Focused Linear LED Stripe for Continuous Web and Line-Scan Inspection
- Narrow uniform stripe matched to the active sensing line of a line-scan camera (±10% intensity uniformity).
- Cylindrical focusing optic collapses LED emission into a sharp band a few millimetres wide at the target.
- Foundation of high-speed web inspection of paper, plastic film, textile and metal foil at several metres per second.
- Rotating cylindrical object inspection (printing rollers, shafts, bottles) where part rotation provides the line-scan motion.
- Strobed operation synchronised with the camera line clock supports kHz line acquisition rates.
- Monochromatic and white spectra for selective contrast and ambient light rejection.
Line lights are specialised LED illuminators engineered to deliver a narrow, intense and highly uniform stripe of light matched to the active sensing line of a line-scan camera. The combination of line-scan imaging and line illumination is the foundation of high-speed continuous inspection of webs, conveyors and rotating cylindrical objects, where the inspection geometry naturally aligns with a linear sensor and the production cadence demands constant-rate image acquisition.
Working Principle of Line Lights
A line light typically consists of a row of high-flux LEDs aligned along a metal profile, combined with a cylindrical lens or specialised optic that focuses the emitted light into a narrow stripe perpendicular to the LED row. The optical system collapses the emission angle in the short dimension to a few millimetres at the target plane, while preserving the full length of the LED row in the long dimension. The result is a stripe of light that can be tens or hundreds of millimetres long and only a few millimetres wide at the working distance of the camera.
The intensity distribution along the stripe must be highly uniform, typically within plus or minus 10 percent across the active length, to avoid intensity gradients in the assembled line-scan image. Achieving this uniformity requires careful selection of LED binning, calibrated optical components and, in critical applications, individual channel control to compensate for residual non-uniformity.
Focused and Collimated Line Lights
Focused line lights converge the emitted light to a minimum width at a specific working distance, producing maximum intensity within a narrow depth of focus. They are preferred when the inspected surface is flat and parallel to the camera optical axis. Collimated line lights produce a parallel-sided stripe that is less sensitive to working distance variations but delivers lower peak intensity. The choice between the two depends on the geometry and the stability of the inspected surface.
Typical Industrial Applications
Line lights are essential for web inspection of paper, plastic film, textile and metal foil at speeds up to several metres per second; inspection of printed media for colour accuracy, registration and print defects; quality control of continuous extruded profiles in aluminum, plastic or rubber; surface inspection of large flat panels such as glass, photovoltaic modules and stamped metal sheets, when the panel is moved past a fixed line camera; inspection of rotating cylindrical objects such as printing rollers, shafts and bottles, where the rotation of the part provides the relative motion required by the line camera; and any application that combines a continuous inspection surface with the need for high resolution along one of the two dimensions.
Selection Criteria and Design Considerations
The length of the active line must match the length of the camera sensor at the imaging magnification of the lens, with a small margin at each end to ensure uniform intensity across the full sensor. The width of the stripe at the target must be matched to the depth of focus of the lens and to any variation in the working distance caused by mechanical tolerances of the inspected surface.
Intensity uniformity is the critical specification for line lights. Even small intensity variations across the line translate into vertical streaks in the assembled image that can mask the defects the system is intended to detect. High-quality line lights provide calibrated intensity profiles and, in advanced versions, software-controlled compensation of individual LED channels.
Spectral content is selected as for any directional illumination. Monochromatic line lights at red or near-infrared wavelengths are common in industrial environments where ambient light rejection through narrowband filters is required. White line lights are used for colour line-scan inspection of printed media and packaging.
Strobed Operation and Synchronisation
For line-scan applications above a few thousand lines per second, line lights must support strobed operation synchronised with the camera line clock. Each pulse delivers the photon flux required by the camera during a single line acquisition, while the LED rests between pulses to dissipate heat. This duty-cycle operation enables peak intensities far above what continuous operation would permit, extending the achievable line rate without sacrificing image quality.
Integration and Limitations
The principal integration challenge of line lights is precise alignment with the camera line. A small angular misalignment between the line of light and the line of sensor pixels causes intensity fall-off at one end of the image and saturation at the other. Alignment fixtures, often integrated into the line light housing, simplify this critical adjustment.
Line lights are not suitable for area-scan applications, where the line stripe would illuminate only a fraction of the sensor field of view. For two-dimensional inspection on stationary parts, panel, bar or ring lights should be selected instead.
RODER Vision LED Line Light Solutions
Line illumination for high-speed line-scan inspection is typically delivered through specialised configurations of LED bar illuminators or through custom-engineered fixtures with cylindrical focusing optics. RODER Vision covers both approaches with its bar illuminator family and its custom illuminator engineering service.
For directional linear illumination with adjustable working distance and angle, the LED Bar Illuminators family includes high-intensity matrix configurations such as the DL1 Series and the DL4 Series, which can be deployed in line-scan inspection cells where the bar geometry matches the camera sensor length.
For dedicated cylindrical-lens line lights, very long active lengths, individual channel control or specific spectral content tailored to a line-scan inspection project, RODER Vision offers Custom LED Illuminators engineered to OEM requirements. Pulsed operation synchronised with the line camera clock is supported by the LED drivers and electronic controllers in the RODER accessory catalogue.