2. DISCOVERY OF INFRARED
The development of spectroscopy in non-visual wavelengths proceeded in
parallel with the development of visual spectroscopy. Infrared light was
discovered by Sir Frederick William Herschel,(born in Hanover, Germany) who
performed experiments with mercury-in-glass thermometers illuminated by
sunlight dispersed through a glass prism (Herschel 1800).
Fig.no. 1. Prism and Thermometer
2
3. Much to his surprise, he found that not only did thermometers
register heat beyond the red end of the visible spectrum, but
the greatest amount of heat was found in this region. After an
initial claim that these “heat rays” were just extensions of
visible light, he became convinced that they were two separate
phenomena.
In April 1800 he reported it to the Royal Society as dark heat: Here the
thermometer No. 1 rose 7 degrees, in 10 minutes, by an exposure to the full red
coloured rays. I drewback the stand, till the centre of the ball of No. 1 was just
at the vanishing of the red colour, so that half its ball was within, and half
without, the visible rays of the sun. And here the thermometer No. 1 rose, in 16
minutes, 83/4 degrees, when its centre was 1/2 inch out of the visible rays of the
sun. Now, as before we had a rising of 9 degrees, and here 83/4 the difference is
almost too trifling to suppose, that this latter situation of the thermometer was
much beyond the maximum of the heating power;while, at the same time, the
experiment sufficiently indicates, that the place inquired after need not be
looked for at a greater distance.
Fig.no. 2. Sir Frederick
William Herschel
3
4. Thomas Young disagreed. Young had already shown that the “actinic rays” off
the blue end of the spectrum (now called ultraviolet radiation), which had
been discovered by Ritter (1803), were as susceptible to diffraction as visible
light and thus represented a continuum of wavelengths. He believed that the
“heat rays” would have similar properties on the other side of the visible. His
interpretation was criticized and it was not until nearly 100 years later, when
it was shown that visible and infrared light had identical responses to
polarization, that the matter was finally put to rest.
4
5. Fig.no. 3. Glass prism, mounted at
end of a brass tube, used by Sir
William Herschel
Fig.no. 4. Replica of Sir William Herschel Mirror
Polisher
5
6. Fig.no. 5. Sir John Frederick
William Herschel
William Herschel’s son, Sir John Frederick William
Herschel, recorded the first infrared spectrum some 40
years after his father’s discovery. He let solar radiation
pass through a prism and shine on an alcohol-wetted
piece of paper covered with soot on its back.
The alcohol caused the paper to be transparent, allowing
the soot to show through, but dried unevenly, permitting
rough observation of the solar spectrum with the
alternating white and black regions on the paper
(Herschel 1840). The spectral features were eventually
ascribed to the presence of water vapor in the
atmosphere and not to features in the solar spectrum.
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7. An important part of an infrared spectrograph is the method used to disperse the
incoming light into the spectrum.
Some types of glass are able to pass wavelengths in the near infrared but
unfortunately glass, in general, is opaque to the middle and far infrared
wavelengths. This requires prisms to be made of other materials.
For more than 130 years, rock salt (NaCl) has been the material of choice, with
various other salts being used for special applications. However, rock salt is
difficult to work with and dissolves easily in water.
Brashear (1886) is credited with being the first person to develop a viable method
for creating prisms out of rock salt.
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8. EARLY INFRARED DETECTORS: THE THERMOPILE AND
THE BOLOMETER
A photograph of a spectrum can be made by allowing the dispersed light to fall on a
photographic plate. Unfortunately, this technique, which was popular in the visible
wavelengths, was poorly suited to the infrared due to the lack of good infrared-
sensitive photochemicals.
Thomas Seebeck discovered, in 1822, that connecting two strips of different metals in a
loop and applying a temperature difference to the two junctions causes a nearby
compass to deflect. He called this the thermomagnetic effect, which was eventually
renamed the thermoelectric effect.
