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Homogeneity and Thickness

Optical Setups

Homogeneity and Total Thickness Variation (TTV) can be determined from a series of laser interferometer measurements. While a standard Fizeau laser interferometer can be used, a short coherence interferometer can accomplish both jobs with fewer measurements, and without requiring sample surface treatments to isolate individual surfaces for measurement.

Contents

Measure Total Thickness Variation with a Short Coherence Fizeau

– Required Equipment

Measure Homogeneity with a Short Coherence Fizeau

– Required Equipment

Measure Homogeneity with a Standard Fizeau

Additional Materials

RECOMMENDED METHOD

Measuring Total Thickness Variation with a Short Coherence Fizeau Interferometer

Total thickness variation can be determined from a single remote cavity measurement, with the front and back surfaces of the optic being the reference and test surfaces.

total thickness variation measurement, short coherence fizeau interferometer

Solid cavity measurement setup for total thickness variation with a short coherence interferometer.

Want a convenient reference to this and other optical test setups? Download the Common Optical Testing Setups guide (PDF).
Download the Optical setups guide
For a remote cavity measurement, interference occurs between the front and back surfaces of the optic, so expensive reference and return optics are not required.
remote cavity measurement

For the TTV measurement, the wedge factor should be 0.5. The OPD data is scaled by (-1/n) to calculate TTV.

Setting the Wedge Factor Tech Note

This tech note explains how to set the Wedge scaling Factor for measuring wavefront error and surface height in common configurations.

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Required Equipment

The diagram below shows the equipment needed to measure total thickness variation with the AccuFiz D. Short coherence AccuFiz D Fizeau interferometers are available in either 100mm or 150 mm apertures, with beam expanders available up to 600 mm.
total thickness variation, short coherence fizeau interferometerjugate system, transmitted wavefront error, optical setups, twyman-green interferometer
Required Components
AccuFiz D Interferometer More Information
Bayonet, 3-Jaw or Shelf Mount Specifications
2-Axis Base with Tip/Tilt Specifications

 

RECOMMENDED METHOD

Measuring Homogeneity with a Short Coherence Fizeau Interferometer

A traditional, long coherence interferometer requires four measurements to obtain homogeneity. With the short coherence AccuFiz D, only three measurements are needed:

    1. A solid cavity measurement, with the front and back of the optic as the reference and test surfaces.
    2. The transmission measurement, with a reference flat and high-quality return optic.
    3. The empty cavity measurement, with a reference flat and high-quality return optic.

    The three measurements are acquired by adjusting the path match distance to the cavity length between the listed surfaces.

    measure homogeneity and optical thickness - setups required

    Setups for measuring homogeneity with a short coherence interferometer: solid cavity measurement (top), transmitted wavefront (center), empty cavity (bottom).

    These measurements are shown schematically in the image below.

    measure homogeneity, short coherence fizeau interferometer

    Required Equipment

    The diagram below shows the equipment needed to measure homogeneity of a flat with the AccuFiz D. Short coherence AccuFiz D Fizeau interferometers are available in either 100mm or 150 mm apertures, with beam expanders available up to 600 mm.
    measure homogeneity and optical thickness
    Required Components
    AccuFiz D Interferometer More Information
    Transmission Flat Specifications
    Bayonet, 3-Jaw or Shelf Mounts Specifications
    2-Axis Bases with Tip/Tilt Specifications

     

    White Paper - Space and Telescope Optics

    Measurement of large optics requires metrology systems that can function despite vibration and other challenges.  Learn how it’s done in this white paper.

    Download

    ALTERNATE METHOD

    Measuring Homogeneity with a Standard Fizeau Interferometer

    With a standard Fizeau, optical thickness and homogeneity can be measured using comparable equipment to that described for the short coherence instrument.  However, the measurement procedures are more complicated because:
    1. Additional measurements are required to complete the process.
    2. The test optic must be flipped between measurements which introduces sources of error. Considerable care is required to align the optics and reduce added error.
    3. The surfaces that are not being measured in a particular setup must be covered in anti-reflective coating in order to remove stray reflections—a messy and time-consuming process that can also potentially damage the optic.

    Because of these complications, a short coherence interferometer is the preferred option for optical thickness and homogeneity measurements.

    Learn about our entire Fizeau family

    Fizeaus Overview
    Download the Optical Setups Guide PDF
    BACK TO OPTICAL TESTING SETUPS GUIDE

    Need Assistance with Measuring Homogeneity or Optical Thickness?

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