1. Fourier Transform Infrared (FTIR) Spectrometer
Subhashree Mishra
ATMS Grad Student, UNR
W. P. Arnott
Physics, UNR
A four channel sun photometer was designed to measure total water vapor column, aerosol optical depth and column ozone. The objective behind building the sun photometer was to create an inexpensive instrument to measure atmospheric parameters that affect global climate. This instrument was developed by Dr. P.W. Arnott at the University of Nevada, Reno. Several improvements were made on the instrument design during the course of the semester to enhance the efficiency of operation of the instrument. Multiple measurements were made using the home built sun photometer on several days. A commercially available 5 channel hand-held sun photometer, designed by Microtops II was used to compare results from the two sun photometers. A complete description of the instrument design is presented along with principle of operation over the four channels representing four wavelengths (950nm, 870nm, 660nm, 470nm). Measurements taken by the instrument are presented along with suggestions for improvement and potential for use in related applications.
Introduction to Atmospheric Instrumentation (ATMS 360)
University of Nevada Reno

2. Energy Levels: Basic Ideas
Basic Global Warming: The C02 dance …
About 15 micron radiation

3. Wavelength and Wavenumber
Wavelength = 1 / Wavenumber
For the IR, wavelength is in microns.
Wavenumber is typically in 1/cm, or cm-1.
5 microns corresponds to 2000 cm-1.
20 microns corresponds to 500 cm-1.
15 microns corresponds to 667 cm-1. Much ‘terrestrial’ IR energy at the wavenumber.

4. Carbon Dioxide Concentration

5. Example Problem: Instantly Double CO2 Concentration
Example Problem: Instantly Double CO2 Concentration. What is the effect on the infrared spectrum at the surface?
Consequence: The Earth’s surface warms because of the additional IR coming to the surface from the Atmosphere.

6. Example Problem: Instantly Double CO2 Concentration
Example Problem: Instantly Double CO2 Concentration. What is the effect on the infrared spectrum from space?
Consequence: The less IR radiation escapes to space when the atmosphere has 800 ppm CO2 because the atmosphere is less transparent to IR emitted by the Earth’s surface. The Earth’s surface temperature must increase to again balance the outgoing IR with the incoming solar radiation.

7. LEDs As Detectors
Each photon with enough energy will normally free exactly one electron, and result in a free hole as well. If this happens close enough to the electric field, or if free electron and free hole happen to wander into its range of influence, the field will send the electron to the N side and the hole to the P side. This causes further disruption of electrical neutrality, and if we provide an external current path, electrons will flow through the path to their original side (the P side) to unite with holes that the electric field sent there, doing work for us along the way. The electron flow provides the current, and the cell's electric field causes a voltage. With both current and voltage, we have power, which is the product of the two.
From

8. LEDs As Detectors: Thermal Noise

9. FTIRs Often Use MCT Detectors: Mercury Cadmium Telluride
HgCdTe or Mercury cadmium telluride (also Cadmium Mercury Telluride, MCT or CMT) is an alloy of CdTe and HgTe and is sometimes claimed to be the third semiconductor of technological importance after Silicon and Gallium(III) arsenide. The amount of cadmium (Cd) in the alloy (the alloy composition) can be chosen so as to tune the optical absorption of the material to the desired infrared wavelength.
(from

10. Outline Introduction Theory Design Applications Measurements
Discussions

11. What is a FTIR Spectrometer?
A spectrometer is an optical instrument used to measure properties of light over a specific portion of the electromagnetic spectrum, 5 microns to 20 microns.
FTIR (Fourier Transform InfraRed) spectrometer is a obtains an infrared spectra by first collecting an interferogram of a sample signal using an interferometer, then performs a Fourier Transform on the interferogram to obtain the spectrum.
An interferometer is an instrument that uses the technique of superimposing (interfering) two or more waves, to detect differences between them. The FTIR spectrometer uses a Michelson interferometer.

12. FOURIER TRANSFORMS
Fourier transform defines a relationship between a signal in time domain and its representation in frequency domain.
Being a transform, no information is created or lost in the process, so the original signal can be recovered from the Fourier transform and vice versa.
The Fourier transform of a signal is a continuous complex valued signal capable of representing real valued or complex valued continuous time signals.

13. Fourier Transforms cont.
The Continuous Fourier Transform, for use on continuous signals, is defined as follows:
And the Inverse Continuous Fourier Transform, which allows you to go from the spectrum back to the signal, is defined as:
F(w) is the spectrum, where w represents the frequency, and f(x) is the signal in the time where x represents the time. i is sqrt(-1), see complex number theory.

