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SWIR microscopy is becoming an important lab tool for real-time inspection using a standard optical inspection microscopy system
* Microscopy
* SWIR cameras
* Semiconductors
* Optics
Optical microscopy has long been a major tool for the scientist and visible microscopy has been the standard wavelength region used by the industry. Advances in imaging techniques and detectors now extend microscopy beyond the reaches of visible wavelengths, which has embraced the age of modern electronic imagers and cameras. New applications using the basic refractive lens principles have grown considerably, most notably with ultraviolet (UV) microscopes for biologists, and near-infrared (NIR) microscopes for materials scientists. This article will examine the use of microscope cameras sensitive in the shortwave infrared (SWIR) range.
THE CAMERA AND MICROSCOPE RELATION
Most modern optical microscopes use refractive glass to examine the specimen at magnifications of up to several hundred fold and suitable to visible wavelengths from nominally 400 to 700 nm. Silicon cameras match well to this range, having natural sensitivity through the visible up to -1100 nm. A shortwave infrared camera operates from -900 nm to 1700 nm (0.9 to 1.7 µm), thereby taking over approximately where silicon gives up. An advantage of SWIR wavelengths is that many materials have unique optical properties in the range, and that optical glass remains transmissive in this region (as long as there are no specific IR-blocking coatings present). Because SWIR cameras often operate with standard glass optics (that is, they often do not require exotic optical materials or optimized optics) they can, conveniently, be used with the many existing lab and quality control microscopes in the field (specialty SWIR lenses and objectives are available and can offer better contrast and throughput to assist in more difficult investigations). Beyond this range, midwave-IR (MWIR) and longwave-IR (LWIR) cameras require the use of reflective specialty optics to pass the wavelengths, and their use becomes a larger capital goods acquisition.
The modern SWIR camera is generally available with a focal plane of nominally 320 x 240 or 640 x 512 pixels and is available with a C-mount optical interface, matching the interface of camera-compatible microscopes. The pixel sizes range from 25 µm to 40 µm square, translating to an array of roughly a 10mm-to-21mm diagonal (the...