IR vision: more than meets the eye

Machine-vision experts say infrared can spot defects better than ordinary cameras or the naked eye. Infrared detectors complement the visible-light spectrum of machine-vision equipment with images of heat sources located on or just below (about 0.001 in.) the surface of an object. Although the systems don't measure temperature directly, IR sensors primarily detect surface emissions such as absorbed energy that is reemitted. This lets IR systems discriminate features from these surface emissions based on material properties or subsurface structures. One advantage of using the IR spectrum is that systems may operate without traditional light sources as do conventional machine-vision cameras. Moreover, IR imaging systems detect heat from both faulty and good parts on a powered-up, functioning unit. For example, an IC package sitting just below the surface of a keyboard emits a certain temperature signature when operating properly, and a different one when the IC is faulty. The resulting change in keyboard surface temperature is easily detectable with an IR system. In other applications, unique thermal patterns may indicate cracks in an object as thermal resistance to heat flow through or around the faults. Simpler IR systems may replace complex machine-vision systems in some cases. For example, some visible-light systems detect surface-finish blemishes on reflective materials. But stray light and light reflections may wash out images or lower contrast. This can make it difficult for the algorithms to detect the defects. However, such defects produce a specific emission signature that an IR system can pick up without external illumination. The resulting amount of washout is minimal, which increases the signal-to-noise ratio, and the defect can be easily and quickly detected. IR may be added to an existing system and share some or most of the major components, such as software and some mechanical components. In most cases, however, cameras and lenses