Optical and Surveying Instruments
Introduction
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The importance of optical instruments has been increasing
in the industrial and scientific world for about two centuries.
The demands of precision, as well as of cheap and rapid production,
and led to the adaption of optical methods in many branches
of engineering today, such methods can be used in a variety
of ways, from tool inspection on the one hand, where meticulous
accuracy is of paramount importance, to certain types of final
inspection on mass-produced articles where a reasonable degree
of accuracy has to be combined with speedy operation. The
instruments based on the optical principle are being used
in the field of surveyign and navigation. In this chapter,
the basic operating principle of certain types of optical
and surveying instruments have been discussed.
Microscope
A microscope may be defined as an instrument for viewing
close objects. In order to increase the apparent size of an
object, we bring it closer to the eye, but the unaided eye
cannot focus on an object which is nearer than a certain distance
which is the minimum distance of comfortable vision. The value
of 250 mm is uuiversally recognised as the standard minimum
distance of comfortable vision. At this distance, a pair of
lines separated by less than about 0.1 mm is seen not as a
pair but only as a single, broadened line which represents
the limit of visual resolution, and it depends on the objects,
the condition of observation and on the quality of the lens.
The product N sin is termed as the Numerical Aperture (NA)
and cannot exceed the value 1.0 when the object is in air.
The value of NA is greater than 1.0 only for an object which
lies in a medium other than air.
Magnification of Microscope
The magnification of a microscope may be defined as the ratio
of the angular size of an image seen through the instrument
to the angular size of an object seen with the unaided eye
at the least distance of comfortable vision.
Types of Microscopes
Following are the three general types of microscope in common
use:
( i ) Simple microscopes (monocular)
( ii ) Compound microscopes (monocular or binocular)
( iii ) stereoscopic microscopes (binocular).
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