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Image Processing Notes
The scans of the Apollo flight films are processed using a standard set of procedures. First, the unexposed portions of the film along the edges of a scanned frame are cropped, and the frame is straightened. Second, the background is removed from all of the scans, by assuming that the average DN values of the unexposed regions at the edge of each raw scanned image represent the background (i.e., film base and fog). Third, a flatfield correction (derived from the actual image data) removes vignetting to the first order. Fourth, the reseau patterns (the small crosses visible on Apollo images published elsewhere) are removed from the images. Fifth, a logarithmic histogram transformation is applied to the image. This is necessary because of the logarithmic response of film, which makes the raw scans appear very contrasty. Since photographic paper also has a logarithmic response and reverses the films response, conventional paper prints have a natural contrast range. The logarithmic histogram correction applied to the scanned images therefore produces a virtual print that simulates the natural contrast of a conventional paper print. Sixth, since the uncompressed images produced by the initial scanning process result in extremely large images, the scale is reduced by a factor corresponding to the square root of 2, which serves to reduce the image size by 50%, and the images are converted from 16-bit to 8-bit. The original, unprocessed raw scans are provided on this website as a full-resolution 16-bit TIFF file. More details about the file formats are provided in a following section.