Mirrors in the paths reflect the light back to the beam splitter, where the two beams recombine, and this modulated beam passes through the sample and hits the detector. In a typical interferometer, one mirror remains fixed, and the other retreats from the beam splitter at a constant speed.
As the mirror moves, the beams go in and out of phase with each other, which generates a repeating interference pattern—a plot of intensity versus optical path difference—called an interferogram.
The interferogram can be converted into the frequency domain via a Fourier transform, which yields the familiar single beam spectrum. The resolution of this spectrum is determined by the distance that the moving mirror traveled. Analyses generally fall into three categories, which are determined by the wavelengths of the radiation.
Midrange IR covers the wavenumbers nm— nm, where strong absorptions from fundamental molecular vibrations are measured. Far IR ranges from nm—1 mm. Infrared radiation is a relatively low energy light. All physical objects give of infrared radiation, the wavelength of which is dependent upon the temperature of the object. This phenomenon is known as black body radiation.
The ideal IR source would emit radiation across the entire IR spectrum. As this is very difficult, a good compromise is a source which emits continuous mid-infrared radiation. Thankfully this can be achieved by most high temperature black bodies. Black body radiation was studied in depth by Max Planck, and it is through his equations that that the spectral energy density at a given wave number from a blackbody source of a given temperature can be calculated.
Not to mention he was the discoverer of the properties of energy quanta. For this Max Planck received the Nobel Prize in Physics in recognition of the services he rendered to the advancement of Science.
Now take a moment to examine the plot of energy density vs. At first glance it would seem that the source temperature should be as high as possible to maximize the results—this is rarely the case. For example consider a typical incandescent light bulb. The tungsten filament glows at a temperature of k, which would emit massive amounts of IR. The bulb portion of a light bulb is responsible for their lack of use as an IR source. The Bulb is made of glass which seals the tungsten filament in a vacuum.
The vacuum is necessary to keep the tungsten from oxidizing at such high temperature, but the glass serves as an IR absorber, blocking its path to the sample.
Any source we choose must be in direct contact with the atmosphere, because of this there are drastic limits on the temperature that we may operate an IR source.
There are several other limiting facts that require consideration when choosing an IR source. The material should be thermodynamically stable; otherwise it would quickly break down and need replacing. This would obviously be an expensive and undesired approach. There is also the possibility that the source may produce an excess of IR radiation.
This would saturate the detector and possibly over load the analog-to-digital converter. This device is commonly and somewhat simply referred to as a Globar. An electric current is passed through the bar which become very hot, prducing large amounts of IR raidiation. A Globar can reach temperatures of K, and in the past required water cooling to keep from damaging the electrical components.
Advances in ceramic metal alloys have lead to the production of Globars that no longer require water cooling. However these newer Globars are typically not operated at as high a temperature as K. Nichrome and Kanthanl wire coils where also once popular IR sources. They too did not require water cooling, ran at lower temperatures than a Globar, and possessed lower emissivity.
Nernst Glowers are an IR source that is capable of hotter temperatures than a Globar. Nernst Glowers are fabricated from a mixture of refractory oxides. Despite being capable of higher temperature than a globar, the Nernst Glower is not capable of producing IR radiation above cm Materials can be quantified using the FTIR materials characterization technique as long as a standard curve of known concentrations of the component of interest can be created.
FTIR Analysis can be used to identify unknown materials, additives within polymers, surface contamination on a material, and more. A simple device called an interferometer is used to identify samples by producing an optical signal with all the IR frequencies encoded into it.
The signal can be measured quickly. Then, the signal is decoded by applying a mathematical technique known as Fourier transformation. This computer-generated process then produces a mapping of the spectral information.
The resulting graph is the spectrum which is then searched against reference libraries for identification. Figure 11 shows a graph of the diameter of the focal spot vs. This was found to be 0. The graph shows that the diameter of the focal spot which corresponds to this value is about 0.
The rough estimate, of the same value made with the formulae on the preceding pages, gives a value of 0. With increasing divergence of the beam, the diameter of the focal spot also increases, as we see, but it has some limit between 1.
The reason for this is that the interferometer itself is blocking high angle rays and they cannot reach the parabola. The maximum value of angle of rays that can get through the interferometer is 0.
This is exactly the region of the curve in Figure 11 that starts to flatten out. Figure 12 shows the energy distribution in the focal plane of the off axis reflector for beams of different divergence angles. Despite universality and wide usage of off-axis parabolic mirrors in FT-IR spectroscopy, they have certain disadvantages. In many applications, especially in the Near IR, lenses could be a good choice. When using Infrared Plano-Convex Lenses , we need to consider the lens material.
We recommend the use of CaF 2 lenses in the whole range where the CaF 2 beam splitter is applicable, 14, — 1, cm-1 or 0. They are somewhat cheaper than CaF 2 lenses. A wide variety of materials are available for the Mid IR. There is usually a choice among performance, expense, durability, birefringence, etc. The hygroscopic nature of some materials can be a major problem. NaCl windows and KBr are two such materials that are commonly used. Some materials are transparent in the visible while others are not; this can be a positive if trying to align in the visible range, or a negative when the material should act as a filter.
A popular, rugged, and transparent material which is used for manufacturing lenses is ZnSe. Anti-reflection coatings can help, but at further expense, and at a reduction of the spectral range. A second issue is dispersion of the lens material.
Lenses are definitely good for limited wavelength range applications. For example, the sensitivity range of an InGaAs detector is from to nm. Using a lens should not pose a major problem, though we do see some dispersion in our labs with fused silica lenses over this range; i. For a wider wavelength range we should position the detector at the shortest focal length position, in other words, in the position of minimum spot size for the shortest wavelength, since this is usually where the system efficiency is the lowest.
These examples show us that the auxiliary optics for an interferometer must be carefully chosen and arranged. Poor choices of components will lead to a reduction of resolution or unnecessary system throughput limitations. Industrial Motion. Custom Motion Solutions.
Vacuum Compatible Motorized Positioners. Optical Posts 0. Lab Supplies. Custom Component Solutions. Diffraction Gratings. Fiber Optics. Optic Accessories. Optic Sets. Custom Optics Solutions. Solar Cell Test Systems. New Optical Sensor Finder. Custom Vibration Isolation Solutions. Baratron Capacitance Manometers. Granville Phillips Vacuum Gauges. Mass Flow Controllers.
Process Automation. Mass Spectrometers. MKS Instruments. Compare All. Figure 1: A Schematic of a generic Michelson interferometer.
Figure 2: Schematic representation of waves and their phases. Figure 3: Two wavelength source case. Figure 4: Broadband source interferogram. The Fourier Transform Algorithm. Table 2 Resolution Values in Wavenumbers and Nanometers. The Relationship Between Resolution and Divergence. Figure 6: Scanning Michelson Interferometer. Figure 7: Detector and optical system.
Detector Optics. Figure 8: Wavelength vs. Source Optics. Figure 9: Light from a point source placed at the focus of a parabola. Figure Section of off-axis parabolic mirror.
Figure Energy distribution in the focal plane of an off-axis reflector. Figure Energy distribution in the focal plane of a CaF 2 lens. Sign In. Email Address: Required. Password: Required. Password Reset. Enter your email address below to reset your account password. Please Note: Your reset password applies to newport. Email Verification Required. Cart Merge. Remove Product. Remove this product from your comparison list? Check Order Status.
0コメント