What Are Newton Gauges and Newton Rings? Precision Measurement Using Optical Flats and Interferometers

Newton gauges and Newton rings are highly precise measurement techniques based on the phenomenon of light interference. By combining these methods with optical flats or interferometers, it becomes possible to measure minute differences in surface flatness, parallelism, and other fine variations.

Measurements utilizing interference phenomena are increasingly used in precision fields because they can visualize surface irregularities or changes in film thickness that are difficult to detect with conventional mechanical measurement methods. They are particularly effective in applications requiring control at the micron to nanometer scale, such as semiconductor manufacturing and optical component inspection.

This article provides a detailed explanation of the basic principles of Newton gauges, Newton rings, and optical flats, the advantages of interferometer-based measurements, and practical applications and examples of their use. It is a valuable reference for those looking to improve measurement accuracy or considering the use of interferometric methods.

Principles and Features of Newton Gauges

First, let us examine the basic mechanism of Newton gauges, which utilize light interference, and the types of measurements for which they are used.

A Newton gauge is a device that reads differences in surface height or thickness of an object by exploiting interference phenomena at the wavelength level of light. Since it does not require mechanical contact, it is particularly suitable for measuring delicate or easily damaged samples. Additionally, it offers a very high measurement range and resolution, allowing detailed assessment of local surface conditions, which makes it widely used in manufacturing processes.

As light propagates through a medium, its phase changes. Even slight variations in thickness or surface unevenness create interference patterns in the reflected light. A Newton gauge analyzes these interference patterns precisely to estimate surface accuracy, film thickness, and other properties with high precision.

Measurements are also sensitive to environmental factors such as temperature and humidity. To obtain accurate results, it is essential to control the measurement environment. Using a vibration-free instrument table and maintaining stable temperature conditions enables measurement accuracy on the nanometer scale.

What Is a Newton Gauge

A Newton gauge is a measurement tool that incorporates optical interference technology, enabling the visualization and quantification of slight thickness variations and surface distortions. It works by illuminating the surface of an object and capturing interference fringes from the reflected light, offering higher resolution compared to other measurement instruments.

Because high-precision lenses and optical systems are required, using a Newton gauge demands more advanced knowledge and careful adjustment than general stereomicroscopes. However, the measurement results are highly reliable, making it invaluable in research and development as well as quality control applications.

Measurement Applications and Examples

Newton gauges are primarily used in machining and inspection processes to check the surface accuracy of components. By detecting minute steps that occur after cutting or polishing, they allow immediate assessment of finishing quality.

They are also widely used for measuring the thickness of thin-film coatings and films. In industries such as optics and semiconductors, as well as during the prototyping of precision components, it is common practice to verify with a Newton gauge whether samples have been formed or coated to the intended thickness.

Accuracy Achievable with Newton Gauges

By precisely reading the interference fringe patterns, Newton gauges can estimate dimensions down to the nanometer scale. However, measurement accuracy can vary depending on the properties of the light source, the surrounding environment, and characteristics of the object being measured.

In particular, substrate expansion due to temperature changes and contamination on the measurement surface can significantly affect the results. Thorough cleaning and stable temperature and humidity control are therefore essential. With these preparations in place, ideal interference patterns can be obtained, improving measurement precision.

Principle of Flatness Measurement Using Newton Rings

Next, we explain the principle of Newton ring generation and how it is applied to flatness measurement.

Newton rings are concentric circular patterns formed by thin-film interference in regions where two transparent surfaces are close together. They clearly reflect variations in the “gap” thickness, making even slight surface irregularities visible as distinct fringe patterns.

Observing Newton rings is effective both qualitatively and quantitatively for checking the flatness of a target object. Minute distortions that are difficult to detect with mechanical touch gauges or dial indicators become readily visible.

Using an optical flat allows interference fringes to be compared with a stable reference surface, enabling high-precision determination of sample flatness. When the instrument is properly adjusted and the sample is correctly mounted, immediate inspections can be performed for in-process quality control.

