Lab 9: Measurement of Shortwave Radiation

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Lab 9 Measurement of Shortwave Radiation

• Shortwave radiation
• Radiation from sun
• 0.3 4 µm wavelength
• Characteristics
• Intensity important for energy balance
• Spectral distribution
• Energy available for specific physiological
  processes
• Affected by cloud cover, particulates, sun angle

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Pyranometer

• Measures total shortwave irradiance
• Two types
• Thermopile measure ?T between black and white
  surfaces (or black surface and ventilated
  pyranometer body)
• Photometric
• R, I, or V proportional to incident radiation

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Important Characteristics of Pyranometers

• Spectral response
• Desire same response from all radiation across
  0.3 - 4µm band
• Thermopile type sensors meet this requirement
  well (but only with protective glass domes) WHY?
• Photometric sensors do not require calibration
  against a thermopile type sensor and knowledge of
  spectral distribution of incident radiation

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Important Characteristics of Pyranometers

• Directional response
• Sensor should follow Lamberts cosine law

Radiation
Radiation
Radiation
?
?
Projected area
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Important Characteristics of Pyranometers

• Linear Calibration
• Ease of calibration function
• Accuracy
• All of factors above affect instrument accuracy

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Thermal (thermopile) sensors

• Accurate (expensive)
• Bulky (long response times)

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Thermopile sensors (contd.)

• Glass dome(s) over sensing surface
• Surface sensitive to a broad range of wavelengths
• Glass dome only lets through solar wavebands
• Improves cosine response
• Protects surface from wind (variable convection)
• Protects surface from weather (precip.)

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Two types of thermopile sensors

• Single domed black white
• Si proportional to temp. difference between black
  and white sections
• Double domed black
• Si proportional to temp. difference between black
  sensor surface and ventilated pyranometer body

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Energy Balance

• Key concept in Environmental Biophysics
• Conservation of Energy
• Energy inputs energy outputs energy storage
• At equilibrium energy inputs energy outputs
  0
• Must take into account all mechanisms of energy
  transfer
• Convection
• Conduction
• Radiation
• Latent heat
• Maybe diffusion, sources, or sinks

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Radiant Emittance
B (W/m2) s T4
Blackbody
Stefan-Boltzman (5.67X10-8 W m-2 K-4)
B (W/m2) e s T4
Real Body
Emissivity (Wavelength dependent)
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Double domed
Energy balance at sensor surface
Assumption Tbase Tinner dome
Absorption of Solar radiation
Longwave radiation Emitted by surface
- k ?T
asSi
sTb4
- s(Tb ?T)4
0
Energy conducted/convected away from the plate
Longwave radiation Emitted by dome
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Double domed
After simplification
k must be constant for linear calibration what
factors could affect k? Simplification depends
on Tinner dome Tbase -- can we be sure this is
true?
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Energy balance of single domed black and white
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Single domed black and white
Energy budget of white areas
aswSi
ew sTd4
- ewsTw4
- k (Tw Ta)
0
Solar radiation absorbed by sensor
Thermal radiation absorbed by sensor
Thermal radiation emitted by sensor
Energy conducted/ convected away from sensor
asbSi
eb sTd4
- ebsTb4
- k (Tb Ta)
0
Energy budget of black areas
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Single domed black and white
After simplification
k still shows up we still need one dome For
this simplification, Tdome not important, so we
dont need outer dome!
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Photometric Sensors

• Voltage, resistance, or current is a function of
  incident radiant flux
• Photoresistors used in photographic exposure
  meters but not suitable for quantitative light
  measurements
• Photovoltaics (solar panels)
• Silicon or selenium sensor for solar radiation
• Gallium arsenide phosphide measures PAR
• LED sensors can sense narrow bands

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Photovoltaic cells

• Light can be filtered to measure limited
  wavebands
• Silicon Cells
• Generally poor cosine response
• mV response a log irradiance
• Sensitive to changes in Tsensor

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PVs contd.

• Solutions to silicon cell drawbacks
• Operate in short circuit mode run current
  through shunt (current sensing) resistor
• Add diffuser to improve cosine response

I
I
R
I
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PVs contd.

• Sabsorbed Sincoming Sreflected
• Can you use a silicon chip to measure Sincoming?
• Can you use a silicon chip to measure Sreflected?

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Todays Lab

• Calibrate shunted and un-shunted silicon cells
  against a thermopile pyranometer
• Look at cosine response of shunted silicon cell
  without diffuser, and LI COR commercial silicon
  cell