■ Erect Image and Inverted Image An 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 figure below) it is referred to as an inverted image (also known as a reversed). Quick Guide to Precision Measuring Instruments
Profile Projectors
■ Telecentric Optical System An optical system based on the principle that the primary ray is aligned parallel to the optical axis by placing a lens stop on the focal point on the image side. Its functional feature is that the image will not vary in size even though the image blurs as the object is shifted along the optical axis. For measuring projectors and measuring microscopes, an identical effect is obtained by placing a lamp filament at the focal point of a condenser lens instead of a lens stop so that the object is illuminated with parallel beams. (See the figure below.)
F
Focal point on the image side
Principal ray
Projection screen
Optical axis
An erect image
An inverted image
Light source (lamp)
Object surface
F
F
Projection lens
Workpiece
Condenser lens
Projection screen surface
Top of the stage
Telecentric contour illumination
■ Working distance Refers 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.
F Workpiece
X-axis movement Y-axis movement
■ Magnification Accuracy The magnification 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 magnification, as marked) to produce a percentage magnification accuracy figure, 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 magnification accuracy is not the same as measuring accuracy.)
Projection lens
L
Workpiece stage
Workpiece
■ Parallax error This is the displacement of an object against a fixed background caused by a change in the observer's position and a finite separation of the object and background planes.
L− M Δ M(%) = ——— X 100 M
Δ M(%): 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 object M : Magnification of the projection lens
Parallax error
■ Type of Illumination ● Contour illumination: An illumination method to observe a workpiece by transmitted light and is used mainly for measuring the magnified 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.)
Projector screen
■ Field of view diameter The maximum diameter of the workpiece that can be projected using a particular lens.
Screen diameter of profile projector Magnification of projection lens used
Field of view diameter (mm) =
Example: If a 5X magnification lens is used for a projector with a screen of ø500mm: Field of view diameter is given by 500mm = 100mm 5
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