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Overview
 The radiometric instrumentation system can assume numerous forms depending upon the measured quantity. In the basic system, a sensor comprised of optical components, detector, and associated signal and control electronics generates a signal which is passed to a data handling system for storage and analysis. Figure 2 illustrates the basic measurement system. Included as a part of the system are associated calibration sources necessary to assure generation of radiometric data. The components are assembled into radiometric systems which are generally divided into three types. First, imaging radiometers (calibrated cameras) which are used to determine spatially resolved radiance distributions in the form of images; second, spectrometers which are used to find spectrally resolved radiance or radiant intensity distributions; and third, radiometers which are used to determine temporally resolved radiance or radiant intensity distributions. These systems will be discussed in Radiometric Systems, while the components or subsystems are presented here.


Figure 2. Radiometric System


Optical Systems
 The first subsystem, following the source and any intervening transmitting media, is the optical system. The optical system includes the collection optic, transfer optics, optical modulator (optional) and perhaps additional apertures or stops. Although at times the optical system contains many elements its function can be depicted in the simple form shown in Figure 3.


Figure 3. Optical System


The purpose of the optical system is to collect and reshape wavefronts to irradiate the image plane (detector). Since the nomenclature related to optics is so broad and diverse a complete listing of the jargon of this field will not be attempted. The nomenclature used with optical systems will be presented within the description of a basic optical system. The text "Optics" by Hecht and Zajac is suggested as a reasonable source of the nomenclature of this field.

For the radiometric sensors under discussion it is important to have a measure of how effectively the optics collect the radiation and to have an accurate knowledge of the size and location of the radiation sources that the optics are viewing. The light gathering and field of view properties are defined by the effective stops, pupils, and spectral transmittance of the subsystem. The quality of the image can also be degraded from an ideal scaled reproduction of the source by aberrations introduced by the non-ideal character of the lenses and mirrors used in the optics of the sensor.

In all optical systems there are apertures which limit the passage of radiation through the system. These apertures include the edges of lenses or mirrors, the edges of detectors and the clear diameters of baffles or diaphragms introduced specifically to limit the image size. The aperture stop is that element in the optical train that limits the angular size of the cone of radiation accepted by the system. The field stop is the element in the optical train which limits the size of the image which can be formed by the system (i.e., defines the field of view).

Generally the light gathering power is defined the f-number or its reciprocal, the relative aperture, of the system. The f-number is defined as the ratio of the effective focal length to the diameter of the entrance pupil. The irradiance at the image plane then varies inversely as the square of the f-number. The f-number depends upon the effective focal length of the optical components of the system. Further, the transmittance of the optical components also limits their light gathering effectiveness. These properties of the system are not influenced by the stops within the system but rather by the shapes, combinations and materials within the optical train. These parameters also influence the image quality of the system. For the purposes of this paper spherical lenses and mirrors treated with paraxial (Gaussian) theory will illustrate the necessary concepts. The term paraxial refers to a system wherein the rays representing the flow of the radiation make very small angles with the optical axis and remain quite close to the axis. The properties of the spherical mirrors or lens can be approximated by the Gaussian lens formula (which has the identical form of the spherical mirror formula) and by the lens maker's formula for a thin lens and can also be treated with ray tracing techniques.

The cases of the flat lens and mirror are simple. In the case of reflection from a mirror the law of reflection merely states that the angle of incidence of a ray of radiation upon a surface equals the angle of reflection of that ray. In the case of refraction Snell's law is used.

The transmittance of the lenses (or reflectivity if using mirrors) must be such that radiation in the desired spectral region be passed through the system. A high transmission over the entire spectral bandwidth of the detector is desirable for all of the optical components except possibly the modular element components. The optical modulator sometimes is a spectral filter which limits the transmission to a narrow band of frequencies of interest. A list of some of the most commonly used near infrared materials is presented in the following table.

