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7. Special Lenses (continued)
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7.3 F-Theta ("Scan") Lens
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Fig. 7.3 shows the principle of the ubiquitous laser scanner (whith laser modulator, light receiving optics, and detector omitted). Only few "point of sales scanner" do not employ this principle.

fig.7.3: Principle of laser scanner bundle (8kByte)


The "Laser" is nearly a point source for monochromatic light; the "Collimator" is a kind of telescope which makes the ray bundle wider and less divergent; the "rotating mirror" makes the bundle perform the scanning movement; and the "Scan Lens" focusses this bundle into an extremely small spot that moves across the scanned field.

Normal task of a lens would be to focus the bundle into the image plane, delivering minimum spot size not only in the field center but all through the allowable field angle domain.

But a "Scan Lens" can do more:
Rotating at constant angular velocity, the rotating mirror would make the focus spot move across the scanned field with some very odd speed:
The speed would be minimum in the field center and increase with the tangent function of the deflection angle.
It is now the scan lens that transforms this tangent function into a linear relation between scan angle and spot location.

This way, speed and pointing accuracy become (nearly) independent of deflection angle.




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Last modified Dec. 2nd, 2004 00:50