1. Introduction
Optical measuring instruments use light and optical principles (reflection, refraction, interference, magnification) to measure dimensions, angles, and surface characteristics with very high precision.
They are widely used in:
- Tool rooms
- Inspection laboratories
- Precision manufacturing industries
2. Advantages of Optical Methods
- Non-contact measurement (no wear or damage)
- High magnification and accuracy
- No mechanical friction or backlash
- Suitable for delicate and small components
3. Principles of Optical Measurement
Optical instruments work based on:
- Reflection of light
- Refraction through lenses
- Magnification of image
- Interference of light waves
4. Types of Optical Measuring Instruments
4.1 Tool Maker’s Microscope
Description
A precision optical instrument used for measuring small components, angles, and profiles.
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Main Parts
- Base and column
- Measuring table (X-Y movement)
- Microscope with eyepiece
- Crosshair scale
Working
- Workpiece is placed on table
- Image is magnified
- Measurements taken using crosshair movement
Applications
- Measuring thread angles
- Inspecting gears and profiles
- Tool inspection
4.2 Optical Comparator (Profile Projector)
Description
Projects a magnified shadow/image of a component onto a screen.
4
Working
- Light source projects image
- Lens magnifies it
- Image appears on screen
- Compared with standard templates
Applications
- Profile checking
- Gear tooth inspection
- Thread measurement
4.3 Autocollimator
Principle
Based on reflection of a collimated light beam.
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Working
- Light beam is projected onto a mirror
- Reflected beam returns to instrument
- Angular displacement is measured
Accuracy
- Very high (seconds of arc)
Applications
- Alignment of machine tools
- Measurement of small angular deviations
4.4 Interferometer
Principle
Based on interference of light waves.
4
Working
- Light waves split and recombined
- Interference fringes formed
- Fringe pattern used to measure deviations
Applications
- Surface roughness measurement
- Flatness testing
- Thickness measurement
4.5 Optical Flats
Description
- Highly polished glass surfaces used with monochromatic light
4
Working
- Placed on surface
- Interference fringes indicate flatness
Applications
- Checking surface flatness
4.6 Laser-Based Measuring Instruments
Description
Modern instruments using laser beams
4
Applications
- Distance measurement
- Alignment
- CNC calibration
Advantages
- Very high precision
- Fast measurement
5. Comparison of Optical Instruments
| Instrument | Accuracy | Application |
|---|---|---|
| Tool Maker’s Microscope | High | Small components |
| Optical Comparator | High | Profile inspection |
| Autocollimator | Very High | Angular measurement |
| Interferometer | Extremely High | Surface & wavelength level |
6. Sources of Error
- Improper alignment
- Lens distortion
- Light intensity variation
- Temperature effects
- Human reading error
7. Applications of Optical Measuring Instruments
- Precision engineering
- Aerospace industry
- Tool and die making
- Quality control laboratories
- Research and development
8. Advantages
- High accuracy and sensitivity
- Non-contact measurement
- No wear and tear
- Suitable for micro-measurements
9. Disadvantages
- Expensive instruments
- Require skilled operator
- Sensitive to environmental conditions
- Need controlled lighting