Abstract: Non-contact gauges and methods of optical measurement are provided which include storing a signature indicative of an error associated with measuring a known dimension and then determining an unknown dimension in relation to this signature. The novel technique enables low cost production of preferred gauge designs using standard, off the shelf components. The non-contact gauge includes a radiation source directed toward the object to be measured and a scanning device, preferably a rotating drum having small slits disposed therethrough, for producing a signal responsive to an image produced by the radiation source upon the object. The device is capable of having a resolution of about 0.0001 inches and an accuracy of about +/0.00025 inches. The device can include a nuclear radiation source, such as X-ray radiation, for determining wall-thicknesses. The designs disclosed further produce cross-sectional images of solid and hollow objects.