APPLICATIONS OF OPTICAL PROJECTORS
The inspection operations and dimensional measurements that
can be carried out with the optical projector, in many respects, are similar to
those discussed in detail in the preceding chapter dealing with engineering
microscopes. However, there are still substantial differences between these two
families of optical measuring instruments, with regard to capabilities and
suitability for particular application tasks and conditions. These differences
will be discussed first from a general viewpoint, disregarding exceptions,
which may be numerous.
Microscopes are instruments intended primarily for tool-room
and gage room applications and require a certain degree of skill and experience
for their efficient operation. Thus, when used to inspect critical dimensions
or for measurements requiring special procedures, they should be assigned to a
competent toolmaker or a specialized metrologist.
Optical projectors are basically production-oriented instruments
and, in fact, are often used on the shop floor by machine tool operators or
trained inspectors. Optical projectors are generally sturdy instruments, less
sensitive than microscopes, easier to operate and even unskilled personnel can
quickly be trained to carry out simple inspection processes on these
projectors are not adaptable to various types of special accessories designed
for microscopes; consequently, certain categories of measurements listed in the
preceding chapter do not apply to optical projectors. While the power of
resolution for linear measurements in the highest grades of measuring microscopes
subdivides the ten-thousandth of an inch, resulting in increments of 50 or 20
microinches, the least scale graduation of even the most advanced types of optical
projectors is in 0.0001-inch units.
On the other hand, optical projectors provide application
advantages in many other respects, in comparison to the capabilities of
engineering microscopes. Examples of such characteristics and operating
conditions are listed below, also with the purpose of pointing out the
preferential areas of optical projector usage.
and weight of the test piece.
Optical projectors are available in sizes that can accept
for staging and inspection heavy parts of considerable outside dimensions.
view on the object.
for medium-size optical projectors this exceeds the comparable capacity of
microscopes in general. A large object field permits the synoptic viewing of
extended areas on the work surface, reduces the need for object displacement or
indexing and offers wider applications for inspection by contour comparison
without actual measurements.
commonly at eye level, permits group viewing and the observation of the
image in unrestricted position under more natural conditions than the viewing
through a microscope eyepiece.
the continued observation of the work progress, guided by screen charts,
without impeding the movement of machine tool members or the handling of the
controls by the operator.
purely visual inspection of toleranced part features can be prepared and
mounted on the screen according to the requirements of the scheduled inspection
operation. On such charts, the tolerance ranges appear as graphically laid out
zones that must contain the pertinent contour of the part being inspected.
only the exchange of the screen against a plate frame, for preparing a
silhouette photo exactly of the size in which the image appeared on the screen.
The silhouette image of the part can also be reproduced superimposed on the
pertinent screen chart, resulting in photos of value for subsequent analysis or
for reference purposes.
by contour drawing.
mounting vellum paper on the glass screen, the contour of the inspected part
can be traced with a pencil and the so-prepared tracing may serve record
purposes, or may be overlaid on the screen image of the subsequently inspected
mating part for observing the resulting contact conditions.
internal surface contours by stylus device.
A stylus attached to a tracer arm follows the optically
hidden contour of the part’s surface and the image of the parallel moving
duplicate stylus, which lies in the optical plane of the instrument, is
projected on the screen; there its position can be compared to the tracing line
of a screen chart displaying the basic contour of the traced element.
of electronic linear measurement information.
Through the use of a simple two-axes
digital electronic display (Figs. 9-7 and 9-8), information can be gathered for
output to computerized video and other statistical process control systems.
Alternating between the more traditional uses and strengths of
the easy-to-operate optical comparator and the recent built-in or add-on
This means that today’s optical comparator can be seen and used as it
has been for decades by projecting a magnified 2D shadow onto a screen for
comparative visual inspection and measurement purposes or it can employ video
and digital cameras, computers, monitors, and sophisticated software making it
an advanced measurement system.
Examples of typical applications, utilizing the characteristic
properties of optical projectors, are shown in Table 9-3, with specimen
sketches illustrating diagrammatically the dimensions being inspected.
TABLE 9-3. EXAMPLES OF
PREFERENTIAL APPLICATIONS FOR OPTICAL PROJECTORS — 1
TABLE 9-3. EXAMPLES OF
PREFERENTIAL APPLICATIONS FOR OPTICAL PROJECTORS — 2
TABLE 9-3. EXAMPLES OF
PREFERENTIAL APPLICATIONS FOR OPTICAL PROJECTORS — 3