Refurbished, Infrared Spectrophotometer

Perkin Elmer 1600	Perkin Elmer 1310
Perkin Elmer Paragon 500	Perkin Elmer RX1

Infrared

Infrared (IR) radiation is electromagnetic radiation of a wavelength longer than that of visible light, but shorter than that of radio waves. The name means "below red" (from the Latin infra, "below"), red being the color of visible light of longest wavelength. Infrared radiation spans three orders of magnitude and has wavelengths between approximately 750 nm and 1 mm.[1]

The infrared portion of the spectrum has a number of technological uses, including target acquisition and tracking by the military; remote temperature sensing; short-ranged wireless communication; spectroscopy, and weather forecasting. Telescopes equipped with infrared sensors are used in infrared astronomy to penetrate dusty regions of space, such as molecular clouds; detect low temperature objects such as planets orbiting distant stars, and to view highly red-shifted objects from the early history of the universe.

At the atomic level, infrared energy elicits vibrational modes in a molecule through a change in the dipole moment, making it a useful frequency range for study of these energy states. Infrared spectroscopy is the examination of absorption and transmission of photons in the infrared energy range, based on their frequency and intensity.

Different regions in the infrared

The infrared band is often subdivided into smaller sections but the divisions are not precise, and are used differently by different authors. One such scheme is:

near infrared (NIR, IR-A DIN)

0.75–1.4 µm in wavelength, defined by the water absorption, and commonly used in fiber optic telecommunication because of low attenuation losses in the SiO2 glass (silica) medium.

short wavelength IR (SWIR, IR-B DIN)

1.4–3 µm, water absorption increases significantly at 1450 nm. The 1530 to 1560 nm range is the dominant spectral region for long-distance telecommunications.

mid wavelength IR (MWIR, IR-C DIN) also intermediate-IR (IIR)

3–8 µm

long wavelength IR (LWIR, IR-C DIN)

8–15 µm

far infrared (FIR)

15–1,000 µm (see also far infrared laser)

Another common scheme is:

· near: 0.75–5 µm

· mid: 5–30 µm

· long: 30–1,000 µm

A third scheme divides up the band based on the response of various detectors[4]:

Near IR (NIR)

From 0.7 to 1.0 micrometers (from the approximate end of the response of the human eye to that of silicon)

Short-wave infrared (SWIR)

1.0 to 3 micrometers (from the cut off of silicon to that of the MWIR atmospheric window. InGaAs covers to about 1.8 micrometers; the less sensitive lead salts cover this region

Mid-wave infrared (MWIR)

3 to 5 micrometers (defined by the atmospheric window and covered by InSb and HgCdTe and partially PbSe)

Long-wave infrared (LWIR)

8 to 12, or 7 to 14 micrometers: the atmospheric window (Covered by HgCdTe and microbolometers)

Very-long wave infrared (VLWIR)

12 to about 30 micrometers, covered by doped silicon

Plot of atmospheric transmittance in part of the infrared region.

These divisions are justified by the different human response to this radiation: near infrared is the region closest in wavelength to the radiation detectable by the human eye, mid and far infrared are progressively further from the visible regime. Other definitions follow different physical mechanisms (emission peaks, vs. bands, water absorption) and the newest follow technical reasons (The common silicon detectors are sensitive to about 1,050 nm, while InGaAs sensitivity starts around 950 nm and ends between 1,700 and 2,600 nm, depending on the specific configuration). Unfortunately the international standards for these specifications are not currently available.

The boundary between visible and infrared light is not precisely defined. The human eye is markedly less sensitive to light above 700 nm wavelength, so longer frequencies make insignificant contributions to scenes illuminated by common light sources. But particularly intense light (e.g., from lasers, or from bright daylight with the visible light removed by coloured gels) can be detected up to approximately 780 nm, and will be perceived as red light. The onset of infrared is defined (according to different standards) at various values typically between 700 nm and 780 nm.

Telecommunication bands in the infrared

In optical communications, the part of the infrared spectrum that is used is divided into several bands based on availability of light sources, transmitting/absorbing materials (fibers) and detectors:

Band Descriptor Wavelength range

O band Original 1260–1360 nm

E band Extended 1360–1460 nm

S band Short wavelength 1460–1530 nm

C band Conventional 1530–1565 nm

L band Long wavelength 1565–1625 nm

U band Ultralong wavelength 1625–1675 nm

The C-band is the dominant band for long-distance telecommunication networks. The S and L bands are based on less well established technology, and are not as widely deployed.

"Heat"

Main article: Thermal radiation

Infrared radiation is popularly known as "heat" or sometimes "heat radiation," since many people attribute all radiant heating to infrared light, but this is a widespread misconception. Light and electromagnetic waves of any frequency will heat surfaces which absorb them. Infrared light from the sun only accounts for 50% of the heating of the Earth, the rest being caused by visible light.[citation needed] Green (or even ultraviolet) lasers can char paper and incandescently-hot objects will put out visible radiation. However, it is true that objects at room temperature will emit radiation mostly concentrated in the 8-12 micron band (see black body and Wien's displacement law). Unlike heat transmitted by thermal conduction or thermal convection, radiation can propagate through a vacuum. Heat is the energy in transient form and flows due to temperature difference.