TRANSMISSION OF SOME IR OPTICAL MATERIALS
MATERIAL TRANS. THICKNESS 50% TRANS. PTS.
(peak) (mm) (microns)
Fused Silicate (glass) 90 2 <0.5-4.2
Sapphire (Al2O3) 93 2 <0.5-7
Calcium Fluoride (CaF2) 95 10 <0.5-9.0
Quartz (SiO2) 95 2 <0.5-4.
Magnesium Fluoride(MgF2) 95 2 <0.5-8.7
IRTRAN 1 (MgF2) 93 1 0.8-7.4
IRTRAN 2 (ZnSe) 74 1 0.6-14.7
IRTRAN 3 (CaF2) 95 1 <0.5-11.5
IRTRAN 4 (ZnSe) 70 1 <0.5-21.6
IRTRAN 5 (MgO) 88 1 <0.5-8.8
IRTRAN 6 (CdTe) 65 1 0.9-31.6
Silicon (Si) 54 2 1.2-16
Germanium (Ge) 47 2 1.8-23
Sodium Chloride (NaCl) 92 2 <0.5-20
Iodide (KRS-5) 72 5 0.6-40
Potassium Bromide (KBR) 92 10 <0.5-27
Potassium Chloride (KCl) 93 10 <0.5-22

Glasses used for visible optics generally are useful to only 2.7 micron, although some high silica glasses or fused quartz have reasonable transmission to 4 micron. Crystals such as germanium and silicon are used frequently, as are other hot pressed crystals such as the Irtrans from Kodak. The transmission of a given optical element is usually known (this transmission is dependent on material and thickness so it is peculiar to a particular optical element) and is used as a scale factor to determine the amount of radiation passed by the given element. Combinations of elements result in an effective transmission that is the product of the transmissions of the individual elements and may include reflection effects. Optical material properties, in addition to spectral transmission, which must be considered when selecting lens materials include the index of refraction, hardness, solubility, thermal expansion and melting temperature.

Reflective optics can sometimes be used to minimize the transmission and chromatic problems associated with lenses. As in the case of transmitting optics, spectral reflectance is not necessarily uniform over a range of wavelengths. The most frequently used infrared mirror is the front surface vacuum deposited aluminum type. It is highly reflective (reflectance greater than 95%) in the 2 to 10 micron range. Usually a coating of silicon oxide protects the aluminum surface with the appropriate thickness to not introduce any interference losses.

A spectral filter may be used to absorb, reflect or otherwise eliminate radiation of all frequencies from the optical path except the particular band of interest. In most near infrared measurement systems the filter of choice is an interference filter of the multi-layer dielectric or Fabry-Perot type. In both types the bandpass phenomenon is based on interference of multiply reflected radiation. The terminology of the filter includes the passband or primary wavelength interval of transmission, the peak transmittance which is the maximum transmittance in the passband and the halfwidth which is the width of the passband at 50% peak transmission.

The phenomena of aberrations are consequences of the shape and composition of all the optical elements. Aberrations can be lumped into two categories: chromatic and monochromatic. Chromatic aberrations cause light of different wavelengths to be brought to a focus at different locations (because of the non-uniform index of refraction as a function of wavelength of the lens). Monochromatic aberrations cause the image to be blurred, smeared or deformed. The five primary monochromatic aberrations are coma, spherical, astigmatism, field curvature and distortion.

Detectors
 In a radiometric measurement system, a sensor comprised of optical components, detector (or radiation transducer) and associated signal and control electronics generate a signal which is passed to a data handling system for analysis. During a radiometric measurement a portion of the radiant flux leaving a source is collected and directed to the field stop of the optical system. Following or coincident with the field stop, is the first element of the next subsystem, the radiation transducer system. Often the edge of the first element, the detector, in this system determines the field stop. The function of the detector is to convert the irradiance incident upon it to a voltage, a current, a change in conductivity, etc. The radiation transducer system includes the detector and signal conditioning elements necessary to convert the output of the detector to voltages of appropriate levels to be handled by the next subsystem, the data acquisition system. Figure 4 lists a number of detectors and their spectral ranges of applicability.

