Ultrasonic testing carries a constraint that has nothing to do with the defects it looks for. Sound does not usefully cross the air gap between a transducer and a part, so conventional ultrasound needs a couplant: gel by hand, a water jet, or the part lowered into an immersion tank. On a flat plate it is a nuisance. On a honeycomb sandwich panel, or an assembly that cannot be wetted, it rules the method out.
Laser-excited acoustics removes the couplant. Before treating that as a straight upgrade over immersion testing, it is worth being precise about what it changes and which problems it genuinely fits.
Is the couplant actually your constraint?
Often it is not. Plenty of composite shops run immersion or squirter systems that work, are qualified, and produce data their customers accept. Replacing a method that is not failing rarely pays for itself.
It becomes the binding constraint in a smaller, recognisable set of cases. Water ingress into a honeycomb core is a defect in its own right, so wetting the part introduces the condition you are inspecting for. Parts that must stay clean, in medical device, food and pharmaceutical production, cannot pick up gel residue. Large or fixed structures cannot be lowered into anything, and on contoured surfaces a hand probe struggles to hold steady contact, which appears in the data as material variation. If one of those describes your work, contactless non-destructive testing deserves a serious look.
How a contactless ultrasonic measurement is made
A laser pulse is directed at the surface. The energy it deposits launches an ultrasonic wave inside the material, so the part becomes its own transmitter and nothing touches it. Detection is the harder half. In place of a coupled piezoelectric probe, the returning ultrasound is picked up by an optical microphone, a sensor that measures sound in air by its effect on light.
That sensor was developed and patented by XARION Laser Acoustics, an Austrian company, and its practical advantage is bandwidth. The XARION optical microphone covers a far broader ultrasonic frequency range than conventional piezoelectric transducers, which are tuned devices by design. Merkel Technologies represents XARION Laser Acoustics in Israel.
The composite defects it suits, and what it will not report
Ultrasound is good at interfaces, because an interface is where it reflects. In composites and bonded structures the defects that matter are mostly interfaces that should not be there: delamination between plies, disbonds in an adhesive joint, separation between a skin and its core. Often nothing is visible from outside, and a visual check passes a part that is already compromised.
What a scan will not give you is the flaw's size in three dimensions. The result is a map across an area with depth information at each point, not a reconstruction you can measure later. How small a feature it resolves, and how fast a surface can be covered, depend on the material and the setup, and are best established on your own parts.
Where laser ultrasound sits next to industrial CT
The two are not competing. An industrial micro-CT system reconstructs a full volume: every void located and sized, comparable against CAD, measurable in software long after the scan. It is the definitive answer for one part, and also a cabinet the part must travel to.
Laser ultrasound answers a narrower question across a much larger area. Is this bond sound, is there delamination here, does this panel pass. The sensible arrangement in a composites shop is often both: ultrasound screens production, CT resolves the parts that fail screening. Teams already using micro-CT in materials research recognise the split.
Where laser ultrasound is the wrong tool
Dense metal is the clearest limit. Porosity in a casting, inclusions in a weld, the interior of a thick metal assembly: those belong to radiography or CT, not to a technique whose strength is thin, layered, low-attenuation material.
Line of sight still applies. Contactless does not mean unobstructed. The laser has to reach the surface above the region of interest, so a feature buried behind another structure inside an assembly is no more accessible than with a probe. Surface finish, coating and colour also affect how the excitation couples in. Ultrasound reports mechanical discontinuities only; elemental questions belong to micro-XRF.
Finally, inspecting a component is not the same discipline as listening to a machine age. Catching a bearing degrading in service is acoustic condition monitoring, which learns an asset's normal running signature and flags drift from it. That is a maintenance question, not a quality question, and one purchase rarely serves both.
Are you inspecting parts or monitoring a process?
Laser acoustics is deployed in two different ways, and buyers sometimes discover late that they were discussing the wrong one. Inspection scans a part and produces a record for that part. Process monitoring follows the laser-material interaction during cutting, welding or drilling, or watches an additive manufacturing build, and produces real-time feedback on whether the process is behaving.
They share a sensor and little else. Integration, acceptance criteria and the person who owns the result all differ, so settle which one you are buying first.
Run it on your own panels first
Contactless ultrasound is narrower than the phrase suggests, and inside that range it beats immersion. Which side your work falls on is a question about your panels. Take three of them: a good one, one with a confirmed disbond, and one of the geometries your current method struggles with.
Judge it on whether the known defects are found, whether anything is reported that is not there, how small a feature stays visible in your layup, and how long a panel takes with fixturing and cleanup counted. Run the same three through your current method for comparison.
Merkel Technologies can arrange that trial with the people who would run the system afterwards. If it shows your immersion tank is still the right answer here, that is worth knowing too.