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Quick Guide to Precision Profile Projectors
Measuring Instruments
Erect Image and Inverted Image F Telecentric Optical System
An image of an object projected onto a screen is erect if it is An optical system based on the principle that the principal ray is
orientated the same way as the object on the stage. If the image is aligned parallel to the optical axis by placing a lens stop on the focal
reversed top to bottom, left to right and by movement with respect point on the image side. Its functional feature is that the image will
to the object on the stage (as shown in the figure below) it is referred not vary in size though the image blurs as the object is shifted along
to as an inverted image (also known as a reversed image, which is the optical axis.
probably more accurate). For measuring projectors and measuring microscopes, an identical
effect is obtained by placing a lamp filament at the focal point of a
F Projection screen condenser lens instead of a lens stop so that the object is illuminated
with parallel beams. (See the figure below.)
Principal ray Focal point on the image side
An erect image An inverted image
FF Light source Object surface Optical axis
Top of the stage (lamp) Workpiece Projection
Condenser lens Projection lens screen surface
Telecentric contour illumination
F Workpiece
X-axis movement Working Distance
Y-axis movement
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
Magnification Accuracy below.
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 Projection lens
screen, with the expected size (calculated from the lens magnification, L
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’.
J (Note that magnification accuracy is not the same as measuring accuracy.)
M (%) = —L—- —M M X 100 Workpiece stage Workpiece
M(%): Magnification accuracy expressed as a percentage Parallax Error
of the nominal lens magnification
This is the displacement of an object against a fixed background
L : Length of the projected image of the reference object caused by a change in the observer's position and a finite separation
measured on the screen of the object and background planes.
: Length of the reference object
M : Magnification of the projection lens
Type of Illumination Parallax error
Contour illumination: An illumination method to observe a Projector screen
workpiece by transmitted light and is used mainly for measuring
the magnified contour image of a workpiece. Field of View Diameter
Coaxial surface illumination: An illumination method whereby a The maximum diameter of workpiece that can be projected using a
workpiece is illuminated by light transmitted coaxially to the lens particular lens.
for the observation/measurement of the surface. (A half-mirror or a Screen diameter of profile projector
projection lens with a built-in half-mirror is needed.) F ield of view diameter (mm) = Magnification of projection lens used
Oblique surface illumination: A method of illumination by obliquely Example: If a 5X magnification lens is used for a projector with a
illuminating the workpiece surface. This method provides an screen of 500 mm:
image of enhanced contrast, allowing it to be observed three- 500 mm
dimensionally and clearly. However, note that an error is apt Field of view diameter is given by 5 = 100 mm
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.)
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