Abstract: A method and apparatus for detecting residual ink in a sample by illuminating by diffused infra-red light a series of spots on a sample and examining a small area on each spot by collecting light in a video camera to form a frame for each spot, digitizing each of the frame and detecting areas in the frames having a brightness below a selected level to determine the total dark area of each frame. The number of such frames processed in a set of frames used to categorize the sample is defined when a certain minimum number of frames have been processed and when a selected degree of convergence of the data collected for the set and the data from the frame currently being processed is obtained, or after a preset maximum number of frames have been processed. The sample is characterised on the basis of the ratio of the dark area to the total area processed.

Abstract: To detect roughness on a travelling surface of a web such as paper a beam of polarized light is directed at an angle onto the web and focused by a focal lens to illuminate a spot on the surface and a specularly reflected component of the light is collected through a second lens focused on the spot. A detector is aligned to receive a portion of the specularly reflected collimated light from the spot passing through a selecting aperture and generates a signal depending on the intensity of the specular light passing through the aperture and received by the detector thereby to provide an indication of the roughness (or smoothness) of the surface.

Abstract: The texture of a travelling web is determined by instantaneously illuminating an area of the web via a strobe light and phtographing the illuminated area by a video camera correlated to operate in conjunction with the strobe. The signal from the camera is digitized, histogrammed and analyzed to determine the characteristics of the web. The strobe light may direct the light through the web or at an angle to the web and the video camera depending on the characteristics to be determined may be positioned to receive specularly reflected light from the web or reflected light or positioned on the opposite side the web to receive light transmitted through the web. In one embodiment the light is polarized and directed at Brewster's angle onto the surface of the web and only similarly polarized specularly reflected light is received by the camera.