|LASER ALIGNMENT THEORY
Modern optical metrology uses precise lines and planes in space from which measurements are made. Because this method creates these features using light it has become known as optical tooling. The 35 year old historical development and current technology of laser instrumentation as used in optical tooling is discussed in detail. This includes the how the measurements are made, applications, and the technology of alignment lasers and position sensing targets developed for sensing position within a laser beam or plane. The various geometries used to make alignment measurements are discussed in detail. Applications are discussed and the challenges each poses are discussed. The challenge of long range alignment and the effect of the turbulent atmosphere on the measurement process is discussed along with methods of handling the associated errors them.
WHAT IS OPTICAL TOOLING?
Optical tooling is a means establishing and utilizing a line of sight (LOS) to obtain precise reference lines and reference planes from which accurate measurements are made with position sensitive targets.  Formerly the measurements were done by a person interpreting a scale or optical micrometer by looking through an alignment telescope; today the lines and planes are created by a laser; the measurements are digital and require no interpretation. Optical tooling uses the principle that light travels in straight lines to produce precise measurements that cannot be reached by manual or mechanical methods. Level lines can be established over great distances so accurately that every point is exactly perpendicular to the force of gravity. Plumb lines can be set to a given level. Right angles can be produced quickly and precisely with auxiliary components. In the assembly, maintenance and calibration of industrial equipment or in the alignment of precision systems, one or as many as four basic questions always must be answered: is it straight, is it flat, is it plumb or is it square? A number of techniques have been developed to make these measurements; however, many of them result in inaccuracies so great that proper operation of the equipment involved will be compromised or seriously endangered. The science of optical metrology and alignment makes it possible to achieve the highest degree of accuracy in answering these four important questions. It is no longer necessary to interpret readings or to make constant adjustments and calculations. In laser alignment applications, direct, precision measurements are made rapidly and consistently.
In aligning several points, a tight wire is often used as a reference line. This technique has numerous drawbacks and introduces inaccuracy. First of all, wire has weight, which causes it to sag; over long distances this sag can become considerable. In addition, wire vibrates, can bend or kink, and when stretched in the area to be measured, equipment cannot be moved around for fear of disturbing the wire reference line. Even a gentle breeze can cause the wire to move sideways a considerable amount; the aerodynamic drag on a thin wire is huge. In laser alignment, the LOS of is established by a laser beam instead of a tight wire. The invisible LOS reference has no weight, cannot sag, kink, or be disturbed, nor is it a safety hazard. It constitutes a precise, unvarying reference, determining straightness to within thousandths of an inch.
In order to determine flatness a shop level and a straightedge was employed in the past. However, over large horizontal areas, the shop level must be moved from part to part. Consequently, one can only tell the degree of flatness of each individual surface upon which the level is place. Whether all objects in a large area are flat is still in doubt. Flatness over a considerable area must be assured in the erection of large machinery, surface tables and large machine tools. Conventional bubble levels simply do not offer the...
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