A type of Target made from retro-reflective material. Retro-reflective material is a type of sheet that reflects light back in the direction of the light source (unlike a diffuse material that reflects light in all directions (white paper is diffuse) and unlike a mirror that reflects light at opposite to the angle of incidence). Retro-reflective material is useful in photogrammetry because if you place as strong light source at the camera (such as a flash) then regardless of the angle of the target, the target will strongly reflect this light.
With targets made from retro-reflective material and a flash on the camera, you can change the camera exposure settings so that the targets in the scene are considerably brighter than the scene. This high contrast makes retro-reflective targets easier to identify and more precise. They are also suited to working at night or in other darkened environments.
Many road-side signs are made with retro-reflective lettering so that when you car headlights illuminate the sign, you can see the letters clearly and brightly. Many shops that make signs can also make retro-reflective targets.