Collimated Parallel-Ray Backlight for Sub-Pixel Dimensional Measurement
- Collimated parallel rays emit from confined aperture via Fresnel or multi-element collimating optics.
- Sub-pixel edge definition independent of object position within the design depth — ideal for precision metrology.
- Best fit for stamped parts, machined components, gear profiles, pharmaceutical container metrology at micrometre accuracy.
- Pair with telecentric lens to complete geometric invariance against depth-of-field position.
- Telecentric angle below 1° for maximum edge sharpness; limits the design depth proportionally.
- Mechanical depth 2–3x larger than equivalent diffuse backlight due to collimating optics geometry.
Telecentric backlight illumination delivers collimated parallel rays from a confined exit aperture, producing the sharp edges with sub-pixel definition that high-precision dimensional measurement applications require. By emitting light only along directions parallel to the optical axis of the imaging system, a telecentric backlight eliminates the edge-blooming effects that limit the accuracy of diffuse backlights and enables measurement repeatability at the micrometre level.
Working Principle of Telecentric Backlights
A telecentric backlight is built around an LED source positioned at the focal plane of a collimating optical assembly, typically a Fresnel lens or a multi-element refractive system. The collimating optics convert the divergent LED emission into a beam of parallel rays that exit the backlight in a narrow angular range, typically a few degrees from the optical axis. The result is a beam that behaves geometrically like the light from an infinitely distant point source. Diffuse-backlight alternatives are available as standard within the LED Backlight Illuminators family, while telecentric variants are engineered on demand within the Custom LED Illuminators portfolio.
When the target is placed in this collimated beam, only rays parallel to the optical axis can pass around the object and reach the camera. Rays diverging from the optical axis miss the camera aperture and are not seen. The geometric occlusion produced by the object is therefore independent of the position of the object within the field of view, and the edges of the object appear with sub-pixel sharpness regardless of the depth at which the object is located.
The Geometric Advantage of Collimation
The principal advantage of collimated backlight illumination is the elimination of edge blooming caused by light rays grazing past the object. In a standard diffuse backlight, light reaches the object from a wide range of angles, and the rays at angles other than parallel to the optical axis can pass around the edges of small features and reach the camera, producing a soft blurring of the edges. In a telecentric backlight, only parallel rays are present, and the edges are reproduced with maximum sharpness.
Typical Industrial Applications
Telecentric backlights are essential for high-precision dimensional measurement of stamped metal parts, machined components and turned shafts, where accuracy at the micrometre level is required; quality control of hole diameters, slot widths and tooth profiles in gears and gearboxes; inspection of small machined parts such as fasteners, pins and connectors; metrology of pharmaceutical container neck diameters and pill geometry; verification of laser-cut profiles in sheet metal; dimensional inspection of injection-moulded plastic components; and any application where the measurement accuracy approaches or exceeds one tenth of a pixel in the camera image.
Selection Criteria and Design Considerations
The active emission area of the telecentric backlight must accommodate the full field of view of the camera with adequate margin to maintain edge sharpness across the entire image. Unlike diffuse backlights, where edge sharpness is degraded across the full field, telecentric backlights maintain consistent sharpness as long as the object is within the collimated beam.
The telecentric angle (the maximum deviation from parallel rays) is the critical specification. A smaller telecentric angle (typically less than one degree) provides better edge sharpness but requires more sophisticated collimating optics and limits the depth of field over which the parallel-ray approximation holds.
The combination with a telecentric lens at the camera completes the precision measurement setup. A telecentric lens accepts only rays parallel to its optical axis, matching the angular acceptance to the angular emission of the backlight. The resulting system is independent of the position of the object within the depth of field, providing the geometric invariance required for repeatable measurement.
Spectral and Driving Considerations
Monochromatic telecentric backlights are preferred for measurement applications because they minimise chromatic aberration of the collimating optics and the camera lens. Red or green wavelengths provide the best combination of LED efficiency, sensor sensitivity and lens correction. Strobed operation is recommended for any inspection of moving targets, to freeze motion within the exposure window.
Integration and Limitations
Telecentric backlights are bulkier than diffuse backlights of equivalent active area because of the collimating optics that must be placed at the focal distance from the LED source. The mechanical envelope of the backlight is typically two to three times deeper than a diffuse backlight of the same emission area, which can constrain integration in confined inspection cells.
The principal optical limitation is the limited depth over which the parallel-ray approximation holds. Outside the design depth, the rays gradually diverge and the edge sharpness degrades. The design depth is typically specified by the manufacturer and ranges from a few millimetres for high-precision compact backlights to several hundred millimetres for large-format telecentric systems. For objects extending beyond the design depth, edge sharpness becomes position-dependent and may compromise measurement accuracy.
The other limitation is cost. Telecentric backlights are significantly more expensive than diffuse versions of equivalent size, reflecting the precision of the collimating optics. The choice of telecentric backlight is therefore justified primarily by precision measurement requirements that cannot be met by simpler diffuse backlights.
RODER Vision Telecentric Backlight Illuminators
RODER Vision engineers application-specific telecentric LED backlight assemblies with calibrated collimating optics for industrial vision inspection requiring sub-pixel dimensional measurement accuracy.
- Application-specific telecentric backlight assemblies and collimating optics — Custom LED Illuminators
- Standard diffuse backlight portfolio for non-telecentric transmission applications — LED Backlight Illuminators
Strobed telecentric backlight operation for moving targets requires precise camera synchronisation — the RODER catalogue includes dedicated LED drivers and electronic controllers compatible with industrial machine vision controllers and PLCs.