Image enhancement of ninhydrin-processed and colored fingerprints

Image enhancement of a pre-processed and colored fingerprint with complex background patterns has high demand in appraisal evaluation of forensic evidence. This paper focuses on image enhancement of ninhydrin-processed fingerprints on periodic-patterned backgrounds. Contrast adjustment usually does not work well for separation between fingerprints and periodic-patterned backgrounds because of similarity in their color scales and/or superposing patterns. As alternative methods of contrast adjustment, two approaches have been proposed. The first one is to transform an RGB evidence image to a CIELAB color space and to utilize hue so that the hue of pixels is converted to a sharp contrast of brightness. The second approach is to conduct Principal Component Analysis (PCA) on an evidence image in RGB and CIELAB color space and to reconstruct PC images. This approach is effective for a case that the hue-based image processing is not sufficient to enhance fingerprints. Moreover, this method can be successful in CIELAB in enhancing a fingerprint which is not able to be enhanced in RGB. It is also suggested that fingerprints and backgrounds in complementary color can be separated. For certain RGB evidence images, the hue-based approach and the PCA-based approach achieve more enhanced fingerprints than the contrast adjustment does. If some periodic patterns remain after the processing of each approach, these patterns can be removed by using frequency filtering based on 2-dimensional Fast Fourier Transformation. It is effective to apply the hue-based approach first and, if the enhancement of fingerprints is not sufficient, then it is recommended to use the PCA-based approach both in RGB and CIELAB color space. Proposed methods are easy to be conducted when an RGB image including a colored fingerprint is given by a common digital camera or flatbed scanner. Our methods can also be used even when an image for background subtraction is not available. Proposed methods expand the capability of utilizing colored fingerprints processed by ninhydrin for the forensic evidence.