The invention relates to a large-
caliber high-gradient optical mirror surface on-line measuring
system which comprises an
optical fiber light source, a converging light path light split
system, a replaceable measuring
grating, a high-precision plane reflecting mirror, a large-
caliber high-gradient optical mirror surface, a digital CCD (
Charge Coupled Device) and a computer
information processing system. In the invention, after light emitted out from a
numerical aperture optical fiber light source passes through the measuring
grating, a group of shadow fringes is generated, and after passing through the converging light path light split system and the high-precision plane reflecting mirror, the shadow fringes are projected onto the large-
caliber high-gradient optical mirror surface and then reflected back by the large-caliber high-gradient optical mirror surface according to the original path; when the shadow fringes pass through the high-precision plane reflecting mirror and the converging light path light split system, another light path is generated, and a conjugate image of the
light source is generated in the light path; after the conjugate image is acquired by the digital CCD, the conjugate image is transmitted to a computer and analyzed and processed by the computer
information processing system; and the
surface shape error of the large-caliber high-gradient optical mirror surface is worked out through comparing the deviations between the returned image and an
ideal image.