Extending line of sight using mirrors

Line of sight has always been one of the defining challenges when measuring complex parts or components with direct scanning technologies. No matter how advanced the hardware or software, if the tracker cannot “see” the surface or feature directly, it cannot measure it. In many real-world applications, for example in aerospace, energy, and research, this quickly becomes apparent. Large assemblies often contain recesses, undercuts, or obstructed geometries that cannot be reached directly by the measurement beam.

The traditional workaround is to reposition the tracker until a new line of sight is achieved. However, this is often not an option. Space constraints on the shop floor, the part geometry, or simply the time it takes to re-establish instrument alignment make moving the tracker impractical.

Looking around the corner with the Leica Absolute Tracker ATS800

With the ATS800, as with any laser tracker, users can use vector targets or sphere-fit adapters in combination with retroreflectors to measure hidden points or features. These methods allow for accurate referencing and provide valuable data, but they are inherently limited. They only support the measurement of discrete single points, which is not always sufficient when complex features need to be captured. Moreover, their effective reach is short. In practice, this means these solutions can solve isolated visibility issues but cannot fully address the broader challenge of scanning hidden areas.

This raises an important question: What if there were a way to extend the line of sight of the ATS800 without moving the instrument, not only for single points but also for scanning? The answer lies in the combination of direct scanning technology with a simple optical principle – using mirrors.

A simple principle

The principle of mirror-based measurements is simple. Instead of requiring direct visibility of a feature, the ATS800’s laser beam is reflected off a mirror placed strategically within the setup. The reflected beam reaches the hidden feature, the data is collected, and then mathematically mirrored across the defined plane of the mirror using Hexagon’s state-of-the-art SpatialAnalyzer software. In effect, the tracker is “looking around the corner.”

For this approach to be feasible, one key requirement must be met: the mirror plane must be determined with very high accuracy. Once this plane is established, SpatialAnalyzer can correctly interpret the reflected data and calculate the position of the hidden feature.

Different methodologies exist for characterising the mirror plane, but one of the simplest and most reliable involves measuring a single reference point twice. The first measurement is taken directly with the ATS800. The second measurement is taken via reflection, with the beam bouncing off the mirror before reaching the same reference point. These two measurements together define a straight line. The line’s midpoint provides the position of the mirror plane, while the line itself defines the plane’s normal vector. SpatialAnalyzer offers a built-in function to create a mirror plane out of two measured points automatically.

Mirror options

With the ATS800, this process can be performed with either retroreflectors or scanning spheres; both accessories provide the precision required to accurately determine the mirror plane, ensuring that subsequent mirrored measurements remain valid.

It is important to mention that not all mirrors are created equal, and the choice of mirror has a direct influence on measurement quality. To ensure reliable results, two main requirements must be considered for the mirror:
• Flatness: The mirror surface must be extremely flat, as even slight deviations introduce errors that propagate into the measurement results.
• Front-surface design: Only first-surface mirrors (where the reflective coating is on the outer surface) are suitable. Standard mirrors, which reflect from the back surface through a glass substrate, introduce secondary reflections and distortions that render them unusable for precision metrology.

Using mirrors will inevitably introduce an additional layer of uncertainty. The degree of uncertainty depends primarily on two factors:

1. The distance between the ATS800 and the mirror.
2. The distance between the mirror and the measured feature.

Theoretically, these errors can be calculated using the ATS800’s accuracy specifications and the geometry of the setup. In simple terms, the accuracy of the mirror plane is directly dependent on the uncertainty of the 2 reference points; therefore the further the mirror is from the tracker or the measured feature, the larger the potential error contribution.

To test this concept, Hexagon engineers performed controlled experiments designed to compare direct and mirror-based measurements. A calibrated scale bar was measured directly in the line of sight of the ATS800, indirectly via the strategically positioned mirror, and with a mix of mirrored and direct points. The 40 repetitions of the scale bar measurement resulted in a standard deviation 0.004mm of and a range of 0.013mm.

A similar additional test was performed, measuring drilled holes to assess the positional accuracy of the mirrored feature measurement. The table below shows the deviation s between a direct measurement and the mirrored results. By comparing the results, the team was able to quantify the effect of using mirrors on measurement accuracy. Initial findings showed that while there is a measurable increase in uncertainty, the results remain within acceptable limits for many industrial applications.

For industries where large, complex assemblies are common, this represents a valuable expansion of capability. Aerospace manufacturers, for example, can use mirrors to measure holes or brackets obscured by structural frames, or research facilities can take advantage of the approach to capture data in confined or hazardous spaces.

A powerful technique for real-world metrology

The use of mirrors with the ATS800 is not just a theoretical exercise, it is a practical extension of the tracker’s capabilities. While it requires careful setup and introduces an additional source of uncertainty, it opens the door to new measurement strategies that address one of the fundamental challenges of direct scanning technologies.

By combining the precision of the ATS800 with thoughtful optical setups, engineers can now measure features that were previously inaccessible, reducing downtime, increasing flexibility, and expanding the range of applications where the ATS800 can deliver value.

What started as a simple optical trick has proven to be a powerful technique for real-world metrology. With the right mirror, the right setup, and the ATS800’s unique strengths, the limits of line of sight can be pushed further than ever before.