Photometric stereo inspection for moving objects

Left: Albedo (surface appearance). Right: Surface Gradient (surface geometry). Photometric stereo separates geometry from appearance, enabling defect detection independent of print, color, and texture. Test images provided by CCS Inc.

Photometric stereo is a machine vision technique used to reveal subtle surface defects such as dents, cracks, scratches, and deformations. Unlike conventional 2D imaging, it evaluates surface geometry rather than surface appearance, making it highly effective for inspecting products with printed graphics, labels, textures, or varying colors.

Photometric stereo implementation and limitation
The technique works by capturing multiple images under different illumination directions and analyzing the resulting shading information to reconstruct the surface shape. By separating surface geometry from appearance, photometric stereo can reveal defects that may otherwise be hidden by printed patterns or texture variations.

However, conventional photometric stereo requires multiple images of the same object position. When inspecting moving objects, even small displacements between image captures can introduce reconstruction errors and reduce inspection reliability. As a result, photometric stereo has traditionally been limited to stationary objects or stop-and-go production processes.



Photometric stereo combines images captured under different lighting directions to separate surface shape from printed or textured features. Test images provided by MSTVision GmbH

Quad shutter control: Capturing four illuminations within a single frame
Basler brings the Quad Shutter Control sensor function in Sony's IMX900 image sensor to the next level by enabling the capture of four illuminations within a single frame.

Extending photometric stereo to moving objects inspection
Basler extends photometric stereo to moving-object inspection through a customized implementation of Quad Shutter Control based on Sony's IMX900 sensor. The technology enables four independently controlled exposure timings within a single acquisition cycle.

In conventional systems, the four images required for photometric stereo are captured sequentially, creating a time gap between acquisitions. When objects are moving, this gap introduces positional differences that can affect reconstruction accuracy.

With Quad Shutter Control, the four exposures are performed within microseconds of one another and synchronized with four directional light sources. This dramatically reduces the effective acquisition interval and allows all required photometric stereo information to be captured while the object remains effectively in the same position.

As a result, reliable photometric stereo imaging can be achieved while the object remains in continuous motion, preserving inspection accuracy without requiring the object to stop.



Motion-induced limitations in conventional photometric stereo imaging

Precise exposure timing for motion-consistent imaging
Quad Shutter Control enables the exposure start timing of each pixel within a 2 × 2 pixel group to be controlled independently. As a result, four separate exposures can be captured from a single acquisition trigger, providing the foundation for photometric stereo imaging of moving objects.

Quad Shutter Control enables the four image acquisitions required for photometric stereo to be performed within an extremely short time window while the object remains effectively at the same position.

For example, a conventional 100 fps camera requires approximately 10 ms between consecutive image captures. By contrast, the Basler implementation allows four exposure timings to be controlled within a single image acquisition at microsecond-level intervals.

When combined with synchronised control of four directional light sources, the effective acquisition interval between the required images can be reduced to nearly zero. This removes the primary bottleneck of conventional photometric stereo systems and enables inspection of moving objects using photometric stereo techniques.



Quad Shutter Control: Capturing Four Illuminations Within a Single Frame

Bringing photometric stereo to high-speed production lines
With Quad Shutter Control, manufacturers can extend the advantages of photometric stereo to applications that were previously impractical due to object motion and throughput constraints.

Typical applications include products with printed graphics, labels, or textured surfaces where conventional 2D inspection may struggle to distinguish appearance variations from true surface defects.

Edited by Induportals Editor, Romila DSilva.