Compact Coaxial Diffusion for Tight Integration Spaces
- A 45 degree internal beamsplitter redirects side LED light downward through the bottom face.
- Delivers full-dome-equivalent diffusion in a low-profile slim aluminum package.
- Camera looks down through a transparent top window along the same coaxial axis as the illumination.
- Ideal for confined inspection cells where full dome vertical clearance is unavailable.
- Beamsplitter introduces approximately 4× photon flux penalty — compensated by higher LED density or strobed operation.
- Combine with cross-polarisation for highly polished coated metals and high-gloss laminates.
Flat dome lights are a compact alternative to hemispherical dome illumination, delivering equivalent diffuse coaxial illumination from a low-profile fixture suitable for confined inspection cells. By combining a semi-reflective beamsplitter with side-emitting LED arrays inside a slim aluminum housing, the flat dome reproduces the cloudy-day effect of a full dome on flat or moderately curved surfaces without the vertical clearance that a hemispherical dome requires.
Working Principle of Flat Dome Lights
A flat dome is built around a 45-degree semi-reflective glass or polymer beamsplitter housed inside a slim rectangular enclosure. LEDs are mounted on one or more inner walls of the housing, directing their emission onto the diffuser, which reflects a portion of the light downward onto the inspected target and transmits the remaining fraction harmlessly to the inactive side. The camera looks through the beamsplitter from above, viewing the target along the same axis as the descending diffuse illumination.
The geometric effect is similar to that of a hemispherical dome: light arrives at the target from a wide range of angles and the apparent brightness of each surface point depends primarily on its diffuse reflectance, not on its orientation. Compared to a full dome, the flat dome covers a slightly smaller solid angle, which makes it most effective on flat or weakly curved targets. On strongly curved or three-dimensional specular targets, the full dome remains preferable.
Beamsplitter Quality and Image Loss
The beamsplitter introduces a transmission loss of approximately 50 percent in both directions: half the LED emission is lost to the inactive side, and half the image signal is lost during the return path from the target to the camera. This 4x flux penalty must be compensated by higher LED density, longer exposure times or strobed operation. High-quality beamsplitters with optimised splitting ratios can reduce this penalty by tuning the reflection-transmission balance to favour the LED emission path, at the cost of slightly higher specular contamination of the image.
Typical Industrial Applications
Flat dome lights are essential for inspection of flat reflective surfaces in confined cells where a full dome cannot be installed: quality control of stamped metal sheets, photovoltaic cells and printed circuit boards; OCR and code reading on shiny labels and embossed packaging; inspection of polished metal components such as razor blades, surgical instruments and coins; verification of coated and laminated products; quality control of mirror-finished plastic parts; and applications where multiple inspection stations must be tightly packed and vertical clearance is limited. The dedicated LED Flat Dome Illuminators family is engineered specifically for this geometry.
Selection Criteria and Design Considerations
The active area of the flat dome must accommodate the full field of view of the camera lens with adequate margin to ensure uniform illumination across the inspection field. The vertical distance between the beamsplitter and the target determines the apparent solid angle of illumination at the surface: shorter distances produce more dome-like diffusion, while longer distances reduce the effective solid angle and shift the configuration toward standard coaxial illumination.
Combination with Polarisation
Flat domes are frequently combined with cross-polarised filters to further suppress residual specular reflections on highly polished surfaces. A linear polariser is placed inside the housing between the LEDs and the beamsplitter, and a crossed analyser is mounted on the camera lens. This configuration delivers diffused coaxial illumination with active glare rejection, which is particularly effective on coated metals and high-gloss laminates.
Integration and Limitations
The low profile of flat dome lights makes them especially attractive for multi-station inspection cells, robotic end-effector cameras and any application with strict mechanical constraints. Integration is generally straightforward, with the flat dome bolted to the camera mounting and the camera looking through the dedicated aperture.
The principal optical limitation is the photon flux penalty introduced by the beamsplitter, which makes flat domes less suitable for high-speed inspection of low-reflectance targets. The second limitation is the reduced solid angle of illumination compared to a full dome, which can leave residual specular reflections visible on strongly curved targets. For these cases, either a full dome or a combined dome-and-coaxial fixture should be considered. Flat domes excel on flat and weakly curved targets where their compactness and ease of integration provide the best practical trade-off.
RODER Vision LED Flat Dome Illuminators
RODER Vision manufactures a dedicated line of compact coaxial diffuse illuminators engineered for industrial inspection of flat reflective targets in confined cells. The portfolio includes standard sizes for general-purpose inspection plus application-specific configurations for OEM integration and multi-station cells.
- Compact coaxial diffuse illumination for flat reflective targets — LED Flat Dome Illuminators
- Full hemispherical dome for strongly curved targets where flat dome is insufficient — Custom LED Illuminators
- Diffuse front panel as alternative for non-coaxial geometries — LED Panel Illuminators
For high-speed inspection lines requiring synchronised strobed operation to compensate the beamsplitter flux penalty, the RODER catalogue includes dedicated LED drivers and electronic controllers and industrial cables and fastening systems compatible with industrial machine vision controllers.