14. Fourier Transforms cont.
A computer can only work with finite discrete signals, not with continuous signals. Thus, we need to define the Discrete Fourier Transform (DFT).
In DFT, the infinite borders of the integrals can be replaced by finite ones, and the integral symbol can be replaced by a sum. So the DFT is defined as:
And the inverse DFT is defined as:

15. FTIR Theory

16. The spectrometer described here is a modified Bomem MB-100 FTIR.
The heart of the FTIR is a Michelson interferometer (figure 2).
The mirror moves at a fixed rate. Its position is determined accurately by counting the interference fringes of a collocated Helium-Neon laser.
The Michelson interferometer splits a beam of radiation into two paths having different lengths, and then recombines them.
A detector measures the intensity variations of the exit beam as a function of path difference.
A monochromatic source would show a simple sine wave of intensity at the detector due to constructive and destructive interference as the path length changes (refer figure 3).

17. In the general case, a superposition of wavelengths enter spectrometer, and the detector indicates the sum of the sine waves added together.
Figure 3 shows some idealized light sources, and the interferograms that they would theoretically produce.
The difference in path length for the radiation is known as the retardation d (OM = OF + d) in figure 1 and 2.
When the retardation is zero, the detector sees a maximum because all wavenumbers of radiation add constructively.
When the retardation is l/2, the detector sees a minimum for the wavelength l. An interferogram is the sum of all of the wavenumber intensities.

18. Figure 1.

19. Schematic of Michelson Interferometer
Figure 2.
Source: MS thesis submitted by Carl George Schmitt, UNR , 1998.

20. Wave Interference

21. Figure 3.
Sample interferograms and their theoretical source intensity Source: MS thesis submitted by Carl George Schmitt, UNR , 1998.

23. Calibration of the FTIR spectrometer
Source: MS thesis submitted by Carl George Schmitt, UNR , 1998.

24. The spectrometer produces a complex voltage at each wavenumber
The spectrometer produces a complex voltage at each wavenumber. A linear model for the spectrometer response is assumed, where A is an instrument offset, and C is a scaling factor,
V= A+CI (1)
If the spectrometer views a perfect blackbody, Eq. (1) gives
V = A+ CBT (2)
where BT is the Planck emission curve for a blackbody of temperature T.
The two unknowns (A and C) can be determined from blackbody measurements at two different temperatures,
V1 = A+ CBT1
V2 = A+ CBT2

25. Solving for the unknowns yields C = (V1-V2)/(BT1-BT2)
and A = {V1(BT1-BT2)-BT1 (V1-V2)}/(BT1-BT2)
Returning to Eq (1), The FTIR voltage of another target (Vtarget) is related to the target radiance (Itarget) by
Itarget=[(BT1-BT2)Vtarget–BT1V2+BT2V1]/(V1-V2)
Thus, with measurements of blackbodies at two temperatures, the calibrated radiance from a target (cloud) can be determined.

26. APPLICATIONS
Identification of inorganic compounds and organic compounds
Identification of components of an unknown mixture
Analysis of solids, liquids, and gasses
In remote sensing
In measurement and analysis of Atmospheric Spectra
- Solar irradiance at any point on earth
- Longwave/terrestrial radiation spectra
Can also be used on satellites to probe the space

27. Source : UV thoughts from http://uvb. nrel. colostate

28. MODIS Solar Irradiance Source : http://en. wikipedia

29. Theoretical Absorption Cross Sections

30. Theoretical Absorption Cross Sections for the indicated gases, averaged to 1 cm-1 resolution for clarity.

31. Measurement Example from Reno

32. FTIR Radiance: Atmospheric IR Window
13 microns
8 microns

33. DEFINITION OF THE BRIGHTNESS TEMPERATURE TB
Measured Radiance at wavenumber v =
Theoretical Radiance of a Black Body at temperature TB

34. FTIR Brightness Temperatures

35. Atmosphere Emission Measurements, Downwelling Radiance
Notes:
Wavelength range for CO2, H20, O3, CH4.
Envelope blackbody curves.
Monster inversion in Barrow.
Water vapor makes the tropical window dirty.

36. RENO FTIR SPECTRA Which day is more moist?
Which day is warmer near the surface?
RENO FTIR SPECTRA

37. Ideal Weighting Function Wi: Where in the atmosphere the main contribution to the radiation at wavenumber i comes from.

38. Downwelling Intensity Emitted by the Atmosphere to the Detector (Radiance)
=cos z dz
emissivity=absdz/cos
B[T(z)] q
blackbody
radiance,
T = temperature. q
ftir
emission
transmission
weighting
function

39. Weighting Functions for Satellite Remote Sensing using the strong CO2 absorption near um. (from Wallace and Hobbs, 2nd edition)