1. Interferometer

2. Principle
Interferometers are optical instruments used for measuring flatness
Determining minute differences in length by direct reference to the wavelength of light.

4. Principle of interferometer

5. Fringes

6. Fringes a b c

7. Fringes A
Here the error is indicated by the amount by which the fringes are out of parallelism with those on the base plate B
If the work piece is concave or convex, fringe pattern will be as shown in Figure (b). C
The surface is flat with slight rounding off at the corner

8. Optical flats Optical flats can be used to measure the flatness
An optical reference flat of known quality
A monochromatic light box
Solvent and cleaning material

9. Typical setup of optical flat

10. Working

11. Fringe Pattern Air wedges Flatness error of zero
Cylinder with flatness error of two fringes
Contact method
Convex Cylinder
Concave Cylinder
Convex Sphere
Concave Sphere
Convex Spheroid
Concave Spheroid
Saddle
Highly Irregular

12. Air wedge Flatness error of zero (a) (a) (b)
Cylinder with flatness error of two fringes (b)

13. Contact method
Convex Cylinder
Concave Cylinder

14. Saddle
Highly Irregular

15. Interferometer

16. Principle
Interferometers are optical instruments used for measuring flatness
Determining minute differences in length by direct reference to the wavelength of light.

18. Principle of interferometer

19. Fringes

20. Fringes a b c

21. Fringes A
Here the error is indicated by the amount by which the fringes are out of parallelism with those on the base plate B
If the work piece is concave or convex, fringe pattern will be as shown in Figure (b). C
The surface is flat with slight rounding off at the corner

22. Optical flats Optical flats can be used to measure the flatness
An optical reference flat of known quality
A monochromatic light box
Solvent and cleaning material

23. Typical setup of optical flat

24. Working

25. Fringe Pattern Air wedges Flatness error of zero
Cylinder with flatness error of two fringes
Contact method
Convex Cylinder
Concave Cylinder
Convex Sphere
Concave Sphere
Convex Spheroid
Concave Spheroid
Saddle
Highly Irregular

26. Air wedge Flatness error of zero (a) (a) (b)
Cylinder with flatness error of two fringes (b)

27. Contact method
Convex Cylinder
Concave Cylinder

28. Saddle
Highly Irregular