2. Spectroscopy: What is it?
“The branch of science concerned with the investigation and measurement
of spectra produced when matter interacts with or emits electromagnetic
radiation.”
• Electromagnetic radiation is passed through or reflected from a sample.
• Some of the radiation is absorbed and some is transmitted or reflected by
the sample.
• Spectrometers separate (think glass prism) and measure the intensity of
the emerging radiation as a function of its wavelength – to enable
analysis of the optical, chemical, and physical properties of the
sample.
3. Spectrometers
• Nowadays, two of the most commonly used spectrometer designs are
Michelson Fourier-transform spectrometers (FT-IRs) (high signal-to-
noise, but bulky and sensitive to disturbance) and miniature dispersive
spectrometers (small & rugged, but low signal-to-noise), which are suitable
to very different applications.
• Dispersive spectrometers disperse the incoming light into a frequency
spectrum, which is directly recorded by a detector array.
• Fourier-transform spectrometers split and recombine the incoming light to
generate an interference pattern which is recorded - the frequency spectrum
is subsequently generated by taking the Fourier-transform of the
interference pattern.
Over the next few slides I’ll explain the operating principles of each of
these spectrometer designs and compare them to our microFTS, a new
kind of Fourier-transform spectrometer.
4. Dispersive Spectrometers
intensity
Optical Bench SPECTRUM
(Un-Folded Czerny-Turner) wavelength or frequency
ENTRANCE SLIT GRATING DETECTOR ARRAY
How they work
Light enters the spectrometer through a slit and
is reflected from a collimating mirror (MIRROR 1).
The collimated light is reflected from a
MIRROR 1 MIRROR 2
diffraction grating to disperse the light into
its separate wavelength components.
The dispersed light is reflected from a second
mirror (MIRROR 2) and refocused onto a detector
array.
The frequency spectrum of the incident radiation
is recorded directly by the detector array.
5. Michelson FT-IR
Optical Bench
MIRROR 1 How it works
(FIXED)
Light enters the spectrometer and is split into two
perpendicular beams at the beamsplitter.
BEAMSPLITTER One beam of light is reflected from a static mirror
MIRROR 2
(MOVING) (MIRROR 1) and the other beam from a moving mirror
(MIRROR 2). The moving mirror introduces a time
delay to the second beam.
The two beams are brought back together at the
SINGLE POINT DETECTOR beamsplitter and interfere to form an interference
pattern that is temporally modulated.
intensity
INTERFERENCE PATTERN Each data point in the interference pattern
corresponds to a time and thus a position of the
time
scanning mirror – the range and speed of the
FOURIER TRANSFORM scanning mirror determines the number of data
ALGORITHM
points in the interference pattern.
intensity
SPECTRUM
Taking the Fourier-transform (FT) of the interference
pattern yields the frequency spectrum of the incident
wavelength or frequency radiation.
6. microFTS
Optical Bench How it works
Light enters the spectrometer and is split into
MIRROR 1 MIRROR 2 two beams at the beamsplitter.
One beam of light is transmitted by the
beamsplitter and reflected from MIRROR
BEAMSPLITTER 2, then MIRROR 1, before being again
transmitted by the beamsplitter and striking
the detector array. The other beam of light is
reflected by the beamsplitter and reflected
from MIRROR 1, then MIRROR 2, before
DETECTOR ARRAY reflecting from the beamsplitter and striking
the detector array. The two beams are
focussed by the curved mirrors to combine
intensity
INTERFERENCE PATTERN and interfere at the detector array.
distance
An optical path difference between the two
FOURIER TRANSFORM beams is introduced by the different
ALGORITHM
distances the two beams travel around the
set-up. The resulting spatially modulated
intensity
interference pattern is spread across a
SPECTRUM
detector array.
wavelength or frequency
Taking the Fourier-transform (FT) of the
interference pattern yields the frequency
spectrum of the incident radiation.
