Infrared radiation imaging

Imaging > Imaging Techniques > Infrared radiation imaging

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Infrared Imaging

Infrared Imaging Terminology
Wavelength Range	Recording Medium	Techniques	Image Produced
700 - 1000 nm -NIR (Near Infrared) -IR-A (CIE1 700-1400 nm)	Film	-Reflected infrared photography -Transmitted infrared photography -False-color infrared photography	-Reflected infrared photograph -Transmitted infrared photograph -False-color infrared photograph
	Digital capture/Electronic systems	-Reflected infrared digital photography -Transmitted infrared digital photography -False-color infrared digital photography	-Reflected infrared digital photograph -Transmitted infrared digital photograph -False-color infrared digital photograph
1000 - 3000 nm -SWIR (Short Wave Infrared) -IR-B (CIE 1400-3000 nm)	Electronic systems	-Infrared Reflectography (IRR)2 -Infrared Transmittography3	-Infrared reflectogram -Infrared transmittogram
3000 - 20000 nm -MWIR (Mid Wave Infrared, 3000-5000 nm) -LWIR (Long Wave Infrared, 7000-14,000 nm) -VLWIR (Very Long Wave Infrared, 12,000-30,000 nm) -IR-C (CIE 3000-1,000,000 nm)	Electronic systems	-Thermal imaging -Thermography	-Thermal image -Thermogram

This table is adapted from Dan Kushel’s Infrared Imaging Terminology Table (2015 version, SUNY Buffalo State course materials).

History of IR imaging

Since the discovery of infrared radiation by Frederick William Herschel in 1800, numerous forms of infrared detection have been developed. These optical infrared detectors fall into two categories: (1) photon detectors, and (2) thermal detectors.  Over the years, advancements in infrared detector technology have been primarily driven by the military, with the seeds of modern infrared detection being developed during World War II.  

The conservation field began using infrared detection technology to image artistic works in the 1930s, when infrared film first became commercially available.  Conservators have used a series of different IR detection devices throughout the 20th and 21th centuries, including infrared film, infrared vidicon television monitor systems, and modified DSLR cameras.  Infrared light and visible light interact with objects differently, absorbing and scattering wavelengths in different ways, which allow for certain aspects of artistic works to be imaged during infrared imaging that are not available to the naked eye. Conservators capitalize on this, and use IR imaging for many purposes, including: to expose underdrawings, to detect changes made during the construction or after the construction of an artistic work, to clarify obscured inscriptions, and to aid in the identification of materials.

Sir William Herschel

Frederick William Herschel discovered infrared radiation on February 11th,1800, while conducting an experiment designed to measure the temperature of different colors of light.  In this experiment, Herschel used a prism to divide sunlight into its constituent colors, and then measured the distribution of energy in each color using a thermometer. The highest temperature that he measured in this experiment was just beyond the range of red visible light, which led him to infer that there must be a form of radiation beyond the red visible spectrum, which is now known as infrared radiation.

Film

In 1873, H. W. Vogel discovered that adding certain dyes to silver halide emulsions extended the spectral imaging capacity of photography, which opened the door to infrared photography. Professor Robert Williams Wood published the first infrared photographs in the Photographic Journal of the Royal Photographic Society in October of 1910.  However, the process of sensitizing photographic plates to red light was difficult, so widespread use of infrared photography did not take off until the 1930s, when infrared film became commercially available.  Conservators began using infrared film to image artistic works in this time period, but faced many challenges, including the need for specialized equipment and techniques.  Furthermore, there was no fast or easy way to tell whether a given artistic work was worth imaging using IR, which made this a laborious, time-intensive, and sometimes fruitless process.  

Vidicon

In the mid-1960s, vidicon video tubes were developed, which allowed infrared images to be viewed on TV monitors.  This technology was introduced to the conservation field by J.R.J. Van Asperen De Boer, who proposed using infrared vidicon television systems to perform infrared reflectography of paintings.  This technology made the reflectogram immediately visible on a television monitor, which could then be photographed for posterity.  The system’s video recorder could also store the imaged content.  Vidicon systems allowed most paintings to be examined in situ. Vidicon systems allowed imaging areas that ranged in size, were effective at revealing underdrawings that were obscured by certain pigments, and had improved IR sensitivity.

Digital Imaging

Solid state sensors were introduced to the conservation field in the 1990s.  These capture devices often have improved IR sensitivity, but have poor spatial resolution, which require mosaicking multiple images in order to get a comprehensive result.  Following the development of digital photography, physical IR film that was used in conservation contexts was eventually discontinued. Now, many conservators use modified DSLR cameras to perform IR imaging.  These DSLR cameras have inherent sensitivity limited to the near infrared spectrum, so are a useful and high resolution option for IR imaging.

Glossary

The following definitions are from Section 6.4.3 of the "AIC Guide to Digital Photography and Conservation Documentation" (Warda et al. 2017, pp. 131-132):

• Reflected infrared digital photography (or a reflected infrared digital photograph): Imaging or images made in the 700-1000 nm range and thus are within the abilities of digital cameras
• Infrared reflectography (or an infrared reflectogram): Imaging or images made in the 1000-2500 nm range and thus are within the ability of typical electronic infrared imagers
• Transmitted infrared digital photography (or an infrared digital photograph): Imaging or images made in the 700-1000 nm range and thus are within the abilities of digital cameras

References and resources

More references and resources to come...

• Dyer, J., Verri, G., Cupitt, J., 2013. Multispectral Imaging in Reflectance and Photo-induced Luminescence modes: A User Manual.

• Warda, Jeffrey, Franziska Frey, Dawn Heller, Dan Kushel, Timothy Vitale, and Gawain Weaver. 2017. “Infrared Photography.” In The AIC Guide to Digital Photography and Conservation Documentation, edited by J. Warda, 3rd ed, 130–47. American Institute for Conservation.
• International Commission on Illumination (Commission Internationale de l’Eclairage)
• “Infrared Reflectograms of Paintings,” Studies in Conservation, volume 11, No. 1, February 1966, p 45. Infrared Reflectography: a Method for the Examination of Paintings, Applied Optics, vol. 7, No. 9, September 1968, pp. 1711-1714.
• “Applications of Transmitted Infrared Radiation to the Examination of Artifacts,” Studies in Conservation, Volume 30, No. 1, February 1985, pp. 1-10.