FT-IR Imaging and Microscopy

Chemical imaging in general is an extremely effective tool to perform a very detailed and spatially resolved chemical analysis based on a sample’s spectroscopic properties. The spectroscopic properties of the sample are then presented as a so-called chemical image.

Each pixel of this chemical image is composed of an entire FT-IR spectrum. By interpretation of this spectral data, a false color image can be rendered to emphasize and characterize the sample’s properties like chemical structure or composition.

There are several ways to create said images. Single-point and line array measurements may allow chemical images to be generated, but Focal Plane Array (FPA) technology easily surpasses both in spectral brilliance and performance.

FPA measurement of a white inclusion in a polymer film (right). Each red square represents one FPA scan and contains 1024 FTIR spectra.

What makes FPA imaging superior?

Basically, you have the possibility to carry out consecutive single point measurements with a small distance between the measuring points. This creates a chemical map of your sample and is sufficient in many use cases. However, the analysis of larger sample areas takes a lot of time with this method. Fortunately, the fully automated LUMOS II minimizes the necessary effort. 

In comparison to FPA and single element measurements, line array detectors are more of a hybrid solution. In this type of measurement, single-element detectors are serially arranged (e.g. 1 x 8) and simultaneously report a line of spectra (linear scans). These spectral lines are "stitched" after the recording in order to gradually obtain a chemical image. FPA detectors, on the other hand, produce a true chemical image of the sample with each measurement. Afterwards, these FPA images can be merged in order to image very large sample areas.

This figure clarifies the principle of the different imaging techniques. On the left you can see the procedure with single point detectors, in the center that of a line array detector and on the right the true chemical image of an FPA detector.

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. This means that in these cases, chemical and visual images can never be properly aligned.

This often leads to the loss of critical sample information. Even worse, depending on their properties and structure, some samples are not even suitable for the above-mentioned scanning methods.

Ultimately, a focal plane array detector has none of the above limitations. It generates chemical images by recording large amounts of data with a single measurement (1024 spectra with LUMOS II and 4096/16384 with HYPERION). The data is recorded in perfect alignment with the visual image, regardless of the sample structure and at amazing speed.



Advantages of FPA imaging:

  • Highest imaging performance: Simultaneously acquire 1024 spectra in every measurement mode with impressive spatial resolution.
  • Unmatched resolving power compared to single-point or line-array measurements.
  • Analyze very large sample areas thanks to the combination of FPA imaging and high degree of automation.
  • FPA imaging produces chemical images in highest definition in shortest time.
  • Add up to two additional detectors to maintain analytical versatility and chose from a broad selection of available detectors.

It takes a deep understanding of FT-IR spectroscopy and imaging technology to provide the best analytical equipment. Easy software, smart hardware and clever automation give you the edge in FT-IR chemical imaging.

The conclusion:

No wonder 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.