Figure 4. Spectral ranges of common detectors and sources


The following table lists some IR detectors and salient features.

COMMON OPTICAL DETECTORS
(A-Thermal Photodetector B-Internal Photoelectric C-External Photoelectric)
DETECTOR
MATERIAL		 SPECTRAL
RANGE		 TYPE RESPONSIVITY
(typical or peak)
Bolometer 1- >100 A 100-1000V/W
CsI 0.12-0.2 C 15mA/W
CsTe 0.12-0.32 C 30mA/W
Ge 0.5 -1.8 B 0.7A/W
HgCdTe 2-25 B 5A/W
InGaAs 0.8-1.8 B 0.7A/W
InAs 1-3.5 B 3.2A/W
InSb 1-5.5 B 3.5A/W
PbS 1-4 B 10A/W
PbSe 1-7 B 0.4A/W
Pyroelectric 0.1 - >100 A 100-45000V/W
Si 0.18-1.2 B 0.5A/W

Numerous figures of merit, operating characteristics and other constraints must be considered when matching detectors and applications. Some of the more common nomenclature is presented in the following table.

Instrumentation Nomenclature
Responsivity R Output signal level per unit of radiant flux (power) on the detector
Spectral Responsivity Rλ Responsivity for a particular wavelength or spectral bandpass
Noise Equivalent Power NEP The level of incident radiant flux (watts) which produces a S/N ratio of unity
Detectivity D The reciprocal of NEP
Specific Detectivity D* Detectivity normalized to a noise BW of Δf = 1 Hz and a detector area of Ad = 1 cm2 {D*=(AdΔf)1/2 D}
Noise Equivalent Irradiance NEI The level of irradiance which produces a S/N = 1
Time Constant τ A measure of the speed of response
Response Time τr
Frequency Response The steady state ratio of the output magnitude to the input magnitude for a sinusoidal input
Quantum Efficiency QE The ratio of induced current to incident flux; often measured in electrons per photon (dimensionless) or amps/watt

The most often used figures of merit for infrared detectors are the spectral responsivity, the noise equivalent power, and the detectivity. Also of extreme importance in many applications is the operating temperature of the detector since the requirement for cryogenic cooling can be problematical.

The responsivity is the ratio between the root mean squared (rms) signal voltage (or current) and the rms incident signal power. Typical units for responsivity are volts or amperes per watt. The noise equivalent power, NEP, is the value of incident rms signal power for a given radiation source, bandwidth, and chopping frequency required to produce an rms signal to noise (S/N) ratio of unity. The NEP is a measure of the minimum power which can be detected. The detectivity, D, is simply the reciprocal of NEP. A more commonly used expression is the dee-star, D*, of the detector. D* is the S/N ratio at a given source temperature and chopping frequency with an amplifier bandwidth of 1Hz for a 1cm2 detector receiving one watt of radiant power. It is therefore a normalization of the reciprocal of NEP to take into account the area and electrical bandwidth of the detector.

The detector time constant is a measure of the speed of response of a detector. This time constant indicates the responsivity of the detector as a function of modulation frequency. Knowledge of this parameter is necessary to choose an appropriate chopping rate or the applicability of a detector in applications with a rapidly transient source.

A wide variety of UV, Vis, and IR detectors are available. Figure 5 provides a summary of the most common detectors.


Figure 5. Detector Summary

Among the more common IR detectors are the pyroelectric thermal type, the photovoltaic indium antimonide (InSb) and the photoconductive mercury cadmium telluride (HgCdTe or MCT) types. Photodiodes and photomultipliers are the most common UV and visible detectors. The thermopneumatic Golay detector is one of the more unusual. It consists of a gas cell into which radiation is allowed to enter. The radiation heats the gas, deforms the cell and causes motion of an attached mirror or alters the character of an integrated capacitor.

