Thickness measurement of multi-layered glass

Paul Boughton

Laminated safety glass, solar panels, flat screens and smartphone displays comprise multiple layers of different transparent materials. Measuring the exact thickness of these individual layers, as well as any air gaps between these layers, during production is a critical but physically complex process and a challenge for measurement technology.

In the manufacture of multi-layer curved glass for aircraft, automotive and architectural applications, precise thickness measurement of the individual layers, and combined overall laminate, are required. If the curvature of the different layers is not precise, when the layers are sandwiched together there could be a risk of stress points developing within the glass, which in turn could lead to defects or delamination of the glass.

For quality inspection and process control of manufactured transparent multi-layer materials, Micro-Epsilon has developed a range of confocal chromatic displacement sensors and controllers, which together, provide multi-peak measurement capabilities for multi-layered glass. The result is improved product quality and increased production yields for manufacturers.

Confocal chromatic sensors offer one-sided thickness measurement and distance measurement of transparent materials such as glass. These sensors work by focussing polychromatic white light onto the target surface using a multi-lens optical system. The lenses are arranged in such a way that the white light is dispersed into a monochromatic light by controlled chromatic deviation. A certain deviation is assigned to each wavelength by a factory calibration. Only the wavelength that is exactly focussed on the target surface or material is used for the measurement.

This light reflected from the target surface is then passed via a confocal aperture to the receiver, which detects and processes the spectral changes. This unique measuring principle enables displacements and distances to be measured very precisely – to nanometre accuracy.

Both diffuse and spectral surfaces can be measured and with transparent materials such as glass, a one-sided thickness measurement can be accomplished along with the distance measurement. And, because the emitter and receiver are arranged in one axis, shadowing is avoided. The white light technology also provides greater stability compared to laser-based technologies, particularly against glass or mirrored surfaces.

Micro-Epsilon’s confocalDT IFS 2405 series of confocal sensors are designed for measurement tasks that require maximum precision – typically R&D, laboratory, quality and production applications. The sensors provide large stand-off distances (up to 100mm), providing users with greater flexibility in terms of the variety of applications in which the sensor can be used. In addition, the tilt angle of the sensor has been increased significantly (up to 34 degrees), which provides better performance when measuring across changing surface features.

The IFS 2405 series comprises five sensors with measuring ranges from 0.3mm up to 30mm. Spot diameter is from 6µm up to 50µm. Maximum resolution is 0.01µm and maximum linearity is 0.3µm.

The confocalDT IFS 2405 series is offered alongside Micro-Epsilon’s confocalDT 2451 controller, which offers a range of outstanding technical benefits, including faster measuring rates, improved signal-to-noise ratios, and up to 100 times faster real time surface compensation for difficult-to-measure surfaces, including mirrored surfaces and glass.

The controller’s integrated multi-peak software enables users to measure the thickness of multi-layer glass. The software is able to evaluate up to five layers by evaluating six measurement values on the boundary areas. In order to accurately determine the thickness of each layer, the controller retrieves the refractive index of each material layer from a database. Each refractive index is corrected depending on the wavelength: three refractive values are used for thickness calibration (beginning, end and middle of the layer).

The controller is simple to set up and operate. The user can remotely configure measurement channels using the intuitive, web-based software via Ethernet and the user does not need to install any other measurement software.

The controller utilises either a white LED as the light source, which enables measuring rates up to 10kHz, or an external Xenon light source that provides up to 70KHz measuring rate. Ethernet, EtherCAT, RS422 and analogue output interfaces are provided.

Most conventional confocal controllers use an integral fan and air vents in order to cool the controller during operation. The confocalDT 2451 uses a passive cooling system that does not require a fan or air vents, which greatly improves the controller’s protection rating and longevity.

Traditionally, most confocal controllers perform poorly when trying to compensate for difficult and changing surface conditions, particularly in high speed surface scanning tasks. However, the confocalDT 2451 controller uses newly developed software algorithms based on Micro-Epsilon’s experience in the design of optical laser sensors. These algorithms enable the controller to compensate in real time, enabling users to scan surfaces very rapidly at high resolution. The controllers also provide high speed triggering that allows them to be synchronised with encoders and other motion control devices. The result is a controller that provides more stable, higher accuracy measurements, down to nanometre resolution if required.

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