Spectroscopy And Remote Sensing June 2007 - SPIE

Unlike absorption and emission spectroscopy, scattering or reflection spectroscopy (Figure 3)identifies properties of the radiated target via the analysis of energy scattered by the target. Thescattering process, much faster than either the absorption or emission process, can be studied asa function of the wavelength/frequency and polarization of the incident electromagneticradiation. Clearly, scattering spectroscopy would be of considerable interest in studying thenature of swamps, forests, and vegetation, which constantly scatter sunlight back into theatmosphere.Figure 3 Scattering or reflection spectroscopySpecific types of spectroscopyDepending on the incident radiation used to illuminate the target, and on the wavelength regionsin which the targets emit electromagnetic radiation, different types of spectroscopy are used andare described as follows: Spark or arc spectroscopy Visible spectroscopy Fluorescence spectroscopy Flame spectroscopy Ultraviolet spectroscopy Infrared spectroscopy X-ray spectroscopy Thermal radiation spectroscopyDetecting and analyzing spectroscopic outputsThe goal of all spectroscopic systems is to receive and analyze the radiation absorbed, emitted,or scattered by the target (gas, liquid, or solid). This detection and analysis is accomplished withinstruments involving prisms, gratings, spectrophotometers, and, in some cases, massspectrometers. The characteristics of prisms, gratings, and spectrophomoters will be describedbriefly, along with the principle of operation of a mass spectrometer.Spectroscopy and Remote Sensing5

The dependence of remote sensing on spectroscopyTo accomplish the analysis and description of the ever-changing crust of the Earth,spectroscopic systems are positioned remotely from their targets—on aircraft, satellites,spacecraft, ships, and, in some cases, near-to-ground platforms.The important point to keep in mind is that in a broad sense, remote sensing is just that,sensing/measuring information received from a target object remotely, from a distance. And inmost cases “remote sensing” in our discussions will be limited to terrestrial phenomena andweather observations.Thus, remote sensing, which by definition does not take place “in contact” with the object ortarget, is sometimes described as “trying to determine the type of animal from its footprints.”For example, we acknowledge that it may be difficult to measure temperatures in the upperatmosphere, so we measure instead the spectral emissions from a certain chemical element, suchas CO2, in that vicinity. Then, using the laws of thermodynamics, we can relate the frequenciesof the spectral emissions of the CO2 molecules—as measured by a spectroscopic system—to thetemperatures in that region of the atmosphere and achieve our goal indirectly.An Overview of CommonSpectroscopic InstrumentsIn this section we begin with a general description of the parameters that characterize thecapability of a spectrometer and then describe in more detail the operations of a prismspectrometer, a grating spectrometer, a spectrophotometer, and a mass spectrometer.Defining parameters for spectrometer capabilitiesIn general, three parameters can be used to describe the capabilities of a spectrometer: Spectral range Spectral bandwidth Signal-to-noise ratioSpectral range refers to the width of the wavelength band or region over which the spectrometercan access and analyze incoming data. In remote sensing, the spectral ranges that are incommon use are these: Ultraviolet (UV) – 0.001 μm to 0.40 μm Visible (VIS) – 0.40 μm to 0.7 μm Near infrared (NIR) – 0.7 μm to 3.0 μm Mid infrared (MIR) – 3.0 μm to 30 μm Far infrared (FIR) – 30 μm to 1 mmFor example, thermally emitted energy from the Earth lies in the MIR range, so thatspectrometers used to examine thermal energy from “hot” spots on the earth would have to beactive in the 3 μm to 30 μm range.6Optics and Photonics Series, Spectroscopy

Spectral bandwidth refers to the width of an individual spectral band—a wavelength orfrequency-width—that can be distinguished from adjacent wavelength or frequency bands. Forexample, if a spectrometer can distinguish between bandwidths of 0.1 μm in the MIR spectralregion (3 μm–30 μm), as compared to a spectrometer with a bandwidth of 10 μm, it is clear thatthe spectrometer with the narrower bandwidth can more accurately differentiate betweenwavelength information contained in the incoming signal, thereby improving the instrument’sresolution.The signal-to-noise (S/N) ratio of the spectrometer is just that—a comparison of the strength ofthe incoming signal to the strength of the background optical-electronic “noise.” Noise is asignal generated from sources other than the one being studied. An effective spectrometer mustdistinguish between the signal from noise and the desired signal from the source of interest. TheS/N ratio is a parameter that indicates how well a spectrometer is distinguishing between thetwo signals. The S/N ratio depends on the strength of the signal being detected, the sensitivity ofthe detector, and the spectral bandwidth of the spectrometer. For example, if a spectrometer is“looking” at only a few strong spectral peaks, an S/N ratio of 5 or so may be sufficient todistinguish between signal and noise. On the other hand, for complex signals with relativelyweak peaks, all crowded together, a higher S/N ratio of 10 may be required. Figure 4 shows theeffects of different S/N ratios in the process of separating signal from noise. For (a) whereS/N 10, all signal strengths are above the noise level and are detectable. In (b) for S/N 1,the signal is not strong enough to be separated from the noise. In (c), for S/N 1, the signalstrength is too low or the noise level is too high, and details of the signal will not be picked upby the detector.Figure 4 Comparison of signal strength and noise strength for S/N ratios of 10, 1, and 1Spectrometers used in remote sensing, with the parameters described above, are found inlaboratories, near the ground, in aircraft, and on satellites. The operating parameters ofspectrometers used in remote sensing will depend on the detailed nature of the target to beexamined (vegetation versus lakes, for example), the size of the field-of-view of the target, thedistance from spectrometer to target, and the nature of the intervening “atmosphere” betweenthe target and spectrometer.Prism spectrometersAn instrument that uses a prism to disperse or separate light into its basic components(wavelengths) is called a prism spectrometer. Its essential elements are shown in Figure 5. Lightfrom a light source to be analyzed is focused onto a narrow slit S. It is then collimated (madeparallel) by lens L1 and refracted by prism P, which typically rests on an indexed rotatableSpectroscopy and Remote Sensing7