In thermal detectors the responsive element is sensitive to changes in its temperature, which are caused by fluctuations in the irradiance. In the bolometric type (e.g. the thermistor is an example), the electrical conductivity is the temperature related variable. In the thermovoltaic detector (for example the thermocouple), a temperature fluctuation of a junction of dissimilar metals results in a generated voltage. With the pyroelectric detector, a change in the temperature of the sensitive element results in the generation of a current proportional to the rate of change of the temperature.

The photoelectric detectors (sometimes call quantum detectors) are based on the internal or external photoelectric effect. The internal photoelectric effect involves the excitation of electrons to higher energy levels in the conduction band by the irradiance on the detector. Electron-hole pairs are generated, either near a semiconductor p-n junction or inside a homogenous semiconductor, which establish a voltage across the junction or changes the conductivity of the semiconductor. The external photoelectric effect is more dramatic and results in the escape of "free" electrons from the irradiated substance. The irradiated substance absorbs sufficient energy to excite electrons on the photoemissive material sufficiently to liberate them from the material. Most quantum detectors used in the infrared are based on the internal effect due to the low energies associated with the low frequency photons. As mentioned earlier, two of the commonly used IR quantum detectors are the InSb type and the HgCdTe type.

Signal Conditioning
 The signal conditioning required by radiometric sensors can be very diverse. It is important that the appropriate conditioning be used or the potential of the detection system may not be fully realized. Texts such as Strobel's Chemical Instrumentation or Diefenderfer's Principles of Electronic Instrumentation are suggested as introductions to the nomenclature of electronic instrumentation. This discussion will only touch the surface of the subject.

One of the important parameters associated with the detector/preamplifier combination is the electrical bandwidth. Here it must be clearly understood that no longer is the concern with the spectral bandwidth which was critically important in the selection of windows, lenses, and detector. This electrical bandwidth is simply the range of the frequencies of the fluctuating current which can flow through detector and electronics. The electrical bandwidth is defined as the range of frequencies within which the amplifier will respond. The frequency range is often measured between the half power (3-dB) points on the output response versus frequency curve for a constant input. The bandwidth of the signal electronics is often tailored for various reasons through the use of electronic filters. High and low pass electronic filters are often employed to remove the ever present 60 Hz noise (from the power grid) and noise of frequencies outside the frequency band of interest.

It is also important that the gain and noise figure of the preamplifier be adequate to assure that the S/N of the detector is not seriously degraded by the remainder of the system. The noise figure is defined as the ratio between the S/N ratio of the input to an amplifier to the S/N at the output. Noise figures of near unity are desirable in the detector and preamplifier, as are high sensitivities or gains in the first stage of a circuit. Noise becomes progressively less critical in amplifier stages with gains greater than unity added beyond the preamplifier.

Another signal conditioning procedure often encountered is the use of a type of correlation analysis known as synchronous demodulation or phase sensitive detection. This procedure, which involves the use of a modulated (usually by a chopper) signal, is encountered in many radiometric systems. The technique involves the use of a stage called a lock-in amplifier. The lock-in amplifier serves two primary roles in this application. First it serves to subtract any offset or drift associated with detector, amplifiers, and filters from the final result. Second it serves to improve the S/N of the detection system. The lock-in amplifier mixes the reference signal derived from the chopper position with the detector signal. Since the signal typically corresponds alternately to that due to the source and that due to a reference, the two signals are correlated. The noise is very effectively discriminated from the output of the amplifier.

