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.
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 ofthese spectrometer designs and compare them to our microFTS, a newkind of Fourier-transform spectrometer.
Dispersive Spectrometers intensityOptical 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.
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.
microFTS Optical Bench How it works Light enters the spectrometer and is split intoMIRROR 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.
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 ~ 10sData 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 FOSSBasically, it’s the best of both worlds: Great signal (almost) like the bulky lab setupswe all know but fast, small & rugged like miniature dispersive spectrometers.
Early Prototype microFTS DETECTOR ARRAY FIBRE OPTIC INPUT BEAMSPLITTER35mm 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.
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-measurementsi. 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.
How can you help?We’ve developed what we think is a really cool instrument, a realstep forward in spectrometer design. While it was developed foratmospheric measurements on Mars, we’d like to find out how itcould 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…