platform. Rays of light corresponding to each wavelength emerge mutually parallel afterrefraction by the prism and are viewed with a telescope T focused for distant vision.Figure 5 Essentials of a prism spectrometerAs the telescope is rotated around the indexed prism table, a focused image of the slit is seenthrough the telescope for each distinct wavelength—at its corresponding angular deviation. Thedispersion and deviation of a light beam incident on a prism are pictured in Figure 6.(a) Typical dispersion curve for refractive index nversus wavelength λ(b) Color separation of white light by a prism(c) A case of large deviation δ and small dispersion D(d) A case of small deviation δ and large dispersion DFigure 6 Examples of dispersion n versus λ, color separations, and different deviations and dispersionsIn Figure 6a we see a sketch of a typical dispersion curve, that is, the variation of the index ofrefraction n as a function of wavelength λ. We see that n is larger for shorter wavelengths. Sincea prism bends the direction of a light ray more if its corresponding refractive index is higher, the8Optics and Photonics Series, Spectroscopy

different colors are bent by different amounts as shown in Figure 6b, with violet light bent morethan red light. The steeper the slope of the dispersion curve shown in Figure 6a for a givenprism, the greater will be the dispersion (separation of adjacent wavelengths).In Figures 6c and 6d, we compare the total bending of the incident ray measured from thestraight-through direction (dashed line), which we call deviation—with the separation(dispersion) of component wavelengths. The comparison is made using three widely separatedwavelengths—seen as red, green, and violet colors—to indicate dispersion D and deviation δ.When the instrument is used for visual observations alone (no measuring of angular deviations),it is called a spectroscope. If means are provided for recording the angularly deviated spectrumcomponents, for example, with a photographic film positioned in the focal plane of the telescopeobjective L2, the instrument is called a spectrograph.To extend the usefulness of the prism into UV and NIR regions, prisms made from quartz andfluorite are used for UV light, and prisms made of salt and sapphire are used for NIR light. Itshould be clear that the function of a prism is to separate (disperse) different wavelengthcomponents as widely as possible. This will happen, of course, if the difference in refractiveindices (Δn) is large for two wavelengths λ1 and λ2 whose wavelength difference (Δλ) is small.Thus the dispersion D is defined to be the ratio Δn/Δλ.D ΔnΔλ(1)where D is the dispersion,Δn is the difference in refractive indexes, andΔλ is the corresponding difference in wavelengths.A closely related parameter in the optical capabilities of a prism spectrometer is called thechromatic resolving power, R. It is given byR b ΔnΔλ(2)where b length of the prism base,Δn dispersion, andΔλR chromatic resolving power.The chromatic resolving power R can be expressed also asR λ(Δλ ) min(3)where (Δλ)min is the minimum wavelength difference that the prism can resolve, that is,distinguish clearly between closely spaced wavelengths λ1 and λ2, and where λ is thewavelength of the radiation incident on the prism.Spectroscopy and Remote Sensing9