Data Acquisition
 The fluctuating voltage occurring at the final stage of the signal conditioning module in the radiometric measurement system is recorded. The act of recording the signal in a form that is amenable to analysis is called data acquisition. One common form of data acquisition involves manually recording a series of voltages taken from meters or oscilloscopes during the course of an experiment. Other forms of data acquisition involve the use of x-y plotters, photographs, and microdensitometers; strip charts; analog tape recorders and digital data acquisition systems. With the exception of the digital data acquisition system the other devices mentioned are only parts of a total acquisition system. For example the tape recorder is only a part of a total acquisition system because little can be done with data in the form of magnetized strips of tape. The analog recorder and several of the other devices mentioned serve only to allow limitless delays in or modifications in the timing of the acquisition process. Digital data acquisition is rapidly becoming the standard and preferred form of data acquisition.

Digital data acquisition can take numerous forms; however the typical system contains a multiplexer, a sample and hold amplifier, an analog to digital converter, an interface to a central processing unit, high speed and bulk memory, and input/output (I/O) devices. The signals enter the data acquisition system through the analog multiplexer. If only one signal is to be acquired this stage is omitted. The multiplexer functions as a bank of switches which allow the signals to be alternately coupled to the sample and hold amplifier. Important considerations are its programmability, range of permissible voltages, and switching speed (multiplexer settling time). The next device, the sample and hold amplifier, captures and buffers the signal for the next stage which is the noise sensitive low impedance converter. Important considerations for the S/H include the acquisition and operative times along with their settling times. The acquisition time is the time required for the output to settle within the rated accuracy after a sample input control signal has been applied. The operative time is the delay between the time the hold control signal is applied and the actual time the circuit enters the hold mode. Following the sample and hold amplifier, the analog to digital converter (ADC) accomplishes the translation from analog to digital form. The ADC may take many forms. Popular types are the integration, successive approximation, tracking, multi comparator ladder and voltage to frequency converters. The questions arising when applying ADC's are usually how rapidly and accurately are the conversions made. Information theory (by Shannon or Nyquist) suggests that the conversions should occur at a rate at least twice the maximum frequency of interest.

From this point, this discussion, if allowed to continue, could fill many volumes; therefore, the interested reader should examine one of the current texts dealing with digital computer systems. The omnipresent PC is frequently the basis for data acquisition and processing systems.

Data Processing
 The signal recorded by the data acquisition system is retrieved and processed in some fashion to yield that final information desired from the measurement system. Currently data processing is synonymous with the digital data acquisition process followed by computer aided data manipulation and graphic presentation of results. The data processing step might be as simple as the addition of units of measure on a strip chart or as complex as the digitization, scaling and Fourier processing required for a Fourier Transform Spectrometer (FTS). Data processing includes viewing the data as it is processed and archiving the data.

As an example of the digital data processing, the treatment of data acquired from a FTS will be explained. As explained previously the first step in the digital processing process is the conditioning of the analog signal to provide the appropriate levels and bandwidths of signals the digitizer. The second step is the actual digitization of the data and transfer to computer memory. The usual next step is the storage of that data on bulk memory for later processing or reprocessing if required. Therefore the next step in the process is the retrieval and probably the reformatting of the data for subsequent file manipulations. Usually at that time a previously created instrument response file is also retrieved. To this point steps are essentially the same regardless of instrument type; however, the next step is peculiar to the FTS. The data file is processed to find phase error in the data, and that information is stored for later use. The data is then adjusted using a special mathematical function (an apodization function) and then Fourier transformed. The phase error is then retrieved and used to phase correct the data. The instrument response is multiplied with the data and finally the data is scaled to remove any geometrical factors (such as range). A following step might be the subtraction of a background file or the manipulation of the data by some atmospheric correction code. The final steps are presentation and archival.

Calibrations
 Instrument calibration and characterization is essential to accurate measurements. The response of the instrument must be determined with known sources; its linearity or non-linearity for possible inputs must be determined; parameters such as field of view, frequency response, and the like must be known. Calibration of a radiometric system with a linear response to input radiation requires a number of steps as shown in Figure 6.


