Infrared sensing, cameras and photography

From Thermal-FluidsPedia

Some digital video sensors and cameras are constructed so that the charge-coupled detectors (CCDs) are sensitive at infrared wavelength ranges well outside the visible. Such equipment must have optical components that are highly transmitting in the infrared; most common glasses are opaque at these wavelengths. Cameras then detect signals that can be calibrated to be proportional to the temperature of locations on a viewed surface (if it is black; otherwise compensation must be made for reflected radiation from the surroundings), allowing scanning for hot spots in electronic equipment, heat leaks from structures to aid in energy conservation, and scanning of power lines to find current leakage. Military night-vision equipment is similar, but uses signal amplification to enhance the weak IR signals emitted by warm objects.

Conventional film cameras can make use of film that is sensitive to radiation in the near-IR region. Because the reflectivity of many materials, especially foliage, varies strongly between the visible and near-IR region, interesting artistic effects can be obtained using IR film. The two photographs shown in figure below of the same scene, one using conventional film and the other using IR film, illustrate how reflectivity can vary. The left-hand photo was taken with standard black-and-white film. The right-hand photo was taken with infrared-sensitive film that extends the film sensitivity to 0.82 μm and a #25 Wratten red filter that cuts out the otherwise transmitted visible light below 0.6 um. It appears that the vegetation clumps have a much higher reflectivity in the IR range than in the normal visible part of the spectrum. IR-sensitive film is often used to provide enhanced contrast, particularly in photos of trees and vegetation.

Comparison of photographs using conventional film (left) and the IR film (right)

References

Faghri, A., Zhang, Y., and Howell, J. R., 2010, Advanced Heat and Mass Transfer, Global Digital Press, Columbia, MO.

Further Reading

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