Frequently Asked Questions

Frequently Asked Questions


  • SRS - SUPPORTING SENSORS Can I check downward facing SRS output using a white panel test?
    Yes, you could check downward facing SRS 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
  • SRS - SUPPORTING SENSORS What is the calibration equation of my SRS sensor?
    Because the response of the SRS 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.
  • SRS - SUPPORTING SENSORS Why does the NDVI or PRI 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 or PRI calculation returns no results when data are zero in the denominator.

  • SRS - SUPPORTING SENSORS Can I use SRS at the plant level?
    Yes, you can use the SRS 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 SRS sensors be used in the field with natural sunlight (field instruments only).
  • SRS - SUPPORTING SENSORS What are some applications for SRS 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
  • SRS - SUPPORTING SENSORS What are some applications for SRS PRI sensors?
    • Relative photosynthetic performance
    • Monitoring plant stress
    • Estimating gross primary productivity of an ecosystem
    • Spatially distributed estimates of carbon flux in conjunction with eddy covariance system
    • Vegetation productivity
    • Drought indication
    • Monitoring plant stress
    • Onset of spring photosynthesis in evergreens
  • SRS - SUPPORTING SENSORS 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.
  • SRS - SUPPORTING SENSORS How do I calculate NDVI?
    Calculate NDVI as:


    NDVI = (ρNIRρred) / (ρNIR + ρred)


    where, ρred and ρNIR 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. Here is an example SRS NDVI calculator: [Download calculator]
  • SRS - SUPPORTING SENSORS Do SRS sensors need to be recalibrated?
    We recommend that the SRS be recalibrated annually. The sensors should be cleaned more frequently than this if needed. Check the SRS manual for cleaning instructions. The recalibration cost is $70 per sensor. Unfortunately, there is not a way to check the calibration in the field. It requires a light source of a known intensity and a calibration standard.
  • ATMOS 14 - SRS - IRT - ECT - PAR - PYR - ATMOS 22 - ATMOS 41 - PHYTOS 31 - ECRN-100 - ECRN-50 How do I assemble the anemometer mast?

    The anemometer mast is suitable to mount METER’s above-ground sensors: ATMOS 41, ATMOS 22, ATMOS 14, PAR, PYR, IRT, SRS, PHYTOS 31, ECRN-100, ECRN-50, and ECT. Additional brackets may be required for some products.

    Anemometer mast kit: 1 extendable mast (fiberglass), 1 guy ring, 1 guy rope (7.6 m nylon rope), 3 guy rope anchors, and 1 mast base with center peg and set screw.

    Mast diameter: 4.5 cm

    Mast adjustable length: 1.3 to 2.3 m

    Tools to bring to the field: measuring tape, long metal screwdriver, mallet, wrench, level, scissors, optional shovel, and lighter.

    Installation:  Position the mast base in the desired installation location, avoiding large rocks. Install the center peg using a mallet. Insert the mast into the base so that the center peg is inside the mast tube. With the mast in the base, turn the set screw so it is hand-tight against the mast. Then, gently turn the set screw with a wrench. Do not over tighten or the fiberglass pole will break. Put the guy ring over the extendable mast. Cut the guy rope into thirds (approximately 2.5 m each) and burn the ends with a lighter to prevent fraying. Position the rope anchors equidistant apart to be about 1 meter away from the mast. Screw the anchors into the soil at the same angle as the guy rope using a long metal screwdriver as a make-shift turning handle. Attach the guy rope to the guy ring in 3 equidistant holes (show below) using knots. Then knot each guy rope in the corresponding anchor. Pull the rope very tight and ensure the mast is level and straight.

    Once the mast is secure, move the adjustable lever up to release the extendable mast. Move the lever down to secure the mast to the correct height. Install instruments following the sensor quick start guides and manuals.

    Limitations: Rocky soils, long-term monitoring sites, locations with high animal traffic. The SRS extension arm may be too heavy for the fiberglass mast.