FT-QPSI is an abbreviation of Front-and-Back Temporal Quantum Phase Shifting Interferometry, a special kind of interferometric acquisition and analysis mode that is provided in Zygo(r) Verifire interferometer systems and can be integrated with the Mxtm metrology software.
It is a sophisticated method of measurement, which is based on the Zygo QPSItm (Quantum Phase Shifting Interferometry) platform – technology created to allow high-precision optical measurements to be made despite the presence of vibration – and is further developed to address more complex optical test conditions with multiple surfaces involved. (zygo.com).
The Core Purpose of FT-QPSI
FT-QPSI has been created to address one of the primary weaknesses of traditional QPSI acquisition:
Classic QPSI deals with single patterns of interference – normally only one optical surface (front or back) at a time.
FT-QPSI can be extended to support multiple surfaces at once including:
Rear and front sides of plano optics.
Total thickness variation (TTV)
Wedge measurements
The measurements of inhomogeneity of material
This is when it maintains strong performance in conditions where similar sources of vibration would otherwise affect the accuracy of the measurement. (zygo.com).
The importance of FT-QPSI in Optical Metrology
Enhanced Vibration Robustness.
Vibration has forever been one of the biggest problems of high-precision interferometry, as interferogram measurements are time-consuming and even the tiniest movements of a micrometer magnitude corrupt measurements. FT-QPSI Mx software includes model-based algorithms that allow reducing the vibration artifact greatly without the need to install vibration isolation hardware.
Multi-Surface Capability
In contrast to conventional QPSI, FT-QPSI works with multi-surface optical components which are mandatory in:
Window assemblies
Thick or thin optic substrates.
Precision parallel plates
This renders it an influential instrument in the production environment where multi-surface optics are rapidly becoming more popular. (zygo.com)
Increased Homogeneity and Thickness Analysis
FT-QPSI does not only measure the shape of the surface, but can also extract:
Optical thickness variation (OTV).
Homogeneity variance of the material.
These play a key role in quality control of high-end optics including precision glass and semiconductor display parts. (zygo.com
Verifire and Mx Software Verifire and Mx Software How It Works
FT-QPSI is not an additional hardware for Mx; it is mostly packaged as part of Zygo Verifire interferometers software.
In the Mx Interface You Can:
As the acquisition mode, choose FT-QPSI.
Measured phase on several reflective/transmissive surfaces.
Carry out elaborate post processing analysis in vibration compensation mode.
FT-QPSI provides the same user-friendly workflow of the other Zygo measurement modes but with enhanced measurement capabilities through the current QPSI platform. (zygo.com).
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Common Uses of FT-QPSI.
FT-QPSI has found application mostly in the following metrology tasks in Optics:
Characterization of multi surface glass and coatings.
Accurate thickness and variation test.
Thin optical homogeneity.
Large-scale inspection of production.
Worried environments (shop floor or around machineries)
Both of these need good interferometry data which even traditional interferometry (particularly standard PSI) cannot give without vibration isolation.
Summary
In simple terms:
FT-QPSI is a vibration-resistant, multi-surface interferometric acquisition in the verifire equipment of Zygo that is operated on the basis of Mx software (i.e., the ability to measure not only the front, but also the back surface, thickness variation, and material homogeneity in the real world).
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This renders FT-QPSI a useful instrument in contemporary optical metrology, especially in situations where a combination of speed, accurateness, and environmental endurance is required.