Comparing Beam-Deflecting Systems

Light beam deflection has become an important mechanism in a number of useful products. Current applications include computer printers, point-of-sale scanners, IR imagers, optical inspection machines, laser displays, facsimile systems and bar-code readers, to name a few.

Scanner types
The most commonly used beam deflectors in this mix of commercial and industrial products are: polygonal mirror scanners, galvanometric scanners, holographic scanners and acousto-optic beam deflectors. Table 1 lists parameters that are key to the design of a scanning optical system and typical performance considerations for the four deflector types.

Rotating mirrors are probably the oldest and easiest to understand of these devices and they continue to dominate the market even in this high-tech era. Galvanometric (galvo) motors and conventional continuous-rotation motors are the most common mirror rotators. Galvos provide an oscillatory rotation over a range of from a few to several degrees and are usually employed in rotating a single mirror surface. Conventional motors are used to rotate polygonal mirrors continuously in one direction.

Holographic scanners most often take the form of a hologon or multifaceted holographic disk which, when rotated by a motor, provides deflection of an incident ray bundle to a scan lens. Although the operating principle is different than that of the rotating mirror, the hologon provides a generic equivalent of the rotating mirror.

Acousto-optic beam deflectors rely on Bragg scattering in an optically transparent crystal. A succession of acoustic waves are propagated through the crystal creating an index-variant grating. A light-ray bundle directed through the crystal will thus be deflected through a small angle that varies in relation to the grating frequency.

All the above devices do in fact deflect light beams and each in its own way is very useful in some application. The intent of this first installment is to contrast all of these types with each other using applications as the basis for comparison.

With the growing use of beam-deflecting devices in commercial applications, choosing system components hinges on application-specific parameters.

Scanner categories
Applications for scanners fall into one of three functional categories. These are: illuminators, collectors and illuminator/collector combinations.

Illuminators are scanners that generate a scanning light beam, usually from a laser. Their purpose is to produce an effect on a photoreceptor. The photoreceptor may take several forms such as film, a display screen, and an electrophotographic surface.
Because the most common source of light for illuminator scanners is a pencil-like ray bundle from a laser, all of the common scanner types are candidates for beam deflection and in some cases, combinations may be in order. For example, in a TV laser-projector system it makes sense to use a fast-rotating polygon to produce 15,750 line scans per second (standard TV rate) and a slower galvo mirror to introduce the vertical deflection at 60Hz.

In this instance the fast line rate is probably a technical challenge (impractical) for a hologon deflector and an acousto-optic deflector may compromise resolution and contrast as a line scan generator. Scanner selection, then is based primarily on performance considerations.

Another illuminator application is the laser printer where 300 dots or more per inch of resolution over nine inches or so is required. That's more dots per line than provided by an acousto-optic device, but polygons, galvos and hologons perform at that level quite comfortably. Selection boils down to speed and cost.

In high-end laser printers producing 6000 scan lines per second or more, speed ranks ahead of cost. In low end printers producing 500 or 600 scans per second, cost outranks speed in the selection process.

Collectors
Collector-type scanners are those that collect rather than emit ray bundles of light. In this case they are used for viewing a scene. Beam deflection allows a stationary detector to sequentially view discrete scene elements usually for the purpose of visual enhancement or for input on an image processor. An example is a forward looking infrared system (FLIR).

FLIR machines frequently use a light-beam deflector to provide a scan of a scene for an IR detector with one or more discrete detection elements. In this case, the deflector must be efficient in the spectral range of 9.14um. The most common tool for displaying the infrared image collected by a FLIR scanner is a conventional color TV monitor. These devices collect thermal images and display them in visible form.

Acousto-optic and holographic deflectors are generally inappropriate for FLIR applications because of their aperture and resolution limitations over broadband viewing (9-14um, typically). Galvos and motor-driven polygons are frequently used in FLIR systems, in combination or separately, depending on the number of detector elements used and the system speed.

Illuminator/collector combinations
Quite a number of scanner systems use the optical scanning subsystems to both transmit and receive light beams. Examples are point-of-sale systems, bar-code readers, facsimile readers, laser microscopes, and the like. In this case a laser is focused and deflected across a target and the light reflected by the target is collected back through the same optics to a stationary detector. A scene is read in this way by monitoring intensity variations in the collected light. The advantage of both transmitting and receiving through the same deflection optics derives from the system's tunnel vision, that is, it sees only one picture element at a time and the noise of adjacent picture elements is easily rejected.

Combined illuminator/collector systems generally require comparatively large receiver elements in order to collect sufficient light back from the scene. In point-of-sale and bar-code systems it may be important to have the beam deflector produce a number of line scans in separate scene locations so as to "catch" the target wherever it may be within a range of several inches. Irregular polygons and hologons provide this feature rather easily.

Calligraphic systems
Calligraphic image generators are devices that create visual images by stroke writing as opposed to raster generators (TV, for example). A stroke writer moves a light beam through a path outlining a scene character at a repetitive rate that accommodates visual persistence. The Lissajous figures created on an oscilloscope are simple calligraphic images. Galvo and acousto-optic deflectors are suitable for this application since they are position-addressable at high speed.

Calligraphic displays have found applications in entertainment and advertising displays, flight-simulator displays and in some cartooning activity.

Performance considerations
In order to select the most cost-effective beam deflection hardware to use for a given performance requirement, the user must consider a number of issues peripheral to the deflector itself. Not that the deflector isn't a key element - it is - but its behavior may have a great deal to do with overall system cost and in the end it's always cost that drives the design of a successful product.

Meet the author
Randy J. Sherman is president of Lincoln Laser Co. in Phoenix, Arizona. Prior to forming the company in 1974, Sherman was involved in rotating-mirror FLIR development with Hughes Aircraft Co. He has held several engineering, production, R&D and management positions in optical technology companies involved in the development of high speed rotating mirror cameras, film and graphic recorders and inspection equipment.

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