Longwave Instrumentation

Longwave Radiation Balance

The earth and atmosphere are major sources and sinks of longwave radiation. Most surfaces on the earth are close to being perfect blackbodies (objects that absorb and re -emit all the radiation striking the object's surface) for the longwave part of the spectrum. The longwave radiation balance (R_lwbal) may be written as:

( 1 )

with and as the incoming (source=sky, clouds, etc.) and outgoing (source=earth's surfaces) longwave components, respectively. The wavelength of these components is usually in the range of 3 to 100 µm.

The energy flux, R, emitted by an object is related to its temperature by the Stefan-Boltzmann law:

( 2 )

where  e_obj is the emissivity of the object,  s is the Stefan-Boltzmann constant (5.68 x 10^-8 W m^-2 K^-4), and T_obj is the temperature of the object in K (Kelvin).  

The total outgoing radiative flux (emitted plus reflected) may be written as:

( 3 )

where   can be calculated from  s T_IRT⁴

T_IRT is the temperature (in K) of the object as measured with an infrared (IR) thermometer.  
 

Using equation 3, the actual temperature of an object can be obtained by solving for T_obj if  e_obj ,   and   are known:

( 4 )

2. Longwave Radiation Sensing

Thermal radiation measurements are based on the exchange of energy between hotter and cooler objects.  Assuming that the infrared instrument is one of the objects, the instrument absorbs or emits energy if it is cooler or warmer than the object that is being sensed, respectively.  An infrared instrument measures radiant energy beyond the sensitive range of the human eyes. All objects (with temperatures above -273^o C) radiate this energy with an intensity relative to the temperature of the object. As an object becomes heated, the peak spectral response will move from the infrared to the visible spectrum, becoming "red hot". Longwave radiation can be measured with an Infrared Thermometer (IRT) (e.g., the Everest model 4000 IRT Transducer with multiplexer or an Everest Model 112 IRT) or a Pyrgeometer (e.g., Eppley Precision Infrared Radiometer (PIR)).

a) Infrared Thermometry
The detector of the infrared thermometer either absorbs or emits energy from the viewed object.  The energy passing through the sensor's optics is either focused or dispersed from the IR-detector.  The IR-detector converts the radiation to a proportional electrical signal that is the electrical analog of the IR radiation.  In the case of the Everest model 4000, with a multiplexer or the Everest Model 112, the longwave radiation is converted to an analog signal proportional to the target temperature and a digital signal which controls the LCD display.  If Everest Model 4000 is not used with a multiplexer, the company supplies algorithms to convert the radiation to a target temperature.

IRT's are fast, simple, dependable and reliable with repeatable accuracy. Also, since there is no physical contact between the detector and target, the energy balance of the target is not altered during measurements.
 It is possible to obtain the incoming and outgoing longwave radiation fluxes by measuring the ground (surface) and sky temperatures with an IRT and then applying the Stefan- Boltzmann law, via Equation 3.

The multiplexer, for the Everest model 4000IRT transducer,  routes the electrical signal from the transducer to a preamplifier in which the signal is linearized and converted to an equivalent digital signal which excites the liquid crystal digital display on the multiplexer.  The analog signal is still available to be read by the data logging system.

If a multiplexer is used, a 110 volt AC or 12 volt DC power supply is needed. (Click on Image to see multiplexer image) If a multiplexer is not used, a positive unregulated filtered DC voltage source between 5 and 10 volts and a regulated -2.367 volt DC voltage source are needed.

b) Eppley Precision Infrared Radiometer (pyrgeometer) - PIR

A PIR is used to measure, hemispherically,  the exchange of longwave radiation between a horizontal blackened surface (the detector) and the target viewed (i.e. the sky or the ground). The PIR has the same circular multi- junction thermopile (detector) as the Eppley PSP which is used as a method of transducing the longwave radiation flux into a electrical response (Figure 1).  The inner silicon dome is coated with a vacuum-deposited interference filter.  Radiation at wavelengths from 0.3-0.4 µm to approximately 50 µm is transmitted into the sensor. The amount of radiation in the visible spectrum 0.3-4.0 µm is  not significant; absorption and re-emission effects are small and have been compensated for in the calibration.

Figure 1 - Sketch of the Eppley pyrgeometer (not to scale) and schematic of the pyrgeometer curcuit. Typical resistance values are R_T = 10.0 KW at 25^oC, R₁ = 35.8 KW, R₂=9.97 KW and R₀ = 35.4 W.

The PIR is designed to compensate for two errors in  measurement: 1) temperature differences between the instrument dome and the actual detector and 2) the temperature difference between the detector and the actual object being sensed.  To correct the first error, thermistors have been attached to the inner dome and  to the detector to measure the temperatures of each object (Albrecht and Cox, 1977).  The PIR also has a thermistor-battery-resistance circuit that can be used to provide an output  to compensated for detector temperature.  The battery is 1.35 Volts.  However, the output from this circuit can be affected by voltage fluctuations.  As an alternative the direct output from the thermopile and the temperature from the thermistor embedded in the thermopile can be used. Basically, three outputs are needed to calculate the longwave radiation:

•  thermopile or the thermistor-battery-resistance circuit output
•  dome thermistor output
•  thermopile thermistor output.

The thermopile output is preferred over the battery circuit because of battery voltage fluctuations.  The thermistors need to be excited with a voltage source (in this case, from the data logger), thus the output is proportional to temperature.

Click here for calculating flux from the PIR.
Click here for example of data from the pyrgeometer.