NDVI Sensor

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? They’re pre-configured for use with the ZENTRA system, so you’re instantly up and running.

Specifications 

Downloads

FAQ's

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.