Spectroscopy

Infrared radiation spectroscopy (see also near infrared spectroscopy) is the study of the composition of (usually) organic compounds, finding out a compound's structure and composition based on the percentage transmittance of IR radiation through a sample. Different frequencies are absorbed by different stretches and bends in the molecular bonds occurring inside the sample. Carbon dioxide, for example, has a strong absorption band at 4.2 µm.

FTIR

FTIR can refer to:

· Fourier Transform Infra Red spectroscopy

· Frustrated Total Internal Reflection

Fourier transform spectroscopy

Fourier transform spectroscopy is a measurement technique whereby spectra are collected based on measurements of the temporal coherence of a radiative source, using time-domain measurements of the electromagnetic radiation or other type of radiation. It can be applied to a variety of types of spectroscopy including optical spectroscopy, infrared spectroscopy (FTIR), nuclear magnetic resonance, and electron spin resonance spectroscopy. There are several methods for measuring the temporal coherence of the light, including the continuous wave Michelson or Fourier transform spectrometer and the pulsed Fourier transform spectrograph (which is more sensitive and has a much shorter sampling time than conventional spectroscopic techniques, but is only applicable in a laboratory environment).

The Michelson spectrograph relies on the same principle as the Michelson-Morley experiment. Light from the source is split into two beams by a half-silvered mirror, one is reflected off a fixed mirror and one off a moving mirror which introduces a time delay -- the Fourier transform spectrometer is just a Michelson interferometer with a movable mirror. The beams interfere, allowing the temporal coherence of the light to be measured at each different time delay setting. By making measurements of the signal at many discrete positions of the moving mirror, the spectrum can be reconstructed using a Fourier transform of the temporal coherence of the light. Michelson spectrographs are capable of very high spectral resolution observations of very bright sources. The Michelson or Fourier transform spectrograph was popular for infra-red applications at a time when infra-red astronomy only had single pixel detectors. Imaging Michelson spectrometers are a possibility, but in general have been supplanted by imaging Fabry-Perot instruments which are easier to construct.

Pulsed Fourier transform spectrometer

A pulsed Fourier transform spectrometer is usually used to measure the spectrum of the light transmitted through a laboratory sample. In a conventional (or "continuous wave") spectrometer, a sample is exposed to electromagnetic radiation and the response (usually the intensity of transmitted radiation) is monitored. The energy of the radiation is varied over the desired range and the response is plotted as a function of radiation energy (or frequency). At certain resonant frequencies characteristic of the specific sample, the radiation will be absorbed resulting in a series of peaks in the spectrum, which can then be used to identify the sample. (In magnetic spectroscopy, the magnetic field is often varied instead of the frequency of the incident radiation, though the spectra are effectively the same as if the field had been kept constant and the frequency varied. This is largely a question of experimental convenience.)

Instead of varying the energy of the electromagnetic radiation, Fourier Transform spectroscopy exposes the sample to a single pulse of radiation and measures the response. The resulting signal, called a free induction decay, is a direct measurement of the temporal coherence of the light and contains a rapidly decaying composite of all possible frequencies. Using a Fourier transform of this, the spectrum of the light can be calculated as for the Michelson Fourier transform spectrometer. In this way the Fourier transform spectrometer can produce the same kind of spectrum as a conventional spectrometer, but in a much shorter time. The principles of the Fourier transform approach can be compared to the behavior of a musical tuning fork. If a tuning fork is exposed to sound waves of varying frequencies, it will vibrate when the sound wave frequencies are in "tune". This is similar to conventional spectroscopic techniques, where the radiation frequency is varied and those frequencies where the sample is in "tune" with the radiation detected. However, if we strike the tuning fork (the equivalent of applying a pulse of radiation), the tuning fork will tend to vibrate at its characteristic frequencies. The resulting tone consists of a combination of all of the characteristic frequencies for that tuning fork. Similarly the response from a sample exposed to a pulse of radiation is a signal consisting primarily of the characteristic frequencies for that sample. The Fourier transform is a mathematical technique for determining these characteristic frequencies from a single composite signal.

Multi-bounce

A multi-bounce attenuated total reflectance (ATR) crystal makes it possible to increase the signal, facilitating surface analysis.

Fellgett Advantage

One of the most important advantages of FTS was shown by P.B. Fellgett, an early advocate of the method. The Fellgett advantage, also known as the multiplex principle, states that a multiplex spectrometer such as the FTS will produce a gain of the order of the square root of m in the signal-to-noise ratio of the resulting spectrum, when compared with an equivalent scanning monochromator, where m is the number of elements comprising the resulting spectrum.

Optical autocorrelation

It has been suggested that this article or section be merged with Autocorrelator. (Discuss)

In optics, various autocorrelation functions can be experimentally realized. The field autocorrelation may be used to calculate the spectrum of a source of light, while the intensity autocorrelation and the interferometric autocorrelation are commonly used to estimate the duration of ultrashort pulses produced by modelocked lasers. (To accurately and definitively determine the pulse length, one needs to perform a full-intensity-and-phase type of measurement, such as SPIDER or FROG.) The laser pulse duration cannot be easily measured by optoelectronic methods, since the response time of photodiodes and oscilloscopes are at best of the order of 200 femtoseconds.

In the following examples, the autocorrelation signal is generated by second-harmonic generation (SHG), the lowest order (and hence the strongest) of all nonlinear optical processes. Higher-order nonlinear optical processes such as third-harmonic generation can also be used in autocorrelation measurements, in which case the mathematical expressions of the signal will be slightly modified, but the basic interpretation of an autocorrelation trace remains the same.