To measure the strain when the specimen is isolated is something I have put a lot of thinking to. One common method is to put an extensometer on the specimen outside the furnace. This requires long test specimens, which is a limitation and not something we want. We want something that can measure a displacement without touching. A laser extensometer came to be the solution. Zwick offered a high-temperature laser speckle extensometer from Messphysik. When the laser hits the specimen a speckle pattern is formed. This pattern changes during loading and by recording it with a camera, the strain is calculated from the displacement.
A laser speckle extensometer. A tunnel is attached on the front to reduce disturbance from air motion.