Quick Guide to Precision Measuring Instruments

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PG43Quick Guide to MeasurementErect Image and Inverted ImageAn image of an object projected onto a screen is erect if it is orientated the same way as the object on the stage. If the image is reversed top to bottom, left to right and by movement with respect to the object on the stage (as shown in the gure below) it is referred to as an inverted image (also known as a reversed image, which is probably more accurate).Magnication AccuracyThe magnication accuracy of a projector when using a certain lens is established by projecting an image of a reference object and comparing the size of the image of this object, as measured on the screen, with the expected size (calculated from the lens magnication, as marked) to produce a percentage magnication accuracy gure, as illustrated below. The reference object is often in the form of a small, graduated glass scale called a `stage micrometer’ or `standard scale’, and the projected image of this is measured with a larger glass scale known as a `reading scale’. (Note that magnication accuracy is not the same as measuring accuracy.)Type of Illumination●Contour illumination: An illumination method to observe a workpiece by transmitted light and is used mainly for measuring the magnied contour image of a workpiece.●Coaxial surface illumination: An illumination method whereby a workpiece is illuminated by light transmitted coaxially to the lens for the observation/measurement of the surface. (A half-mirror or a projection lens with a built-in half-mirror is needed.)●Oblique surface illumination: A method of illumination by obliquely illuminating the workpiece surface. This method provides an image of enhanced contrast, allowing it to be observed three-dimensionally and clearly. However, note that an error is apt to occur in dimensional measurement with this method of illumination.(An oblique mirror is needed. Models in the PJ-H30 series are supplied with an oblique mirror.)Working distanceRefers to the distance from the face of the projection lens to the surface of a workpiece in focus. It is represented by L in the diagram below.Parallax errorThis is the displacement of an object against a xed background caused by a change in the observer's position and a nite separation of the object and background planes.The maximum diameter of workpiece that can be projected using a particular lens.Field of view diameterΔM(%): Magnification accuracy expressed as a percentage of the nominal lens magnification L :Length of the projected image of the reference object measured on the screen :Length of the reference objectM :Magnication of the projection lensParallax error Projector screen Projection lensWorkpiece stageWorkpieceLFFAn erect imageFFAn inverted imageProjection screenTop of the stageFWorkpieceX-axis movementY-axis movementField of view diameter (mm) = Screen diameter of prole projector Magnication of projection lens used L− MΔM(%) = ̶̶̶ X 100 MExample: If a 5X magnication lens is used for a projector with a screen of ø500mm:Field of view diameter is given by 500mm = 100mm 5Prole Projectors