Glass, Plastics & Transparent Materials

Transparent and semi-transparent materials are among the hardest subjects in machine vision. Because light passes straight through them, standard front lighting produces weak, ambiguous contrast. Moreover, the same surface both transmits and reflects, which confuses simple setups. Therefore, specialised geometries and wavelengths are usually required. This sector hub introduces the inspection tasks for glass, plastics and clear films. In addition, it links each application note to the recommended technique and to the matching RODER Vision products. In short, it is the entry point for engineers lighting glass, PET, acrylic, polycarbonate and clear films. Moreover, it gathers the sector know-how in one structured place.

Why Transparent Materials Are Difficult

Low contrast is the first problem. A clear defect inside a clear part reflects very little light. Consequently, it almost disappears under ordinary illumination. Therefore, the lighting must convert the defect into a bright or dark signal that the camera can detect.

Transmission is the second problem. Light travels through the body of the material, so reflections come from several surfaces at once. However, a backlight turns this behaviour into an advantage. By placing the source behind the part, internal flaws appear as clear shapes against a bright field.

Surface glare is the third problem. Glossy plastic and polished glass create strong specular highlights. As a result, scratches and coating defects can hide in the glare. For this reason, dark field, diffusion or polarisation are often added to the design.

The Main Inspection Tasks

This sector groups several recurring tasks. First, internal-defect detection finds cracks, inclusions and bubbles inside the material. Next, surface inspection looks for scratches, coating flaws and contamination. In addition, completeness checks confirm that moulded plastic parts are fully formed.

Dimensional control is equally relevant. Here a backlight gives the sharp silhouette needed to measure clear parts. Furthermore, clarity and haze checks assess optical quality on lenses and films. Finally, foreign-body detection finds particles trapped inside transparent containers or liquids.

Injection moulding adds a specific task. Short shots leave a cavity partly unfilled, which weakens the part. Because the missing material changes how light passes through, dark field exposes the defect clearly. Therefore, completeness inspection is a frequent moulding requirement.

Recommended Illumination Techniques

Each task maps to a specific geometry. For internal flaws and dimensions, backlight illumination gives the cleanest, highest-contrast image. To reveal scratches and short shots, dark field lighting lights only the defects against a dark background. On glossy surfaces, diffuse lighting suppresses glare.

Wavelength opens further options. For example, short-wave infrared sees through some plastics that block visible light. By contrast, ultraviolet can excite fluorescence in coatings and contaminants. Therefore, the spectrum is chosen to match the material and the defect.

Polarisation completes the toolkit. Since glass and gloss create polarised reflections, a polariser and analyser remove much of the glare. Consequently, faint surface defects regain contrast. In addition, a stable driver keeps brightness constant for repeatable thresholds.

Materials, Speed and Reliability

Different materials behave very differently. Glass, PET, acrylic and polycarbonate each transmit and reflect in their own way. Consequently, the lighting recipe must be tuned per material and per defect. Nevertheless, the underlying principles stay the same across the sector.

Speed and stability matter as always. Because many clear products run on fast lines, pulsed lighting freezes motion and avoids blur. Moreover, current-stabilised drivers keep contrast constant over long shifts. As a result, marginal defects are caught consistently rather than intermittently. In practice, a short validation trial on real samples confirms the best recipe before deployment.

Application Notes in Glass, Plastics & Transparent Materials

The following application notes describe real transparent-material inspections. In each case, the note explains the problem, the recommended lighting and the related RODER products.

Matching RODER Vision Products

Most transparent-material tasks use a focused set of illuminator families. For internal flaws and dimensions, choose LED Backlight Illuminators. To create dark field on surface defects, use low-angle LED Ring Illuminators or LED Bar Illuminators. For focused or specialty wavelengths, consider LED Spot Illuminators.

Where SWIR or UV is needed, RODER Vision supplies custom configurations on request. Beyond catalogue options, the company also provides engineering support for OEM and custom geometries. In practice, the right mix of backlight, dark field and wavelength turns an invisible flaw into a clear signal. Therefore, start from the application note that matches your task, and then follow the links to the technology and product pages.

Frequently Asked Questions