Vision Lighting for Glass and Transparent Material Inspection

Glass and transparent materials are the final frontier of machine vision. Unlike opaque objects, they transmit, refract and polarise light. Therefore, a high-resolution camera alone is not enough. Instead, success depends on the right lighting strategy. This page explains the main techniques for transparent inspection and links the matching RODER products.

The stakes are high across industries. For example, pharmaceutical vials, automotive windshields and optical lenses all demand zero defects. Consequently, the lighting decides whether a critical flaw is caught or shipped. As a result, geometry, wavelength and polarisation must be chosen with care.

The Unique Challenges of Transparent Inspection

Glass is designed to let light pass through. Therefore, standard front lighting is almost useless. Moreover, three problems recur on every transparent part. Specifically, they involve reflection, contrast and refraction.

• Specular reflection: polished surfaces create hotspots that blind the sensor.
• Low contrast: clear bubbles and scratches share the background colour.
• Refraction: curved glass bends light and mimics false defects.

Transmission Mode: Backlight for Internal Defects

Transmission mode is the most common, effective setup. Here a uniform LED backlight sits directly behind the object. Consequently, the system works by light occlusion. Therefore, any internal impurity stands out clearly.

As the light passes through the glass, a stone, inclusion or bubble blocks it. As a result, the defect appears as a dark spot on a bright field. In turn, the software triggers an instant reject. Moreover, this is the standard method for fill level and contaminant checks in food and beverage.

Reflection Mode: Coatings and Surface Scratches

Transmission looks through the glass, but reflection looks at the surface. Therefore, reflection mode suits coatings and fine scratches that do not cross the thickness. For example, anti-reflective and hydrophobic layers need this approach.

The light strikes the surface at a chosen angle. Consequently, the camera captures the reflected beam. Then, any coating disruption or scratch alters the pattern. As a result, the defect shows up as a bright glint or a dark void.

Darkfield for Inclusions and Micro-Bubbles

Sometimes defects are smaller than 0.1 mm. In that case, plain backlight can wash them out. Therefore, darkfield illumination is the better tool. Here the source sits at a low angle, so direct light misses the lens.

In perfect glass, the camera sees a black image. However, a micro-bubble or inclusion scatters light into the lens. As a result, the defect glows brightly against a pitch-black background. Consequently, this method excels at high-precision optical inspection.

Edge Lighting for Flat Glass Panels

Large flat panels suit edge lighting, also called side lighting. Here light is injected into the glass thickness from the sides. Because of total internal reflection, the light stays trapped inside the panel.

However, a crack, chip or etched mark frustrates this reflection. Therefore, light escapes exactly at that point. As a result, the defect lights up brightly. Consequently, the system finds structural cracks that could cause breakage.

SWIR and Polarised Light for Hidden Flaws

Some defects stay invisible to the eye and to standard sensors. Therefore, short-wave infrared imaging works in the 900 to 1700 nm range. For example, it sees through plastics that block visible light. In addition, it detects moisture or liquid levels through dark glass, and certain infrared-reactive coatings.

Internal stress is another hidden risk. Because glass is a frozen liquid, fast cooling builds tension. Consequently, these stresses can shatter the part later. However, polarised light reveals them. By placing the glass between crossed polarisers, stress appears as coloured fringes. As a result, high-tension parts are rejected before assembly.

RODER Vision Products for This Application

Transparent inspection relies above all on uniform, stable backlights. Therefore, the four backlight series below are the recommended choices. Each one offers high uniformity, stable current control and rugged housings.

Surface, scratch and stress checks need other modes. Therefore, RODER also supports reflection, darkfield and polarised setups. For these, use LED Ring Illuminators, LED Bar Illuminators and dedicated optical filters and polarisers. Beyond catalogue options, RODER Vision provides engineering support for SWIR and custom geometries. In practice, the right mode turns invisible glass flaws into clear signals. Therefore, define the defect and the material first, and then choose the matching illuminator.

Frequently Asked Questions