1. A method for measuring the degree to which a printed image on a first side of a sheet is visible when illuminating and viewing a second side of the sheet, the method comprising: a) creating a calibration image of a reference object containing no image by illuminating the reference object at an initial illumination level; b) determining an average gray level of the reference object and adjusting the illumination level to achieve a predetermined average gray level; c) illuminating the sheet at an illumination level the same as that used to create the calibration image and creating an image of the sheet; d) measuring the ratio of the pixel intensities of the image of the sheet with the corresponding pixel intensities of the calibration image; and e) calculating a mean value of the ratios of the pixel intensities.