As a prerequisite contrasting stripes are printed on the test sample. Depending on the material under test and the type of experiment, the stripes can be simple sticky tape or paint brushed or sprayed on the sample.

A laser beam driven by a rotating deflector, scans continuously along the main axis of the sample.

Diffuse scattering of the laser light occurs at the stripes. The scattered laser light is observed by a receiver. When the laser crosses the stripes, the intensity of the scattered light changes. The time of these changes is measured by a specially developed multi-stop counter. Because the speed of the rotating deflector is known, the measured time can be used to calculate the position of the stripe in the observation area.

At the beginning of an experiment the positions of the stripes are used to define the reference length for further strain calculation. (With our data evaluation software, the reference length can be determined at arbitrary locations of the recorded data vectors.)

When the load is applied, the sample is strained. Then the positions of the stripes change. These positions are continuously observed during the experiment and with the reference length from the beginning, the strain is determined.

During the past years this technique was significantly improved and allows us to build reliable, easy to use and very accurate laser extensometers. The great flexibility of this technique is a basis for one of our major innovations to the market of strain analysis. Our laser extensometers can observe not only 2, but up to 25 or 63 fringes during an experiment. As a result the locally resolved behaviour of the samples under load can be analysed very easily.

More details are available in the page Locally Resolved Strain Measurement.