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Zakres odczytu
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Among the many characteristics of any bar code scanner, perhaps one of the most commonly documented specifications is depth-of-field (DOF). Referring to the dimension over which a scanner can successfully read a bar code, DOF can largely determine whether or not a scanner is appropriate for a given application.
A scanner is comprised of optics, mechanics, and electronics which work together to determine the operative scanning volume. However, some of the most notable limitations of a scanner are imposed by the optical subsystem, consisting of the outgoing path of light and the light collection mechanism.
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The Visible Laser Diode, simply known as the VLD, is at the heart of the outgoing path (see "Technically Speaking," The VAR Side, April, 1995). In order to "see" a bar code, the highly divergent laser light emitted by the diode must be focused to a very specific spot size and remain within those limits throughtout the system's working depth. This laser beam diameter is a major determinant of the minimum element width of a bar code that a scanner can successfully read. Simply stated, the more narrow the bar code, the more narrow the laser beam needs to be. Ideally, the perfect beam would be of constant size and shape over an infinite distance, but this is never completely the case. The profile of a real beam constructed from a VLD (fig. 1) converges to a focus at a predetermined point in space (d), then diverges. This occurrence, in part, gives rise to the inherent nature of "sweet spots" within the DOF, and also explains why larger bar codes can typically be read over a greater distance.
As the focused laser beam is swept across a bar code, it is either reflected back toward the scanner by the light-colored spaces, or is absorbed by the dark bars. The diffusely reflected light from the spaces is then collected and focused onto a photocell. From here, the encoded reflections are transferred into an electrical signal (see "Technically Speaking," The VAR Side. May, 1995). Collection and focus are generally achieved through lenses, mirrors, or a combination of the two. The focal length, defined as the distance from a lens which will focus a bundle of parallel light rays to a point, is chosen for each scanner through calculation and careful experimentation.
Determining the focal length for the optical system is complicated by its interaction with the mechanics and electronics and further by the need to operate over an extended range
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The prescribed focal point for a lens is constant only for light rays which propagate parallel to one another. As shown in Fig. 2, an angle is subtended between the bar code and the collection lens, dependent upon the distance between the two, which alters the point at which the image is focused. This ultimately changes the image size intercepted by the detector. Additionally, the design must compensate for the Inverse Square Law. This law of physics states that the illuminance of a point source (the reflected light in this case) varies with the square of the distance between the source and a receiver (Fig. 3). With these factors accounted for, the appropriate collecting mechanism can be chosen to operate over the same defined distance as does the focused laser beam.
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So remember, as you peruse your next specifications sheet, the depth of field is not just some arbitrary distance. Rather, it is a predetermined, well-defined boundary within which the system has been designed to operate consistently on a wide variety of bar code sizes, contrasts, and styles.
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