CERN’s surveyors send sparks flying

A few weeks ago, we published an article on the three-dimensional laser scanner technique used at CERN to produce 3D images of the LHC tunnels and experiments (see the article here). Photogrammetry is another technique in the CERN surveyors’ arsenal.


The ATLAS wheel during a photogrammetry measurement campaign. The white spots (see red arrows) dotted across the wheel are the retro-reflective "dot" targets.

Used in a number of fields including topography, architecture, geology and archaeology, photogrammetry is a stereoscopy technique whereby 2D images taken from different angles can be used to reconstruct a 3D image of an object. Surveyors at CERN have been using this technique for over 15 years as a way of gaining precise information on the shape, size, deformation and position of the LHC detectors and their composite elements.

The photogrammetry used at CERN is relatively “light” in terms of the equipment required, which makes it very portable. In fact, all the surveyors in the field need is a set of retro-reflective targets, which they place all over the object being measured, and a camera with a flash linked by Wi-Fi to a laptop that processes the data. This portability means that the object can be photographed from all angles: from the ground, from scaffolding, from a ladder, or from a cherry picker, etc. “With this technique, we are able to evaluate the conformity of prototypes or components that are under construction at CERN or elsewhere in the world,” underlines Antje Behrens, surveyor for the ALICE and CMS experiments. "For example, we carried out checks during the manufacture of the ATLAS detector feet in Russia and the CMS endcaps in Japan.”

Photogrammetry measurements were made of the Alpha Magnetic Spectrometer (AMS), now installed on the International Space Station. Photo: Dirk Mergelkuhl.

Four types of retro-reflective targets are used: simple reflective dots; bands, used on flat surfaces; bar codes, whose appearance in several images allows the surveyors to position the photos correctly in relation to one another; and lastly, the cross, which is photographed first and serves to position the images in a specified coordinate system, which is then integrated into the CERN repository. Covered with these reflectors, the detectors sparkle under the flashes of the surveyors’ camera.

The surveyors need an identifiable reference from which to extrapolate the object’s dimensions, so they define the scale of the photos using one of two methods: including a ruler of a known length in the pictures; or, for a more precise measurement, measuring the distance between two of the targets using a laser tracker. The level of precision that can be obtained using photogrammetry is remarkable. For example, when a deformation analysis was performed on the CMS tracker endcap, which is 850 millimetres in diameter, the margin of error was just 0.03 mm!

For more information on the subject, read the CMS Times article: Surveyors measure new CSC layer with photogrammetry.

by Anaïs Schaeffer