Example 1Jocelyn is a photonics tech working with a prism spectrometer whose prism is made of flint glass.Her supervisor has asked her to calculate the dispersion, the chromatic resolving power, and theminimum resolvable wavelength difference for this particular prism for a wavelength around530 nm.SolutionJocelyn measures the prism base b to be 5 cm (5 107 nm) and from appropriate tables for flintglass, she finds that n 1.7328 at λ 486.1 nm and n 1.7205 at λ 589.2 nm. She sees that theoperating wavelength of 530 nm is about halfway between the two endpoint wavelengths in thetable she is using.From Equation 1 she finds the dispersion D to beD Δn 1.7328 1.7205 0.0123 – 1.19 10–4 per nm.Δλ486.1 589.2 103.1 nmFrom Equation 2, she finds the resolving power R to be( )( 4R b Δn (5 107 nm) 1.19 10Δλnm) 5950.From Equation 3 she finds (Δλ)min around λ 530 nm to be(Δλ)min λ 530 nm 0.09 nm, about 1/10 of a nm, the minimum wavelength difference the5950Rprism can resolve around λ 530 nm.Jocelyn notices from Equations 2 and 3 that if she were to use a flint prism with a base of 10 cm, theresolving power would be doubled and the value for (Δλ)min would reduce to 1/20 of a nm. But thatmight make the flint prism too large and bulky.Grating spectrometersThe diffraction grating, which serves as the heart of a grating spectrometer, is a simple and mostuseful optical device. The grating consists of a grid of fine parallel lines, uniformly spaced on apolished reflecting or transmitting surface. The lines generally are ruled with a fine diamondpoint and generally number several thousands of lines per centimeter. The reflection ortransmission of light, by the uniformly separated portions of the surface, causes diffraction andinterference effects, as discussed in Module 1-5, Basic Physical Optics, in Course 1,Fundamentals of Light and Lasers. These effects lead to the formation of a spectrumcharacteristic of a given light source.The essential makeup of a grating spectrometer is shown in Figure 7. The similarity betweenthis arrangement and that of the prism spectrometer shown earlier in Figure 5 is readilyapparent.10Optics and Photonics Series, Spectroscopy

Figure 7 Essentials of a grating spectrometerThe collimator directs parallel light onto the grating. The grating, in turn, serving as thedispersing element, separates the bundles of parallel light of different wavelengths and directsthem into the telescope. The observer adjusts the telescope for proper focus and views thecharacteristic line spectra or band spectra of the source in the focal plane of the telescope.The diffraction angle θ shown in Figure 7 is unique for each wavelength of light in the lightsource. As described in Module 1-5, Basic Physical Optics, the wavelength of light λ, thediffracting angle θ, the grating constant d, and the diffraction order m are all related in thegrating equation.mλ d(sin i sin θ)(4)where i is the angle of incidence of light measured with respect to the normal of the gratingsurface and d is the grating spacing. For example, if there are 3000 lines/cm ruled on the gratingsurface, d 1 cm 3.33 10–4 cm distance between centers of the ruled lines.3000The collimated beam illustrated in Figure 7 is perpendicular to the plane of the grating so thatfor this orientation angle i is zero and sin i is zero. Thus, for light normally incident on thegrating, the grating equation reduces simply to Equation 5.mλ d sin θ(5)where m 0, 1, 2, . . . refers to the different orders of diffraction. The essential geometryrelating λ, θ, and d for order m is illustrated in Figure 8.Spectroscopy and Remote Sensing11

Figure 8 Diffraction for order m at angle θ at normal incidenceThus, for a given order other than the zeroth order) (m 0), the incident light is diffracted (bent)at an angle θ, given from Equation 5 by sin θ (mλ)/d. Since d is the grating constant and m is aconstant for a given diffraction order, the angle θ changes with wavelength. For each order (m 1, 2, 3, etc.), the different wavelengths are separated out and observed as a distinct spectrum.However, since spectral lines in adjoining orders may overlap, one must be careful ininterpreting the spectral lines observed in the focal plane of the telescope. For example, a redline of 700-nanometer wavelength in third order is diffracted through the same angle as thegreen line of 525-nanometer wavelength in fourth order since sin θ 3(700)/d 4(525)/d isidentical for each wavelength. For visible light, there is no overlapping of the first and secondorders since, with λ1 720 nm and λ2 400 nm, the red end of the first order (m 1) falls justshort of the violet portion of the second order (m 2). When photographic observations aremade, however, these orders may extend down to 200 nm in the ultraviolet, and then the firsttwo orders would overlap.One can avoid this complication through the use of suitable color filters to absorb thewavelengths from the incident light that would overlap in the region under examination. Forexample, a cutoff filter that transmits only wavelengths longer than 600 nm might be used toblock the shorter wavelengths at order m 2 or higher from disturbing the observations in thewavelength region near 700 nm. In any case, a little common sense and attention to color, whereorders do overlap, will help in avoiding incorrect identification of wavelengths in unknownspectra.Example 2Sean is using a grating spectrometer to analyze the spectral light from an emission spectrum put outby a glowing gas. The emitted light is incident on the grating surface along the grating normal(i 0 ), and the spectral line Sean is looking for diffracts at

Visible spectroscopy Fluorescence spectroscopy Flame spectroscopy Ultraviolet spectroscopy Infrared spectroscopy X-ray spectroscopy Thermal radiation spectroscopy Detecting and analyzing spectroscopic outputs The goal of all spectroscopic systems is to receive and analyze the radiation absorbed, emitted, .