While a single pair of metal strips produces only a very small voltage, multiple junctions
can be connected in series to boost the voltage output. Such a device is called a
thermopile.
The first practical thermopile was invented by Leopoldo Nobili and Macedonio Melloni.
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9. 9
In 1880, Samuel Langley and Frank W. Very took on the task of designing an infrared
detector specifically for dispersive spectroscopy.
The result was the bolometer, which consisted of two thin platinum strips covered
with lampblack . One of the strips was exposed to the incoming radiation and the
other was shielded. The resulting difference in temperature resulted in a small
difference in electrical resistance that could be measured by a galvanometer.
Over 20 years, Langley was able to improve the sensitivity of his bolometer by 400
times
Bolometers are still popular today, especially for detecting the far infrared
wavelengths.
Fig.no. 6(a)Thermopile’s
prototype
invented by Nobili.
Fig.no.6(b) Incomplete version of
the Nobili−
−Melloni thermopile.
10. 10
The Michelson interferometer was invented by Albert Abraham
Michelson(1891).
The first use of an interferometer to measure infrared radiation was
by Rubens & Wood (1911), who used quartz plates as mirrors and
recorded the interferogram of the far infrared spectrum of a
Welsbach (gas) mantle (found in modern camping lanterns).
They had to guess at the spectral components and they synthesized
sample spectra which could then be matched with the recorded
interferogram.
Practical use of the interferometer for spectroscopy would have to
wait for the invention of the digital computer.
Michelson interferometers
Fig no. 7. A basic
Michelson interferometer,
not including the optical
source and detector.
11. 11
The first discussion of numerically computed Fourier-transform spectroscopy (FTS) was
presented by P. B.Fellgett in 1951. Fellgett also noted that FTS provided a multiplex
advantage over standard dispersion spectroscopy by increasing the signal-to-noise ratio by
N , where N is the number of spectral wavelengths being sampled. This occurs because with
a prism or diffraction grating spectrometer with a single detector, most of the energy in the
incoming light is ignored at any given time. However, with FTS all of the energy from the light
is used at all times.
The first digitally computed infrared spectrum was produced by H. A. Gebbie, G. A. Vanasse,
and J. Strong in 1956 and the first astronomical observation was of the Sun in the far-
infrared.
Today, with the availability of sensitive infrared detectors and fast digital computers, FTS is a
common,and often preferred, method of spectroscopy.
12. 12
1800-Herschel discovered IR radiation
1905-Coblentz’s experiment led to IR spectra of
organic and inorganic compound
1930-Interest increased in IR application to
Analytical chemistry
1947-Wright and herser developed double beam
dispersive IR
1969-Digilab commercial FT-IR with dedicated
minicomputer
1700-Fourier developed mathematical
transform method.
1980-FT-IR combined with personal computers
to make widely used, versatile, and cost
effective method of analysis.
1891-Michelson published design of his
interferometer.
1949-Fellgett discovered multiplexing advantage
of FT.
1965-Cooley and tukey discovered fast fourier
transform algorithm.
HISTORY OF IR SPECTROSCOPY
13. REFERENCE:
WISEweb: Wide-Field Infrared Survey Explorer, available at:
http://www.nasa.gov/mission_pages/WISE/main/index.html
(accessed on 12 November 2017)
Michelsonweb: Fourier Transform Spectrometer, available at:
scienceworld.wolfram.com/physics/FourierTransformSpectrometer.htm
(accessed on 13 November 2017)
IRSHandbookweb: Spitzer: IRS Instrument Handbook, available at:
http://ssc.spitzer.caltech.edu/irs/irsinstrumenthandbook/
(accessed on 12 November 2017)
MIPSInstHandbookweb: Spitzer: MIPS Instrument Handbook, available at:
http://ssc.spitzer.caltech.edu/mips/mipsinstrumenthandbook/
(accessed on 12 November 2017)
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