A chemical image contains a lot of molecular information in each of its pixels. In the case of infrared (IR) imaging this means an entire IR spectrum. As a result, the image created visualizes the chemical properties of the investigated sample, based on the infrared data.
This spectral data can then be used in multiple ways to answer certain analytical questions. The creation of false-color images to emphasize and characterize the properties of a sample is a standard application, for example. In short, An FT-IR image provides a clear representation of the sample's chemical composition.
Usually, an FT-IR microscope is used to obtain said images and there is no restriction on the IR technique used. You can find IR images obtained in ATR, reflection and transmission.
The easiest way to generate an FTIR image is to perform single IR measurements with defined distances on a sample. By combining the infrared with spatial data, even rudimentary problems such as questions about the homogeneity of a coating can be answered. This is called single point mapping.
However, to create chemical FT-IR images more effectively, special infrared detectors are required. Basically there are two approaches: Line array or focal plane array detectors.
While line array detectors are rather a cheap hybrid solutions, FPA detectors are state of the art. They take high-resolution images of defined pixel format, e.g. 64 x 64 pixels, in a single shot. Thus, such a one-shot-image would consist of more than 4000 IR spectra !
This allows to achieve spatial resolutions in IR microscopy up to the physical diffraction limit of infrared light!.
For line array measurements, single-element detectors are serially arranged
(e.g. 1 x 8) and simultaneously report a line of spectra (linear scans). These spectral lines are then "stitched" after the recording to obtain a chemical image. Although a line array might provide faster results than single-point measurements, there are major trade-offs in spectral quality and data handling. In addition, ATR imaging is unreliable at best and only feasible with impractical accessories.
FPA detectors, on the other hand are made of a 2D array of IR detectors
(e.g. 32x32, 64x64, 128x128, etc.). This way, they collect a true chemical image of the sample with each measurement at once without stitching. Ultimately, a focal plane array detector has none of the above limitations. The data is recorded in perfect alignment with the visual image, regardless of the sample structure and at superior speed.
The picture below shows the working principle of single element vs. line array vs. focal plane array detectors. As you can see, single element and line array approaches are consecutive methods that step by step collect the imaging data.
FPA technology naturally exceeds the speed and spatial resolution of line array and single-point measurements. The applicability is unlimited, the spectral data obtained is always of the highest quality and the measurement times are as short as technically possible.
1. What is chemical imaging?
Chemical imaging is a method for spatially resolving the chemical properties of a sample in 2D or 3D images. With this technique it is possible to obtain information about the material properties, the structure and the origin of the examined samples.
2. What is FT-IR imaging?
FT-IR imaging is one way to create said spatially resolved chemical images. Each pixel of these images consists of a whole IR spectrum. By interpreting the individual spectra, interesting sample regions can be detected and evaluated.
3. How do you create FT-IR images?
Common methods are sequential single point or line array measurements, as well as the direct acquisition of 2D images by a focal-plane array (FPA) detector. While FPA detectors offer the superior solution, highly automated single-point measurements are an economical alternative.
4. How does an FPA detector work?
The principle of an FPA detector is analogous to that of a digital camera. Instead of visible light, however, a defined array of pixels is illuminated by infrared light, with each detector pixel recording an independent, spatially resolved IR spectrum.
5. Do FPA detectors require apertures?
No, an FPA detector does not require any apertures. Each pixel of the detector functions as an aperture and thus records a spatially IR information directly. This allows much faster and higher resolution measurements compared other detector techniques.
6. Is it possible to adjust the spatial resolution of an FPA?
The spatial resolution of an FPA detector depends on the size of the individual detector pixels. However, adjacent pixels can be combined to form a "larger pixel" and thus the spatial resolution is reduced, also improving spectral quality.
7. Are there different FPA sizes?
FPA detectors are available in different array sizes. Size should be selected according to the optical system (microscope). For example, the LUMOS II is optimized for a 32x32 pixel array, while the HYPERION 3000 is designed for a 64x64 or 128x128 pixel arrays. With the latter it is possible to record an impressive number of more than 16,000 spatially resolved spectra in one scan.
8. Is a larger FPA better?
No, because the size of the FPA detector depends exclusively on the optimal illumination provided by the microscope. A homogeneous illumination of the detector array is important to ensure a consistently high spectral sensitivity both in the center and at the edges of the detector.
9. When does a larger FPA have advantages?
The larger the FPA detector area, the more spectra are recorded simultaneously. Since the spatial resolution is independent of the array size, this means that a 128x128 FPA detector covers an area 16 times larger than a 32x32 detector array in a single measurement.
10. Can FPA be combined with any measurement technique?
Yes they can. FPA detectors offer advantages in transmission, reflection and attenuated total reflection (ATR). Especially when used with ATR technology, this type of detector achieves an exceptionally high spatial resolution.
11. Why is the resolution of FPA measurements in ATR increased?
The combination of a high refractive solid-state lens (germanium ATR crystal) and an "aperture-free" FPA detector increases spatial resolution by a factor of 4 compared to transmission measurements. This effect is also called an immersion lens.
12. Are FPA measurements applicable to all samples?
Since FPA measurements can be combined with all measurement techniques, in principle all types of samples can be analyzed this way. Gases, liquids and other volatile substances cannot be analyzed microscopically due to their kinetic properties.
13. What are typical applications of an FPA?
Typical applications can be found in all areas of industry and research. Starting with the analysis of microplastics, particles and contaminations over the characterization of complex chemical structures, such as biological tissue, pharmaceutical products up to multilayer laminates and lacquers. In short, this detector technology is used wherever very high spatial resolution and the analysis of large sample areas are indispensable.