Measurement of Shortwave Radiation

1
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

2
Pyranometer

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

3
Important Characteristics of Pyranometers

• Spectral response
• Desire same response from all radiation across
  0.3 - 4µm band
• Cant have influence from thermal radiation
• 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

4
Important Characteristics of Pyranometers

• Directional response
• Sensor should follow Lamberts cosine law

Radiation
Radiation
Radiation
?
?
Projected area
5
Important Characteristics of Pyranometers

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

6
Thermal (thermopile) sensors

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

7
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.)

8
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

9
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

10
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)
11
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
12
Double domed
After simplification
k must be constant for linear calibration what
factors could affect k? Simplification depends
on Tinner dome Tbase How can we be sure this
is true?
13
Energy balance of single domed black and white
14
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.
15
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
• Used for measuring PAR

16
Photovoltaic cells

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

17
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
18
PVs contd.

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

19
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