7. Comparison Table
Instrument Format Michelson Dispersive microFTS
Static-Imaging Fourier
Grating-based, diode array, or
Also Known As FT-IR or FT-NIR spectrometers
CCD spectrometers
Transform Spectroscopy
(SIFTS)
Moving Parts? Yes No No
Relative Cost High Low Low
Fast ~ 100ms Fast ~100ms
Slow ~ 10s
Data Acquisition Speed (limited by detector read-out (limited by detector read-out
(limited by mirror scan speed)
rate) rate)
Signal-to-Noise Ratio /
High Low Medium
Sensitivity
Internal Wavelength
Yes No Yes
Calibration Possible?
Size And Weight Large and heavy Compact and light Compact and light
Spectral Region Of
MIR to NIR NIR , Vis and UV MIR, NIR, Vis and UV
Operation
Bruker, PerkinElmer,
Examples of Market Ocean Optics, Avantes,
ThermoFisher Scientific, ABB, N/A
Leading Brands ThermoFisher Scientific
FOSS
Basically, it’s the best of both worlds: Great signal (almost) like the bulky lab setups
we all know but fast, small & rugged like miniature dispersive spectrometers.
8. Early Prototype microFTS
DETECTOR ARRAY
FIBRE OPTIC INPUT
BEAMSPLITTER
35mm
A an early prototype
instrument, operating in the visible
spectral region.
MIRROR 2 The instrument is fibre-optic fed
MIRROR 1 and uses a off-the-shelf optical
components and a CCD detector
array.
9. Early Performance Specifications
Spectral Region UV-Vis IR
Detector(i) 2d CCD array Linear PZT pyroelectric array
2 to 17 µm
(5 100 to 600 cm-1)
Wavelength
200 to 1 050 nm or
Range(ii) 2 to 20+ µm
(7 000 to 350 cm-1)
@ 200 nm < 1.2 nm @ 10 µm 0.1 µm
(50 000 cm-1) (300 cm-1) (1 000 cm-1) (10 cm-1)
Resolution(iii) @ 500 nm < 2.9 nm @ 20 µm 0.2 µm
(20 000 cm-1) (120 cm-1) (500 cm-1) (5 cm-1)
Better than 1 part in 25 000 per ⁰C
Stability(iv) Equivalent to: 0.04 nm per ⁰C @ 1 000 nm (0.4 cm-1 per ⁰C at 10 000 cm-1)
SNR(v) 500:1 100:1
Size ~ 160 x 115 x 45 mm
Mass ~ 0.5 kg
Note that these are indicative specifications, based on laboratory-measurements
i. The system will incorporate alternative detector technologies, such as linear PbSe (lead salt) arrays, 2d VOx
microbolometer arrays, and others.
ii. Wavelength range is dependent on beamsplitter material and detector type. For the IR model the ranges quoted
are for ZnSe or KBr beamsplitters.
iii. Instrument resolution is determined by the maximum optical frequency (or minimum wavelength) in the recorded
spectrum. The instrument resolution was measured as the FWHM of a laser source.
iv. Measurement of instrument stability was limited by the experimental set-up. It is anticipated that it will be an
order of magnitude better than the value quoted. A calibration laser can be incorporated into the instrument for
enhanced stability (equivalent to the use of a HeNe calibration laser in a Michelson-interferometer).
v. SNR measurements were made as the ratio of the central peak of a laser line to an average background noise
level.
10. How can you help?
We’ve developed what we think is a really cool instrument, a real
step forward in spectrometer design. While it was developed for
atmospheric measurements on Mars, we’d like to find out how it
could be useful for us Earthlings.
• Do you have some thoughts on how to implement the
proposed applications listed in the challenge page below?
• Can you think of any new applications?
• It’s a pretty modular setup – How could we modify the
instrument to be even more useful or useful in a different
setting?
Thanks a lot guys! Remember to use those marbles…