Principle of Newton Ring Formation

Newton rings are interference patterns that occur when light undergoes multiple reflections in a small gap between a flat glass and a spherical glass surface, or in a thin gap between two flat surfaces. Part of the light from the source reflects off the surface of the glass, while another portion passes through the gap and reflects off the sample surface. When these reflected beams overlap again, they form interference fringes.

Fundamentally, bright and dark fringes appear at locations where the gap thickness corresponds to integer or odd multiples of the light’s wavelength. The subtle transitions in these periodic patterns allow detailed analysis of surface distortions and thickness deviations.

Procedure for Flatness Evaluation Using Newton Ring Observation

In practice, an optical flat is first placed on the object being measured, and light is either directly illuminated onto it or observed through a transparent optical flat. The flatness is then estimated by counting and measuring the spacing of the concentric rings formed by the reflected light.

Quantifying flatness involves carefully analyzing the interference fringe patterns obtained over the area of interest. Using dedicated software or comparison charts enables precise verification of flatness at the nanometer scale, which is difficult to achieve by visual inspection alone.

Role and Selection Considerations for Optical Flats

Optical flats are essential items for interferometric measurements, and their selection and handling have a significant impact on measurement accuracy.

Optical flats used in interference measurements are transparent substrates that are polished to extremely high flatness. They effectively provide an “ideal flat surface,” allowing minute gaps between the optical flat and the measured object to be visualized as interference fringes.

Selecting materials with a low coefficient of thermal expansion helps minimize the effects of slight temperature changes on the surface shape. It is important to choose suitable materials, such as fused silica or optical glass, according to the application and measurement environment.

If the surface of the optical flat itself does not maintain high flatness, accurate measurement results cannot be obtained. Surface scratches or the presence of fine particles can disrupt the interference pattern, making careful control of storage conditions and cleaning procedures essential.

What Is an Optical Flat

An optical flat is a flat plate made from materials such as optical glass or fused silica, finished using the highest-level polishing techniques. It functions as an almost perfect flat surface in the visible light range, playing a crucial role in standardizing interference phenomena.

Optical coatings are generally not applied, but in some cases, special coatings may be added to achieve high reflectivity or anti-soiling properties, depending on the application. Understanding the characteristics of each option and selecting the appropriate variation as needed is recommended.

Material and Manufacturing Specifications (Thickness, Parallelism, etc.)

Fused silica, with its excellent thermal properties, is widely used in interferometric measurements because its dimensional changes due to temperature variations are extremely small. Optical glass is also easy to handle and cost-effective; however, its higher thermal expansion requires caution during long-duration high-precision measurements.

In addition to flatness, other factors such as parallelism and surface roughness should also be checked. Thin flats can deform easily, so it is important to select an appropriate thickness based on the usage environment and how the optical flat will be handled.

Storage and Handling Considerations for Optical Flats

To maintain the high optical precision of an optical flat, it is essential to store it in a dust-proof case or a dedicated storage box. When not in use, wrap the flat in a soft cloth or clean paper and handle it carefully to prevent scratches.

Dirt or fingerprints can affect the interference fringes and lead to measurement errors. For cleaning, it is recommended to use alcohol or specialized optical cleaners and to wipe with materials that are unlikely to cause scratches, such as lens tissue or soft lens cloths.

Examples of High-Precision Measurement Using Interferometers

For more advanced measurements, interferometers enable highly precise analysis.

An interferometer works by splitting light and reflecting it along different paths, then observing the resulting phase differences to analyze surface shapes and thicknesses. While based on the same principle as Newton gauges and optical flats, interferometers can capture more complex interference patterns from multiple directions simultaneously in some cases.

A major advantage of interferometers is that certain models can measure large areas at once, and there is abundant software available that automatically analyzes and quantifies interference fringes. Improvements in setup and operability allow some systems to acquire large amounts of data in a short period of time.

