Optical Comparator

• This is the commercial measuring instrument
• This utilizes a plunger tilted mirror, objective lens,prism and observing eye piece to provide a high degree of magnification
• The mirror is mounted on a knife-edge
• It can be tilted about the fulcrum by any linear vertical movement of the contact plunger

• A beam of light passes through a graticule suitably engraved with a linear scale
• The movement of mirror causes this scale to move up (or) down past a translucent screen inside the observing hood of the instrument
• The eye placed near the eye piece views the image of a small scale engraved on scale after reflection from the plunger actuated mirror
• The” plan” view for the mirror is shown in fig

• In the focal plane of the eye piece , a fine reference line ( index 0) is provided
• The system of lenses is so arranged that the image of the scale is projected in the same focal plane
• Thus with movement of scale the image can be measured with reference to the fixed line
• The division of the scale image opposite the index line indicates the amount of movement of contact plunger

• The image of the scale and the index line could also be viewed through a projection system
• The overall magnification of the comparator is given by (2f/d)x eye-piece magnification
• Where `f` is the focal length of the lens and `d`is the distance between the knife-edge and the plunger

Principle of an optical comparator

• In 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

• The image of an index is projected on a scale on the inner surface of a ground glass screen
• The whole system could be explained diagrammatically from fig which gives very simple arrangement
• In this system mechanical amplification is L2/L1
• Optical amplification is ( L2/L1)X2

• It is multiplied by 2 because if mirror is tilted by an angle δө, then the image will be tilted by 2xδө
• Thus overall magnification of this system is
• 2x ( L2/L1) ( L4/L3)
• The important point in optical comparators is that mirror used must be of front reflection type and not of normal back reflection type

Advantages

• Optical magnification provides high degree of measuring precision due to reduction of moving members and better wear resistance qualities
• Optical magnification is also free from friction,bending,wear etc.

•  An illuminated scale is provided that enables readings to be taken without regard to the room lighting conditions
•  These are also used to magnify very small parts such as needles, saw teeth, screw threads etc.