Close this search box.

NIR Plant Stress Test: Investigating the Impact of Light and Water on Plants


The primary objective of this classroom experiment is to deepen students’ understanding of how different wavelengths of the electromagnetic spectrum, particularly Near Infrared (NIR), can be used to assess plant health. By conducting a hands-on experiment with  plants, students will explore how the NIR reflectance changes over time in response to variations in light and water conditions.


  • STELLA spectrometer
  • Computer with appropriate USB cable
  • Leafy green plants
  • Water
  • Plant tray
  • Sheet of Polystyrene Foam
  • Full spectrum grow light (if indoors) or sunshine
  • Excel software for data analysis


Place the plants on a tray, positioning them in full sunlight or under full spectrum grow lights.

Divide the plants into two groups and arrange them close together, with the leaves leaving little to no gaps when viewed from above.

Activity Steps

Introduction (15 minutes):

  • Discuss the importance of understanding plant health and the role of different wavelengths, especially NIR, in assessing vegetation.
  • Explain the experimental setup and the variables being tested: water availability and light exposure.

Setup and Initial Measurements (10 minutes):

  • Take initial measurements from both groups of plants

Daily Measurements (15 days):

  • Perform calibration measurements each day before taking plant measurements.
  • Take measurements of calibration medium (polystyrene foam) and plants from same measured height and angle.
  • Water one group of plants regularly while withholding water and/or sunlight/grow light from the other group.
  • Use the STELLA spectrometer to take daily measurements from both groups, controlling for variables such as angle and distance.

Data Analysis (60 minutes):

  • At the end of 15 days, gather all the measurements.
  • Convert measurements to reflectance before comparing.
  • Use Excel to create graphs mapping out curves of reflectance for each day.

Discussion (20 minutes):

  • Lead a class discussion on the observed trends in NIR reflectance over the 15-day period.
  • Encourage students to analyze and interpret the data, considering the differences between the groups.

Conclusion and Reflection (10 minutes):

  • Summarize key findings from the experiment.
  • Discuss the implications of the results for understanding plant stress and health.
  • Encourage students to reflect on the broader applications of using NIR spectroscopy in agriculture and environmental monitoring.

This classroom experiment engages students in a long-term investigation, allowing them to observe and analyze the effects of water availability and light exposure on plant health through NIR reflectance. The use of real-time measurements and data analysis in Excel enhances students’ scientific inquiry skills and critical thinking.

Calibration Procedure

  • Keep foam board as clean as possible
  • Keep STELLA as still as possible when taking calibration measurements
  • Take measurements from same angle (nadir being ideal)
  • Measure and maintain distance from calibration medium (foam board)
  • Take at least 20 measurements (green: sample and average mode)
  • Avoid any shadows and if possible, avoid taking measurements on overcast days
  • When taking measurements in overcast conditions, take frequent calibration batches as close to other measurements as possible

STELLA Reflectance Formula

STELLA spectral sensor data are reported in irradiance units (uW/cm²) and need to be converted to radiance and then reflectance using the formulas below.

Radiance = (Irradiance * Distance²)/Area

Reflectance = Radiance from the plant/Radiance from the white reference

Additional Resources