Sector-Controlled Hemispherical Illumination for Direction-Sensitive Inspection
- 4, 8 or 16 independently driven sectors mounted on the inner surface of a hemispherical dome.
- All-sectors-on mode produces standard cloudy-day dome; single-sector mode produces directional illumination.
- Radial slices control azimuth; concentric rings control elevation; combined arrangements deliver 2D angular control.
- Best fit for high-end photometric stereo, directional defect classification, machining-trace and weave inspection.
- Sub-100 μs per-sector switching enables photometric stereo at 100+ fps with 4–8 sectors.
- 3–5x cost premium over single-channel domes — value realised only through application-specific algorithms.
Spherical multi-segment illuminators divide a hemispherical dome into independently controlled sectors, providing fine spatial control over the direction of incidence at the target. By selectively activating one or more sectors, the illuminator can generate any combination of directional and omnidirectional lighting from the same fixture, supporting advanced inspection techniques such as photometric stereo, direction-sensitive defect classification and adaptive illumination optimisation. The multi-segment dome bridges the gap between standard dome illumination (which is symmetric) and directional illumination (which is fixed), offering the flexibility of both in a single unit.
Working Principle of Multi-Segment Domes
A spherical multi-segment illuminator consists of a hemispherical structure with multiple independently driven LED sectors mounted on the inner surface. Typical configurations divide the hemisphere into 4, 8 or 16 sectors arranged in concentric rings or in radial slices. Each sector is controlled by an independent LED driver that can be activated, deactivated or dimmed independently of the other sectors. Application-specific multi-segment dome assemblies are engineered within the Custom LED Illuminators portfolio.
When all sectors are activated simultaneously, the multi-segment dome produces standard cloudy-day illumination equivalent to a uniform dome. When only one sector is activated, the dome produces directional illumination from a specific azimuthal and elevation angle. By cycling through individual sectors in sequence, the system performs photometric stereo with the angular precision determined by the sector resolution.
Sector Arrangement and Angular Resolution
The arrangement of sectors determines the spatial resolution of the illumination control. Radial slice arrangements provide azimuthal directional control suitable for direction-sensitive surface defect inspection. Concentric ring arrangements provide elevation control suitable for switching between bright field and dark field on the same fixture. Combined radial and concentric arrangements provide full two-dimensional angular control, supporting the most sophisticated photometric stereo and adaptive illumination strategies.
Typical Industrial Applications
Spherical multi-segment illuminators are essential for high-end photometric stereo inspection where surface normal reconstruction requires precise angular control of the illumination; quality control of complex three-dimensional targets where adaptive illumination must respond to the specific shape of each part; inspection of directionally oriented surface features such as machining traces, weave patterns and grain structures; verification of embossed and engraved features where the optimal illumination direction depends on the orientation of the feature; quality control of polished and finished surfaces where the optimal angle for defect visibility varies across the surface; research and development applications requiring full control of the illumination geometry; and any application that combines the requirements of dome illumination (shadow-free imaging) with the requirements of directional illumination (feature enhancement).
Selection Criteria and Design Considerations
The number of sectors is the primary specification. More sectors provide finer angular resolution but require more LED drivers and more complex control electronics, with corresponding cost implications. Typical industrial multi-segment domes use 4 to 16 sectors, which represent the practical sweet spot between flexibility and cost.
The switching speed of the sector drivers determines the maximum acquisition rate for sequential multi-sector imaging. Industrial-grade controllers support switching times below 100 microseconds per sector, allowing photometric stereo at camera frame rates above 100 fps when using four to eight sectors. Multi-channel sector drivers are part of the RODER LED drivers and electronic controllers catalogue.
Calibration and Algorithmic Considerations
Multi-segment dome systems require calibration of the angular position and the intensity of each sector relative to the camera. Calibration uses a reference target imaged under each sector individually, with the resulting parameters stored in the vision controller. The calibration must be updated periodically to compensate for LED ageing and any mechanical drift in the dome geometry.
The image processing algorithms exploit the multi-sector capability through techniques such as photometric stereo, direction-sensitive defect classification and adaptive illumination optimisation. Each technique requires specific software development tailored to the application, and the value of the multi-segment dome is realised primarily through the algorithmic exploitation of its capabilities.
Integration and Limitations
Spherical multi-segment illuminators integrate as standalone fixtures with multi-channel LED drivers, typically controlled through a single digital interface to the vision controller. The physical dimensions are comparable to those of standard dome illuminators of equivalent active area, with the added depth of the multi-channel driver electronics.
The principal limitation of multi-segment domes is the cost, which scales with the number of sectors and the sophistication of the control electronics. A 16-sector dome can cost three to five times more than a comparable single-channel dome, which limits its adoption to applications that genuinely require the multi-sector capability.
The second limitation is the algorithmic complexity required to exploit the multi-segment capability. Without sophisticated image processing software, a multi-segment dome operates as a standard dome and provides no additional value compared to a single-channel equivalent. Investment in the algorithm and software development is essential to justify the cost of the multi-segment fixture.
RODER Vision Spherical Multi-Segment LED Domes
RODER Vision engineers application-specific spherical multi-segment LED dome assemblies with multi-channel sector drivers for industrial vision inspection requiring direction-sensitive illumination control.
- Application-specific 4, 8 or 16-sector multi-segment dome assemblies — Custom LED Illuminators
- Multi-azimuthal ring geometries as a compact alternative for sector-controlled inspection — LED Ring Illuminators
- Multi-channel sector drivers with synchronised microsecond switching — LED Drivers and Electronic Controllers
Multi-segment dome installations require shielded multi-channel cabling for reliable sector synchronisation — the RODER catalogue includes industrial-grade cables and fastening systems engineered for advanced multi-sector deployments.