Know how to select the right camera—tips for engineers
Steve Scheiber, Contributing Technical Editor -- Test & Measurement World, 9/25/2007 8:39:00 AM
A series of “tech tips” on Edmund Optics’ Website illuminates the nuances of camera features and capabilities to helps engineers select the best camera for particular applications. You’ll find three such tips on the page linked here.
The first tip explores the tradeoffs between the camera’s resolution and contrast specifications. According to the author, the two camera characteristics are often defined separately, but in selecting a camera they are intimately linked. Resolution is defined as the level of detail at which a camera can present a target object. Contrast reflects the range of white, black, and shades of gray in the camera’s image. Spatial resolution defines a camera’s resolution at a fixed level of contrast.
The article then moves from the theoretical to the practical, discussing the limitations of real lenses by exploring the concept of diffraction limits. This unavoidable artifact of lens optics prevents any camera from depicting, for example, a square wave with complete accuracy. The diffraction limit gets worse as the camera increases the f-stop setting (reducing the size of the lens aperture). In actual use for inspection, however, many industrial lenses operate wide-open and are not constrained by the diffraction limit. In those cases, optical errors and lens manufacturing tolerances become more critical.
The remaining two tips on the page consider a camera’s depth-of-field and the distortion inherent in any image acquired through a real lens.
Depth of field specifies how well an image remains in focus as it is placed closer to or farther from the lens than the optimum focus distance. The need for good depth of field depends a great deal on the nature of the inspection task. Refocusing a camera during an inspection represents an unacceptable time-waster. Depth of field increases as the aperture gets smaller, so each situation requires setting the f-stop to balance depth of field and diffraction limit requirements.
Distortion measures the amount that a point on an acquired image appears displaced from its actual location on the object under inspection. Because no information is actually lost, digital reconstruction can minimize its effects.
Engineers must carefully evaluate each of these camera/lens characteristics to choose the best compromise for the target application.
www.edmundoptics.com/TechSupport/DisplayArticle.cfm?articleid=288#2.1
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