Laser Scanners - Some Major Scanner Types
Drum scanners have been used extensively as reprographic expose units. Optically they offer the simple advantage of on-axis imaging at a fixed distance between objective and film. Normally, such on-axis imaging is diffraction limited. Optical difficulties such as curved fields, speed changes and magnification variations, along the scan line are minimal.
Mechanical translation, combined with rotation of the imaging spot (expose optics) is needed to expose over the required format (see drum scanner schematic above) . The drum on which the film/media is placed may be internal as above, or external (which requires drum rotation). The uniform movement (or rotation) of a heavy housings by mechanical means is the main disadvantage of drum scanners. As shown above, the whole optics housing is rotated to produce the raster line. Alternatively by rotating the mirror only (shown in green), on an air bearing, much higher speeds can be achieved. Since any astigmatism in the beam is also rotated by the mirror, spot errors can vary along the raster line, which is a disadvantage.
The other disadvantages of drum scanners are:
- The film or media has to be curved to conform to the drum, which excludes the use of inflexible media.
- Complex media transport mechanisms are required.
- Precision engineering is needed to ensure that the axis of the drum and the translational axis of the optics are coincident and collinear (within limits).
As with all scanner types, the design, particularly in terms of depth of focus, must also cater for drum waviness and film thicknes variations.
In some systems, where the image spot is subdivided into smaller spots, telecentric imaging has been used to ensure system insensitivity to film plane deviations. Telecentric imaging is a widely applied technique, whereby the chief rays of the output beam(s) are maintained parallel to the optical axis, normal to the film plane. Thus film plane deviations have a reduced effect on spot magnification.
Flat Field Flat Bed Systems
The linear scan line (fast axis) in flatbed reprographic systems is generally created by opto-mechanical means (alternatives such as diode array and image-bar type systems are ignored here), however, as in drum scanners, the translation in the slow direction is always done mechanically.
The primary task of the scanning optics is to convert an angular deflection, into a linear velocity. The ideal characteristic takes the form:
dx = f * da
Where a constant angular change da, is transformed to a constant linear change dx.
In practice, lens characteristics take the form:
dx = f * Tan(da)
So that equal angular increments result in larger linear increments, at the edge of the scan. This results in positive "pincushion" distortion which can be optically compensated by introducing negative distortion in the lens design.
Without auxiliary optics, even a compensated lens does not produce telecentric imaging. This is because the principle rays vary in angle, relative to the film plane, at scan line. Magnification errors caused by variations in film plane position will thus vary across the scan line. Toroidal lenses can be used to improve the tolerance to such film plane errors.
Other pixel placement errors (jitter) occur whenever there is a synchronisation error between the X,Y image spot coordinates in space and the electronically stored pixel coordinates. Speed variations in the polygon and optical errors in the facets are primary causes of mismatch. Using start/end of scan sensors, certain jitter errors can be corrected along the scan line by adjustments to the data clock frequency. Typically, residual pixel placement errors of +/- 1% are acceptable.
The design of the imaging objective must take into account spot size and scan linearity requirements and other factors such as available space etc.
In pre-objective scanners, the scanning element (generally a polygon) scans the beam over an angle a, into the imaging objective, which images over a straight scan-line field L . Because the pre-objective lens must provide a well corrected image over the whole scan line, it requires a complex design. Note that the optical scan angle a, is twice the mechanical scan angle. When these objectives are used with collimated input beams, which is normally the case, spot size is limited only by the lens aperture and back focus distance. Again auxiliary optics are needed for telecentric imaging.
In post-objective scanners, the polygon is placed after the imaging objective. Because imaging is always on axis, the design of a post-objective lens is relatively simple. Satisfactory optical performance can be obtained with a four element lens. Since the mirror (polygon facet) pivot point is at a constant distance to the focused image spot, the swept field describes a circular arc. The highly convergent beams, which occur in high resolution systems, can make it difficult to use post-objective scanning. This is because the mirror folding of highly convergent becomes increasingly difficult and results in geometric or space problems.
Using a novel arrangement of hyperbolic and parabolic mirrors as auxiliary optics Wim Van Amstel, has shown it is possible to obtain a flat field and telecentric imaging over an extremely wide format.
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