Additionally, many modern interferometer designs facilitate not only flatness and parallelism measurements but also three-dimensional shape observations. In research facilities and industries working with advanced technologies, the resolution and measurement speed of interferometers can directly influence development efficiency, making them a highly valued tool.

Basic Structure and Principle of Interferometers

A typical interferometer works by splitting a coherent light source with a beam splitter, directing one beam as a reference light and the other as a sample light, reflecting each, and then recombining them. Phase differences between the two beams are detected as interference fringes, from which surface height variations and film thickness can be determined.

The device often incorporates light sources such as lasers or LEDs that provide stable output at specific wavelengths. Achieving high precision requires careful attention to the coherence of the light source and minimizing vibrations in the sample stage.

Simultaneous Measurement of Flatness and Parallelism

Using an interferometer allows multiple measurement points to be acquired simultaneously, enabling flatness and parallelism to be checked all at once. In processes that require precise alignment, quickly detecting deviations in parallelism helps identify potential processing errors early.

In manufacturing environments with multilayer components, verifying the parallelism of different layers improves process reliability, which is another advantage of interferometers. In fields where even minor errors directly impact the final product’s performance, interferometric measurement is an indispensable tool.

Comparison with Other Measurement Methods

Contact-based measurement devices, such as touch probes, allow relatively easy acquisition of 3D shape data of components. However, when measuring fine surfaces, there is a risk of damaging the sample, and the probe diameter can limit measurement resolution. Interferometric methods, by contrast, do not contact the sample, enabling measurement with minimal impact on the surface condition.

Non-contact optical profilometers or laser scanners also exist, but interferometers are generally better suited for high-precision measurements over wide areas and for evaluating transparent materials. It is important to consider the resolution and variety of the measurement target to select the most suitable method for the intended application.

The Importance of Understanding the Difference Between Flatness and Parallelism

In evaluating geometric accuracy, it is crucial to correctly understand the difference between flatness and parallelism.

Flatness indicates how close a single surface is to being perfectly flat, quantifying variations in the surface profile, including minute bumps and depressions. Parallelism, on the other hand, measures how accurately two surfaces are maintained in a parallel relationship with each other.

A misunderstanding of this distinction can lead to incorrect interpretation of a product’s dimensional chain, potentially affecting assembly accuracy and defect prevention measures. Accurate measurement of both parameters contributes to higher quality and reliability of the final product.

Interferometric measurement provides strong observational capability for both flatness and parallelism, allowing rapid and precise verification even for complex components. Understanding which parameter is more critical helps in selecting the most suitable interferometer for the application.

Definition and Measurement of Flatness

Flatness indicates how closely a single surface approaches an ideal plane. When using an interferometer, the surface profile is estimated from the interference fringes between an optical flat and the sample, and the amplitude of these variations is evaluated to see whether it falls within an acceptable range.

While flatness can also be measured with 3D coordinate measuring machines or optical scanners, methods that utilize light-wave interference, such as Newton rings or interferometers, offer the advantage of non-contact, highly detailed measurement. The appropriate measurement technique should be selected based on the size, shape, and material of the sample.

Definition and Measurement of Parallelism

Parallelism refers to the degree to which two surfaces maintain an ideal parallel relationship, considering allowable tilts or gaps between them. By reflecting interference light from each surface and observing shifts in the interference fringes, precise evaluation of parallelism can be achieved.

This is especially critical for multi-layer optical components, where improper parallel alignment of layers can significantly affect optical performance. Combining interferometers with high-precision stages allows for smooth measurement and adjustment processes.

Optical Material Specifications and Surface Quality

The refractive index and surface quality of the materials used significantly affect the accuracy of interference measurements.

In optical systems, key material properties include refractive index, Abbe number, and transparency. Materials that exhibit significant absorption or scattering in the wavelength range used for measurement can make it difficult to accurately read interference data.

