Infrared (IR) microscopy, also known as infrared microspectroscopy, is a type of light microscopy that uses a source that transmits infrared wavelengths of light to view an image of the sample.
Unlike other optical microscopes with absorbent glass optics, an infrared microscope has reflective optics to allow the microscope to cover the entire spectral range of infrared light.
An infrared microscope usually comprises of a Fourier Transform Infrared (FTIR) spectrometer, an infrared detector, and an optical microscope.
The FTIR spectrometer enables the microscope to use infrared spectroscopy to make a chemical analysis of the sample. The infrared detector may detect infrared light at a single point, a linear array or a focal plane array to view different sections of the sample.
Some infrared microscopes use a focal-plane array (FPA) method of detection, in conjunction with step-scan interferometry to create the image of the sample. With this technique, the contrast of the image depends on the response of the sample to the specific wavelengths of infrared light selected at the time of analysis. This allows both spatial and spectral information about the sample structure to be collected.
Infrared microscopy is a versatile analytical technique with many practical uses. For example, it is commonly used in gemological testing.
- Diamond typing: identification of irradiated and HPHT-annealed diamond
- Polymer treatment detection: recognition of treatment in emeralds and jadeites
- Origin detection: recognition of natural or synthetic origin or alexandrite and emerald
It is also commonly used for chemical imaging. In this setting, the response of the sample to the infrared light determines the contrast of the image. The operator selects different wavelengths that correlate to specific IR absorption bands with certain molecular resonances to enhance the image contrast.
Progressions and limitations
Infrared microscopy already has many different applications, particularly in routine testing of gems and scientific research. In recent years, infrared microscopy has advanced to enable rapid measurements of large sample areas, even those with a high lateral resolution.
In the future, the capabilities of the infrared microscope could be combined with other analytical techniques, such as chemical, Raman, and UV-visible analyses. This could help to further enhance our understanding of the microscopic world around us by uncovering details about characteristics, history, color, and origin of samples.
The spatial resolution of infrared microscopy is limited by the diffraction of the wavelength of the IR light. For example, most IR microscopes in practical use have a spatial limitation of 1-3 times the wavelength of the light source, although this can vary based on the technique and microscope in use.
Reviewed by Susha Cheriyedath, MSc