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Non-contact Surface Metrology-Introduction to LUPHOScan Ranges

This article is about non-contact metrology for 3D acquisition of form errors, based on a multi-wavelength interferometric approach.

What are the Methods of Measuring Optical Components?

Taylor Hobson offers two methods/approaches to measure optical components:

What is Non-contact Metrology: Multiwavelength Interferometry (MWLI)?

Interferometrical point sensors can easily achieve accuracies within a few nanometers and record changes in path in the picometer range. With the appropriate collimation, very large measuring ranges are possible. However, one is limited to half of the wavelength as unambiguity range when using a homodyne interferometer.

Multi Wavelength Interferometry (MWLI) uses several independent, discrete wavelengths that share one optical path, and calculates a synthetic beat wavelength. This way, the very good resolution as well as the very large measuring range of homodyne interferometers, can be combined with an unambiguity range extended by various magnitudes.

For example, when using two wavelengths λ1 and λ2 to measure the same distance, each wavelength will deliver its own interference signal. This corresponds to two independent homodyne interferometers with high accuracy but small unambiguity range λ1/2 and λ2/2, respectively. Thanks to the MWLI technology, however, the unambiguity range can be expanded by calculating the synthetic beat wavelength of both discrete wavelengths. The size of the achieved unambiguity range is equal to half of the synthetic beat wavelength Λ, which can be more than 1 mm.

Multiwavelength Interferometry
Fig1: Multi Wavelength Interferometry (MWLI)

What are the Properties of Multiwavelength Interferometers?

• Non-contact, optical measurement
• Extremely high accuracy (< 2 nm, 2 σ)
• Large working distance (up to 30 cm)
• Large working range
• Absolute measurements (tolerates signal interruption and measures step heights up to 1.25 mm)
• Different surface types and materials (transparent, opaque, specular, polished, rough)
• Varying reflection coefficients (from 0.1% up to 100%) due to phase evaluation 

Ground Lens Non-Contact Measurement
Fig2: Ground Lens (Non-contact Measurement)

What is LUPHOScan and 3D Form Measurement of Aspheres and Freeforms?

The LUPHOScan system is a turnkey solution (see Fig3) to measure different types of surfaces, starting from flats or spheres (up to hemispheres), aspheres, both convex and concave or with inflection points. You can measure freeforms, as well as surfaces with small steps like diffractive optical elements, large steps like Fresnel lenses, interrupted surfaces like D-shaped or a rectangular lens, elements with a central hole or any other kind of discontinuity.

Non-contact 3D Profilometer LUPHOscan
Fig3: Non-contact 3D Profilometer LUPHOScan

The background idea of the system was to combine the advantages of two worlds: the tactile measurement with a great geometric flexibility and a non-contact, accurate interferometric measurement. Like this we came up with a LUPHOScan which is a
scanning interferometer based on an optical point sensor.

What is The Working Principle of LUPHOScan Non-contact Interferometer?

Linear and rotational stages are responsible for the positioning of the object sensor above the surface. Here, two linear stages, in radial (R) and in vertical direction (Z), position the sensor in the measurement volume, while a rotational stage (T) facilitates the vertical measurement in relation to the surface.

The object sensor is measuring the distance to the object under test. While the geometrical design shape of the surface is being captured, the object rotates by means of an additional air-bearing spindle. This leads to a measuring trajectory along a spiral path on the design surface.

However, during the measurement not just the distance from object to sensor must be measured, but in addition, the exact position in the measurement volume for each acquired data point. To achieve this, (as shown in Fig4) three reference sensors and a robust metrology frame with highly accurate reference mirrors are necessary. Thanks to the shown positions of two linear and one cylindrical reference mirrors, the absolute position of the object sensors in the measuring volume can be measured uninterruptedly. The reference sensors are positioned in a way that Abbe errors of the first order (caused by mechanical movement of the stages, they influence the measuring distance) can be compensated completely.

3d Non-contact Profilometer LUPHOscan Working Principle
Fig4: Working principle

System Configuration and Accuracy

The LUPHOScan systems standard sizes go from 120 mm, 260 mm to 420 mm maximum measurable diameter of the surface under test. Larger systems measuring 600 mm or up to 850 mm in diameter are available, too. On the other end, there is a smaller system especially for the smartphone camera market that is designed to measure 50 mm max. The LUPHOScan (standard sizes) have an absolute PV form accuracy of 30 nm up to 30°, 70 nm up to 70° and 100 nm up to 90° surface slope (3 sigma each).

The typical cycle time for a standard size object of a LUPHOScan 260 system is below 8 minutes including setup time. Thanks to the scanning approach, the system has a great flexibility and does not need any hardware setup changes with varying object design: Just enter the aspheric coefficients of the object under test to the software (or load from a previous measurement) and start the measurement. There is no limit in deviation to sphericity.

Example: Fig5 shows repeat measurements of a hemisphere over 13 hours, of course without any recalibration in-between. In red the measured power error and in blue the PV value are shown. PV easily stays in a band of +/-10 nm, power in a band of +/- 20 nm, which highlights the long-term stability of LUPHOScan systems.

Data of a Hemisphere Repeat Measurement
Fig5: Example Data of a Hemisphere Repeat Measurement

What are The Applications of LuphoScan 3D Non-contact Optical Measurement System/Profilometer?

  • Aspheres, spheres, flats
  • Segmented objects
  • Annular objects
  • Cylinder and axicons
  • Asphero-diffractive und Fresnel lenses
  • Freeform optics
  • Correlation of front and back surface
  • Mechanical alignment

For more details about LUPHOScan non-contact interferometers, Contact Us


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