Surfaces with scratches or high roughness can easily distort interference fringes, so it is desirable to select materials that are well-matched in both optical and visual quality. This consideration applies not only to the sample and optical flats but also to all optical elements within the measurement system.

For high-power laser-based interference measurements, neglecting a material’s laser resistance can lead to thermal damage or changes in refractive index. In particular, for long-term laser exposure in research, development, or production lines, selecting glass or crystal materials with high light resistance is crucial.

Effects of Refractive Index and Lens Power

If the refractive index is not appropriate, the light path can change significantly, affecting both the appearance and intensity of the interference fringes. This is particularly important when observing at oblique incidence angles or when measuring birefringent materials, where pre-measurement correction calculations are often required.

Even small lens effects, such as curvature (optical power) or slight tilts, can reduce measurement accuracy. The more precision is sought in interferometric measurements, the more crucial it becomes to thoroughly understand these optical properties.

Evaluation of Surface Quality and Roughness

In high-precision interferometric measurements, even minor scratches or contamination on the sample surface can disturb the interference fringes, so it is essential to thoroughly manage surface quality before measurement. Ideally, handling should be done in environments with minimal dust and contamination, such as cleanrooms.

Parameters indicating surface roughness (e.g., Ra, Rz) should be assessed prior to conducting interferometric measurements, as this facilitates smoother analysis of the results. For surfaces that are relatively rough, alternative measurement methods may be more suitable, so it is important to clearly define the measurement objectives and choose the appropriate technique.

Key Points for Checking Abbe Number and Laser Resistance

The Abbe number is an indicator of a material’s dispersion characteristics, quantifying how focal length and refractive index vary with the wavelength of the light source. When stable measurements are required across a broad wavelength range, it is advisable to select materials with a high Abbe number.

In experiments or inspection lines using high-power lasers, glasses and crystals exhibit varying levels of laser resistance. To avoid thermal distortion or damage, it is essential to verify in advance the allowable irradiation intensity and compatibility with the intended wavelength range.

Applications of Newton Gauges, Newton Rings, and Optical Flats

Here, we introduce specific examples of how these techniques are applied across various industries.

Interferometric measurements using Newton gauges and Newton rings are in demand not only in semiconductor manufacturing and precision component processing but also in fields such as medical devices and optical communications. This is due to the increasing need for nanometer-scale dimensional control as microfabrication technologies continue to advance.

In recent years, with the rise of high-performance digital cameras and lenses, there is a growing demand for precise surface accuracy and uniform coatings on optical components. Interferometric measurements are one of the most reliable methods for ensuring such quality.

In research and development settings, a wide variety of interferometers are used to evaluate the optical properties of new materials and to analyze the fine structures of samples. Collecting multidimensional measurement data in this way helps shorten development cycles and accelerate innovation.

Use in Research and Development

In the development of new materials and during the prototyping of optical components, interferometric measurements are essential. By accurately assessing surface precision at the product launch stage, improvement cycles can be executed quickly, contributing to faster R&D progress.

Furthermore, when combined with other measurement devices such as 3D coordinate measuring machines or spectrometers, interferometry allows for comprehensive evaluation of dimensions, optical properties, and material characteristics. This integrated approach significantly enhances overall quality and reliability.

Summary

To make the most of the technologies and methods introduced so far, proper understanding and careful management are essential.

Interferometric measurements, represented by Newton gauges, Newton rings, and optical flats, utilize phenomena at the wavelength level of light to obtain highly precise information about surface flatness and thickness. Being non-contact and capable of extremely fine measurements, these techniques have become indispensable across a wide range of industries.

However, the full potential of these methods can only be realized by comprehensively understanding and managing multiple factors, including the measurement environment, material and glass selection, and data analysis techniques. By diligently optimizing each of these aspects, it becomes possible to accurately measure in the nanometer scale.