The instruments comprises a red (671nm/655nm) and/or a green (532nm) diode pumped solid-state system or semiconductor diode laser, expansion and collimating optics, and a proprietary camera-based detection system. The mechanical construction of the mapper is entirely built using carbon fibre. This allows for an extremely rigid and thermally inert system as well as having significant weight advantages as compared to more conventional engineering materials. The power outputs of the lasers are around 200mW depending on the system. This makes them Class 3B lasers. The emitted beam is expanded and collimated before illuminating the target surface at an angle approximately 30° to the target normal. Laser beams are monochromatic (single wavelength), highly directional and are spatially and temporally coherent. This coherent property of a laser beam, where all emitted waves are in phase with each other, enables its use for strain measurement. The beam incident on the target is reflected by its surface. Most surfaces are not optically flat. The surface roughness causes the reflected waves of light to interfere with each other creating a grainy effect that is known as speckle. The detection system comprises an image shearing head, a phase stepper, an imaging lens and a camera. The strain field is produced by saving an image of the unstressed target, and subtracting all subsequent images from this reference image. As a mechanical or thermal stress is applied to the target surface, the phase of the speckle changes corresponding to the stress. This is visible as fringes in the subtracted video. Download our specification PDF here. |
