company news about Say Goodbye to Defocus | Spectral Confocal Enables In-line Micron-level Thickness Measurement of Transparent Filmsa

In the film industry, thickness measurement of transparent materials has long been an industry pain point. For PET packaging films, optical protective films and lithium-ion battery separators, micron-level thickness deviations can directly affect light transmittance, mechanical strength and electrochemical performance. Traditional laser triangulation tends to suffer from defocus on transparent materials; ultrasonic methods lack sufficient precision; the gravimetric method only supports offline sampling inspection and cannot realize in-line monitoring. Spectral confocal technology perfectly addresses the challenge of high-precision thickness measurement for transparent films.

Spectral confocal is a high-precision displacement measurement technology based on wavelength encoding. It converts chromatic aberration, a flaw in traditional optics, into a core means of distance measurement. Compatible with a wide range of materials including transparent, translucent, high-reflective and low-reflective surfaces, it achieves submicron measurement accuracy.

White light passes through a dispersive lens assembly. Light of different wavelengths is focused at distinct positions along the optical axis: short-wavelength blue light lies close to the lens, while long-wavelength red light is farther away from the lens. Only light rays that precisely hit the focal point produce strong reflections; all other stray light is filtered out by the pinhole aperture. The spectrometer identifies the central wavelength of the reflected light, from which the position of the measured surface can be calculated. This delivers stable measurement and strong anti-interference performance.

When the light beam hits the transparent film, two reflected signals are generated: The reflection from the top surface forms the first spectral peak. The light penetrates the film and reflects off the bottom surface to form the second spectral peak. The optical path difference is calculated from these two peaks. Dividing this value by the film’s refractive index n yields the actual physical thickness: Physical thickness d = optical path difference ÷ refractive index n

Advantages: It acquires both position and thickness in a single measurement. Non-contact and non-destructive, it requires no machine shutdown and is suitable for in-line monitoring on production lines.

Spectral confocal sensors can improve detection capability for complex surfaces and microstructures via different spot size options:

• Small spot: suitable for measuring tiny targets, fine contours and surfaces with complex profiles.
• Large spot: ideal for rough surfaces and highly scattering materials. It suppresses the influence of local texture to deliver more stable signals.
• Four-spot mode: Multiple light spots perform independent measurements followed by data computation, reducing measurement errors caused by surface unevenness, tilt and local defects.

his advantage enables spectral confocal technology to be suitable not only for thickness measurement of flat films, but also for inspection of protective films, optical coatings, composite films and transparent materials with relatively complex surface structures.

From "struggling with defocused measurement" to "reading distance via wavelength", spectral confocal technology redefines the measurement method for transparent films with a single beam of white light. It exposes micron-level thickness deviations completely, and enables in-line, non-contact, high-precision quality control to be truly implemented on production lines.

As films are manufactured thinner and thinner with ever-higher precision requirements, spectral confocal is more than just a measuring instrument. It serves as critical support for transparent materials to achieve superior quality and higher yield rates.