Theoretically, when a plane light or collimated light is radiated to a fine grating part, the reflected light make an images of diffraction grating is repeated at regular distance. This image is called self-image or Talbot image. This apparatus used this phenomenon to measure the quality of linear scale. The Talbot image of linear scale is observed by a CMOS camera and a 10X objective lens. When the scale was moving by a motorized stage under OM, the effect of Talbot image will moved also. The changes of pixel’s intensity will be sinusoidal. The quality of linear scale will be verified by uniformity and matching of these two curves.