Other Fizeau 

   4-inch Horizontal

   4-inch Vertical 

   Grazing Incidence

   How Phase Works





The Model 4VPS shown below, is an example of the Vertical Fizeau interferometer with integral Tip/Tilt Table whose height can be adjusted to accommodate larger assemblies, or to minimize the air gap that can result in drifting fringes.

While requiring fine pitch screws to facilitate accurate adjustment of the test piece, in a vertical interferometer, the Tip/Tilt table must also be ruggedly built since it may be required to rigidly support heavy components or assemblies.


All interferometers require a Tip/Tilt device in at lease one position in the optical path.

In vertical format interferometers such as the Model 4VPS at the left or the smaller Model 2VP PHASE MITETM shown below, this device is usually configured as a table which supports the test piece.

In horizontal format interferometers such as the Model 339ir Infrared Interferometer shown below, it usually takes the form of a fixture or bracket mounted on the optical table.

If only Plano work is involved, only two axes of adjustment are required, Tip and Tilt.

If Spherical surfaces are to be measured, however, X, Y and Z axes are also required.


When doing Plano work, the Tip/Tilt Table is used to bring the test surface parallel to the reference surface of the interferometer.  Centration is not too important since plano surfaces do not really have an optical axis.

When a spherical test piece is being measured, a 5-axis fixture is required to adjust of the orientation and position of the spherical surface so that:

  • The optical axis of the test piece coincides with the optical axis of the Transmission sphere. This requires four axes of adjustment -- Tip, Tilt, X adjust and Y adjust.
  • The Z adjust is required to position the test piece so its center of curvature coincides with the focal point of the Transmission Sphere being used.
  • If radius of curvature is to be measured, some type of scale must also be provided along the Z-axis to determine the distance between the "Cat's Eye" position and the null position (the position with the fewest possible number of fringes)  i.e. when the center of curvature of the test surface coincides with the focal point of the Transmission sphere.
  • The Cat's Eye position occurs when the center of the test surface is located at the focal point of the transmission sphere. 

The image at the left shows changes in the interferogram of of a test piece as the Tip /Tilt Table is adjusted.  At the beginning of the cycle there are many fringes, and as the Tip/Tilt is adjusted to bring the test surface parallel to the reference surface, the number of fringes steadily decreases, until at the last frame of the sequence, one fringe pretty well covers the entire interferogram. 

This technique often called "fluffing out" the fringes is one that is performed routinely in the day to day use of the interferometer.

 Model 339ir Infrared

  Model 2VP

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This page last updated April 23, 2013