Figure 6. Calibrations

First the calibration requirement is determined from the intended instrument application (e.g., if the instrument is to be used for a transmission measurement no calibration is required). Then a suitable calibration source is positioned to overfill or under-fill the field of view of the instrument, depending upon the type calibration required; and signals are recorded with sources at two or more different known output conditions. A multiplicative instrument response is derived from these measurements. The output of the source for the two different conditions is determined (from measurements with standard detectors, manuals, calculations, etc.) and the difference is calculated. The difference between the signals recorded when exposed to the two sources is then determined. The instrument response is the ratio of the two differences. As an example, the instrument signal might be measured in volts and the source radiant output in watts per steradian within a specified spectral band. The multiplicative response would then be a number with units of watts per steradian per volt within the specified bandpass which could be used as a multiplier of voltages measured while viewing unknown sources to yield the inband radiant intensity attributable to the source.

Figure 7 is an illustrative step by step description of the calibration of an infrared camera and includes the steps described below.


Figure 7. Inband Radiance Calibration of Camera/Radiometer

The example is of an IR calibration using a blackbody source which permits ready calculation of the source output using the Planck equation

L(λ) = [εC1] / [πλ5(e[C2/(λT)] - 1)]

Calibration of IR cameras, radiometers, or spectrometers for either radiance or radiant intensity measurements consists of measuring a known source to determine the response of the instrument. In these infrared calibrations, the known sources are Planckian radiators which behave nearly like blackbodies. Their radiance is calculable (within an error band related to the accuracy of their measured temperature, emissivity and uniformity) using the Planck equation.

In the case of a radiance calibration the source is situated nearby (possibly with an evacuated path to the sensor) to overfill the FOV of the instrument (its range to the instrument is not needed for further calculations). For radiant intensity calibrations the source is situated to underfill the instrument FOV. In this type of calibration the distance to the source and the area of the source are required for subsequent calculations. The instrument output, a voltage, is recorded while the source is being viewed by the instrument. Typically several sources (varying temperatures, ranges, areas, etc.) are viewed and at least two measurements are required.

In the case of the IR cameras, using an additional filter, the calibration calculation steps include:

a. Calculating the radiance of the blackbody at small wavelength intervals.
b. Determining the normalized spectral response of the cameras.
c. Determining the normalized spectral transmittance of the filter used.
d. Determining the normalized spectral transmittance of the lens used.
e. Calculating the product of the spectral responses of the camera, filter, and lens. This function is then normalized.
f. The normalized spectral response is then multiplied with the appropriate Planck function and integrated over the spectral range. The integral is the amount of power causing a response in the instrument under calibration. The units of the radiance integral are (power/unit area of source/unit solid angle of collecting optics). Atmospheric effects may also be treated within this step. The spectral transmission of the intervening atmosphere is included as a factor in both integrals.
g. The above steps are performed for at least two conditions and the multiplicative instrument response is the ratio of the difference of calculated power reaching the detector from the two conditions and the difference in voltages measured for the two conditions.
h. The radiance of the test target is then the product of the instrument response and the voltage measured while viewing the target.

In the case of the spectrometer the calibration steps are similar. As in the previous case the Planckian radiation from a calibration source is calculated and measured. In this case the calibration is done in narrow steps of wavelength (or wavenumber) and the response is a function of wavelength (or wavenumber). The instrument response is the ratio of the difference in power from calibration sources at two different temperatures and the difference in measured signal generated in response to the two sources. In this case the geometry of the calibration setup has to be considered if the instrument is being prepared for a radiant intensity measurement. The power reaching the spectrometer, since the FOV is under-filled, also depends upon the area of the source and the range to it. The radiant intensity of a test target is then the product of the instrument response, the measured voltage, and the square of the range to the target.

Data Analysis
 While data analysis can be very elaborate and is generally performed some time after the measurements, immediate minimal data analysis should be accomplished along with the generation of data to establish error bounds and to identify anomalies with their causes (such as effects due to known perturbations, like instrument effects, that appear in the data).

