10-04-2017, 08:09 PM
In mechanical optical comparators small displacements of the measuring plunger are
amplified first by a mechanical system consisting of pivoted levers. The amplified mechanical
movement is further amplified by a simple optical system involving the projection of an image.
The usual arrangement employed is such that the mechanical system causes a plane reflector
to tilt about an axis and the image of an index is projected on a scale on the inner surfac^ffta
ground-glass screen. Optical magnification provides high degree of measuring precision Mp
to reduction of moving members and better wear resistance qualities. Optical magnificatioijis__
also free from friction, bending, wear etc. /rt^r ^ ?T /
Principle of Optical Comparator
Fig. 5.14. Principle of Optical Comparator.
The whole system could be explained diagrammatically by Fig. 5.14, which gives very
simple arrangement and explains the principle of above comparator.
In this system, Mechanical amplification = tyli
and Optical amplification = x 2.
It is multiplied by 2, because if mirror is tilted by an angle 60, then image will be tilted
by 2 x 66. Thus overall magnification of this system = 2 x (Z^i) (J4//3).
Thus it is obvious that optical comparators are capable of giving a high degree of
measuring precision owing to high magnification and the reduction of moving members to
minimum. Further these possess better wear resistance qualities as the only wearing members
are the plunger and its guide and the mirror pivot bearing. Another advantage of the optical
comparators is that provision of an illuminated scale enables readings to be taken without
regard to the room lighting conditions. The point of importance in optical comparator is that
mirror used must be of front reflection type and not of normal back reflection type. In normal
back reflection type there are two reflected images, one each from front and back. Thus the
reflected image is not well defined one, as one bright and other blurred image are observed. If
front reflection type of mirror is used, then it requires considerable care in its use to avoid
damage to the reflecting surfaces.
