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Square Continuous Diffuse Illumination (SCDI)

Square-section continuous diffuse illumination tunnel with a glass bottle inside, camera viewing from above through rectangular aperture

Cloudy-Day Diffuse Illumination Along Elongated Reflective Targets

  • Square-section diffusing tunnel surrounds the object from four sides with longitudinal invariance.
  • Light arrives from every direction visible to each point along the full length of the target.
  • Camera mounts on top, side or end depending on the inspection geometry.
  • Inner cross-section should be at least 3 to 4 times the largest target dimension.
  • Dominant geometry for bottles, vials, cans, extruded profiles and cylindrical packaging.
  • Strobed and line-scan variants for high-speed inspection on continuous conveyors.

Square Continuous Diffuse Illumination, commonly abbreviated SCDI, extends the cloudy-day effect of dome illumination to elongated targets that cannot be inspected inside a hemispherical fixture. By wrapping the inspection area with a square-section diffusing tunnel open at the front and rear, SCDI delivers uniform diffuse light from all sides along the full length of an elongated object such as a glass bottle, a metallic profile or a cylindrical container.

Working Principle of SCDI

An SCDI fixture is constructed as a square or rectangular tube with diffusing inner walls, LEDs mounted in concealed positions and a camera aperture either above or to one side. The inspected object travels through the tunnel along its longitudinal axis, receiving light from all four inner walls simultaneously. Because the geometry is invariant along the tunnel axis, the illumination at every position along the length of the object is identical, producing a uniform inspection condition that does not depend on the position of the target within the tunnel.

The diffusing walls are typically lined with a high-reflectance, high-diffusion white material that scatters the LED emission across the full interior volume. The resulting light field combines the angular characteristics of a hemispherical dome (illumination from every direction visible to each point of the target) with the longitudinal invariance required for continuous inspection of elongated parts.

Geometric Relationship Between Tunnel and Target

The cross-section of the tunnel must accommodate the largest expected target with sufficient margin to maintain uniformity. As with a dome, the inner dimensions should be at least three to four times the maximum cross-sectional dimension of the inspected object. The length of the tunnel determines how much of the target is uniformly illuminated at any moment and is typically chosen to cover the field of view of the camera with a small margin at the leading and trailing edges.

Typical Industrial Applications

SCDI is the dominant illumination geometry for inspection of cylindrical containers in the beverage industry, including glass and PET bottles for surface defects, foreign objects, fill level and label registration; quality control of metal cans for dents, scratches, seam integrity and printed labels; inspection of pharmaceutical vials and syringes for cracks, contamination and dimensional anomalies; surface inspection of extruded aluminum, plastic and rubber profiles; and quality control of reflective elongated industrial products where uniform shadow-free illumination is required along the full length of the part. SCDI fixtures are engineered as application-specific units within the Custom LED Illuminators portfolio.

Selection Criteria and Design Considerations

The cross-section of the tunnel must accommodate the target with adequate margin, and the length must accommodate the camera field of view. The position of the camera aperture, either at the top, on one side or at the end of the tunnel, determines the inspection geometry. A top-mounted camera with a side-traversing target provides line-scan compatibility for continuous web-like inspection. An end-mounted camera with the target advancing along the tunnel axis provides through-tunnel imaging of cylindrical containers.

Spectral content is selected according to the target. White SCDI fixtures are standard for colour inspection of packaging and printed labels. Monochromatic versions, typically red or near-infrared, are preferred when narrowband filtering at the camera is used to reject ambient light, or when the target requires a specific wavelength for optimised contrast.

Strobed Operation and Line-Scan Compatibility

SCDI fixtures support strobed operation, which is essential for high-speed inspection lines where containers travel at several hundred per minute. The strobed pulses, synchronised with the camera exposure, freeze motion within microseconds and deliver the photon flux required for short exposure times. Line-scan compatible SCDI variants integrate continuous high-intensity illumination matched to the line-scan acquisition rate, providing constant illumination of the active line as the target advances through the tunnel.

Integration and Limitations

The principal mechanical constraint of SCDI is the need to convey the inspected object through the tunnel without contact with the inner walls, while preserving consistent positioning relative to the camera optical axis. Conveyor design and guide rails must be coordinated with the tunnel dimensions, and any vibration or mispositioning of the target degrades the uniformity of the image.

SCDI is less efficient than directional illumination in terms of photon utilisation because the diffused emission spreads light in all directions. For applications that require very short exposure times on low-reflectance targets, alternative geometries such as multiple strobed bar lights or focused panel illuminators should be evaluated. SCDI remains the optimal choice when the target is elongated, reflective and benefits from longitudinal invariance of the illumination, conditions that are common throughout the beverage, pharmaceutical and packaging industries.

RODER Vision SCDI and Tunnel Illuminators

RODER Vision engineers application-specific SCDI and tunnel illumination units for industrial inspection of elongated reflective targets in the beverage, pharmaceutical and packaging industries. Each unit is sized to the target dimensions, conveyor speed and inspection geometry of the specific production line.

For high-speed inspection lines requiring strobed or line-scan synchronised operation, the RODER catalogue includes dedicated LED drivers and electronic controllers compatible with vision controllers and PLCs.