Analysis of Radiation Wavelength

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Analysis of Radiation Wavelength

• So far, only considered radiation in lump sum
• Sun
• Black bodies
• Now, consider individual wavelengths
• Incoming radiation
• Reflected radiation
• Leaf/canopy reflectance
• Clarify some radiation issues
• Show how absorbance (a) is not constant with
  wavelength
• Investigate how absorbance changes with surface

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Electromagnetic Spectrum
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Why study spectral reflectance?

• Reflectance in visible range dominated by
  absorbance of chlorophyll and other pigments
• Relative abundance of water and other compounds
  (nitrogen, carbon, etc.) identified in
  not-so-near infrared region
• Source of ongoing exploration
• Identify important individual wavelengths
• Develop inexpensive sensor to read in those
  wavelengths

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Why Spectroradiometry?

• Unlike other measurements, conducted on large
  scale (airplanes, satellites)
• Study large areas in short time
• Develop models with data
• Non-destructive
• Use spectral data to determine
• Vegetation type
• Stress level
• Evapotranspiration rate

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Problems with Field Spectroradiometry

• Scale dependence of vegetation spectra
• Needle/branch/crown/canopy
• Viewing geometry and illumination sources
• This is critical
• Any difference between reflectance calibration
  angle and sample view angle will yield differing
  results
• Ground-truthing
• Requires considerable field/lab measurements
• Can be season specific
• Spectral library
• Must have information on file to compare spectra

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Overcoming problems with Spectroradiometery

• Develop spectral libraries
• Grad students
• Increase knowledge on leaf to canopy scale
  correlation

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Other uses for spectroradiometer

• Identifying true colors
• Camouflage
• Develop proper growth chamber environments
• Study activity of reaction centers of leaves in
  carbon cycle
• Soil mineral analysis
• Water activity (water potential) in food/soil
• Emerging technology at Decagon, in literature
• Still very expensive (lab tool only)

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Spectroradiometers

• Two ranges to consider
• Visible and shortwave near infrared
• 300 to 1000 nm
• Includes reflectance in visible range
• Uses
• Determine plant health, etc.
• Useful for analysis of light quality
• General not useful for library analysis
• Shortwave not-as-near infrared
• 1000 to 2500 nm
• Absorption bands of important molecules
• Absorbance/reflectance correlated with relative
  abundance

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Spectroradiometers How they work

• Mechanics
• Visible/shortwave near infrared
• Photons enter into instrument and strike concave
  (grating) mirror
• Mirror spreads light into individual wavelengths
• Wavelength bands incident on GaAsP detectors
• Voltage from detector proportional to radiation
  taken in at individual wavelengths

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Visible Near Infrared Sensor
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Spectroradiometers How they work

• Mechanics
• Shortwave not-so-near infrared
• Same process but...
• Mirror moves on a pivot
• Moves through single slit
• Single sensor measures energy at each wavelength
• Stepper motor moves through each wavelength

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Shortwave Infrared Sensor
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Accessories for Spectroradiometer

• Calibration for radiant intensity
• Spectroradiometers come ready to measure
  reflected radiation only
• Require factory calibration to give radiant
  intensity (W/m2)
• Additional cost
• Factory calibration allows instrument to be
  pointed toward light source
• Cosine corrected head for measuring irradiance
• Solar
• Growth Chamber
• Greenhouse

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Commercial Instrumentation

• Factors
• Cost
• Resolution
• Signal to noise
• Detector types
• GaAsP (more expensive but better resolution)
• CCD (closed-circuit detector)
• Array of silicon sensors sensitive to changes in
  radiant energy
• Less expensive but more noise than GaAsP

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Analytical Spectral Devices, Inc.

• Range of devices/Cost
• Field Spec. Hand Held
• 300 to 1000 nm
• Visible range (1 nm resolution)
• Shows light quality
• Chlorophyll absorbance
• 10,000
• Field Spec. Pro
• 300 to 2500 nm
• Includes high spectra for molecular absorbance
  bands
• 30K to 50K depending on detector type

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Ocean Optics, Perten etc.

• Provide lower cost spectrometers
• Resolution is lower (2-5 nm)
• Data quality may be just as useful depending on
  your needs

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FieldSpec Pro Data (300 to 2500 nm)
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Measurement Technique

• Dark current
• Measure nominal voltages on GaAsP detector
• White balance
• Measure radiation reflected from Spectrolon panel
• Gives maximum reflected radiation AT THAT VIEW
  ANGLE
• Measure surface at that view angle and source
  radiant intensity

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FieldSpec HandHeld (300 to 800 nm)
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Todays lab

• Collect spectra and photographs of a few surfaces
• Analyze spectra to see if what we see is really
  what the surface is reflecting
• Look for other interesting phenomena