Line Lights for Web Inspection: Maximizing Throughput in Continuous Production

Web inspection runs at high speed in continuous production. Paper, film, foil, textiles and nonwovens all move as a wide, endless web. Therefore, line scan cameras are the natural choice for this task. However, these cameras place very strict demands on the lighting. This page explains those demands and lists the matching RODER Vision illuminators.

The goal is constant, uniform and stable light across the full web width. As a result, real defects stand out while false signals stay low. Moreover, the system must hold this performance for hours or even days. Consequently, throughput and quality both depend on the line light.

Line Scan Camera Illumination Requirements

Line scan cameras work at line rates from a few kilohertz to over 100 kHz. Therefore, each line exposure is extremely short. At 50 kHz, for example, each line is exposed for about 20 microseconds. At 100 kHz, that drops to roughly 10 microseconds.

This short window has a clear consequence. Specifically, the light must deliver enough photons during each brief exposure. Otherwise, the image stays dark and noisy. Therefore, line lights for web inspection need very high intensity along a narrow strip. As a result, the camera reaches the target grey level even at full speed.

Line Light Geometry for Web Inspection

The geometry of the line light decides which defects become visible. In practice, three fundamental geometries are used. These are bright-field, dark-field and transmitted illumination. Each one reveals a different class of defect, so the choice depends on the target flaw.

Bright-Field Illumination

In bright-field mode, the light reflects directly into the camera. Therefore, a flat, intact surface looks bright and uniform. However, a scratch or pit scatters light away from the lens. As a result, that defect appears dark against a bright background. This geometry suits general surface checks on smooth webs.

Dark-Field Illumination

Dark-field mode uses a low, grazing angle instead. Consequently, a smooth surface reflects light away and looks dark. However, raised edges, embossing and particles scatter light back into the lens. Therefore, these features light up brightly. This geometry excels at fine surface texture and small raised defects.

Transmitted Illumination for Transparent and Translucent Webs

Some webs let light pass through them. Clear film, thin paper, woven fabric and nonwovens are common examples. For these, transmitted light gives the best contrast. Here the line light sits below the web and the camera above. Therefore, thickness changes, holes, inclusions and contamination show as brightness variations.

Uniformity Across the Web Width

Uniformity is the most critical specification for web line lights. Any change in brightness along the line appears in the image. Consequently, it shows as a grey-level variation across the web width. This false variation can hide real defects. Equally, it can trigger false alarms across the profile.

Therefore, the illuminator must stay even from end to end. A high-uniformity line light keeps the background flat. As a result, the defect threshold can be set tightly. In turn, the system detects small flaws without raising false rejects.

Synchronisation: Continuous Mode vs Strobe Mode

Line scan systems most often use continuous illumination. In this mode, the camera integration time sets the exposure. Meanwhile, the light simply stays on. Therefore, no trigger signal is needed for the illuminator, which keeps the setup simple.

However, continuous mode places a heavy demand on stability. Specifically, the output must stay constant for the whole run. That run may last hours or days on large rolls. For very high speeds, strobe mode is an alternative. Then the light pulses in step with each line, which freezes fast motion.

Wavelength Selection for Web Materials

The best wavelength depends on the web and the defect. For most opaque webs, red or near-infrared light works well. Typically, this covers the 625 nm to 850 nm range. Because silicon sensors respond strongly here, the signal-to-noise ratio stays high.

This choice has a second benefit. Specifically, longer wavelengths reduce the visibility of web texture. As a result, subtle surface defects are not masked by the material pattern. For other tasks, blue or white light may give better contrast. Therefore, the wavelength should match the defect type.

Thermal Stability in Long-Duration Runs

Long runs expose a hidden risk. As LEDs heat up, their output tends to drift. Consequently, the grey level can change during the inspection. Therefore, thermal management is essential for stable results.

RODER Vision line illuminators include HTTM© thermal management technology. The HTTM system controls the LED junction temperature from switch-on. Therefore, the unit reaches equilibrium quickly and holds steady output. As a result, no warm-up delay is needed and the brightness stays constant throughout the run.

RODER Vision Illuminator Families for Web Inspection

The families below suit line scan web inspection in continuous production. Each one offers high intensity, tight uniformity and stable output. Therefore, the right choice depends on the geometry and the web material.

The right choice always starts from the defect and the material. For opaque surface defects, the DL5 and DL6 matrix illuminators fit best. Transparent webs instead favour the BL3 backlight, which gives strong transmitted contrast. Glossy finishes call for the FD2 flat dome, since it reduces glare. In every case, RODER Vision provides engineering support to size the illuminator to the web width and the camera. Therefore, define the speed, the web and the defect first, and then select the matching light.

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