Error Analysis
 The methodology and arithmetic of error analysis is presented in numerous references and will not be included herein. However, the key to performing error analysis for radiometric instruments is to correctly identify and estimate the magnitude of error associated with each error source. The errors must also be classified as bias or precision. Finally the elemental errors have to be summed (with some reasonable mathematical scheme) to determine the final uncertainty limit. Possible sources of error associated with the radiometric instrumentation (not to be confused with the error associated with measured data obtained with the instrument, which has many contributions other than that due only to the instrument) include:

a. the calibration source parameters: temperature, emissivity, uniformity;
b. geometry parameters: source range, source area, and instrument FOV;
c. the atmospheric transmission during calibration;
d. the spectral characteristics of the detectors, filters, lenses and windows; and
e. the gain, bias, drift, linearity, frequency response, sensitivity, dynamic range, spatial and spectral resolution of the instrument and data systems.

Typical errors to be expected with well behaved instruments are on the order of 5% for IR instruments and over 20% for UV instruments. The errors associated with the total measurement are larger containing many sources of error as shown in Figure 8.


Figure 8. Error Sources

Atmospheric Corrections
 Radiometric measurements are often made with substantial intervening atmospheric paths between the sensor and source. The intervening atmosphere contributes effects which may, in part at least, be removed. The correction of the measured quantities depends upon accurate knowledge of the atmospheric effects. Numerous computer codes exist which may be used to quite accurately predict the effect of the atmosphere in the IR and substantial improvements are being made for UV codes. Localized weather conditions must be known accurately, since the absorption depends on numerous factors such as temperature, pressure, humidity, etc. Figure 9 shows the atmospheric transmission over a 1 kilometer path for the spectral region between 0.5 and 15 microns.

Figure 9. Atmospheric Transmission

The absorption bands apparent in the figure correspond to the near infrared molecular band centers tabulated below. Common to most signatures is the atmospheric absorption due to the water and carbon dioxide molecules. The absorption due to water is responsible for the loss of transmission in the spectral regions (or bands) near 2.69 and 6.27 micron in the figure, as well as, to a lesser degree, in several other places. The carbon dioxide absorption band near 4.26 micron is also very apparent. In many types of rocket and jet plumes these molecules are abundant and are also major contributors to the infrared plume radiation. While scattering of radiation by these molecules is a contributor, absorption is the major loss mechanism for infrared transmission. With only a few feet of atmospheric transmission, losses can be significant. Less complete but more graphic losses can be observed near 3.2 microns where the water molecule also absorbs some radiation. Here it can be seen that molecular absorption is a rapidly varying function of wavelength. The molecular absorption is a function of the concentration and path length. The figure shows some spectral regions with virtually no transmission and others, called windows, which transmit significant infrared radiation for significant distances. It is within these windows that many radiometric instruments operate.

MOLECULE BAND CENTERS
(microns)
CO2 1.96, 2.01, 2.06, 2.69, 2.77, 4.26, 4.68, 4.78, 4.82, 5.17, 15.0
CO 4.66, 2.34, 1.57
H20 0.94, 1.1, 1.38, 1.87, 2.66, 2.73, 3.2, 6.27
NO2 4.50, 6.17, 15.4
N2O 2.87, 3.90, 4.06, 4.54, 7.28, 8.57, 16.98
OH 1.00, 1.03, 1.08, 1.14, 1.21, 1.29, 1.38, 1.43, 1.50, 1.58, 1.67, 1.76, 1.87, 1.99, 3.87, 4.14, 4.47
SO2 4.0, 4.34, 5.34, 7.35, 8.69

Another issue to be considered is the target background. The background is any radiation additional to the source radiation which can confuse the source measurement. The background is therefore just another, albeit unwanted, source within the field of view. The importance of considering the background in any measurement cannot be overstated. The atmosphere provides the background for many measurements and emits and scatters radiation. Especially in the UV in daylight conditions, scattered sunlight may be the dominant contributor to a scene. In the IR, emission from clouds may confuse the scene.

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