target surface (cone base) and the detector (cone apex), as shown below, where the target is defined as a circle from which 98% of the radiation detected by the radiometer is emitted.

Sensor FOV, distance to target, and sensor mounting angle in relation to the target will determine target area. Different mounting geometries (distance and angle combinations) produce different target shapes and areas, as shown below.

How do I calculate NDVI?: Calculate NDVI as:

NDVI = (ρ_NIR − ρ_red) / (ρ_NIR + ρ_red)

where, ρ_red and ρ_NIR are percent reflectances in the red and near infrared (NIR). ρ at each waveband is calculated as radiance/irradiance at each waveband, where radiance is reflected radiation intensity in that waveband and irradiance is incoming radiation intensity in that waveband.