Shedding light on vision systems

Before setting up a vision system, understand the basic parameters involved. The fundamental parameters of imaging systems include the resolution of the object, the field of view, and the depth of field. The working distance from the object to the lens is also important, as is sensor size. Primary magnification is obtained by dividing the field of view by sensor size. A dual-magnification imaging system with in-line laser illumination is used in wire-bonding inspection. A single lens routes light through two cameras to generate the high-magnification image on the top screen and the lowmagnification alignment image on the bottom. Image contrast depends on object spacing. The two dots at the top blur slightly when seen through a lens. If the dots are moved closer together, their blurs overlap and contrast decreases. The spacing where this happens is the limiting resolution. The modulation transfer function (MTF) is one of the best tools for evaluating the imaging quality of a system. The MTF of a lens measures its ability to maintain contrast at a particular resolution level. All too often, machine-vision systems are built that either fail