Forensic Uses of Deflection (Y) Modulation and X-Ray Dot Mapping

  • Samarendra Basu


The image processing in the scanning electron microscope by deflection (Y) modulation allows augmentation of depths and topographic features of line crossings that naturally occur in many kinds of handwritten documents. A critical evaluation of this method, in comparison with the secondary and the backscattered electron imaging methods, has been made in order to establish that deflection modulation is advantageous for sequencing of intersecting lines of pencils and ball-point pens which leave relatively deep and equal impressions in the paper. The most reliable features of sequencing were less frequent with plastic-point pens. These results have indicated that the sample condition of line crossings is improved by the hardness of the writing tip, by increased writing pressure, and by thick or non-dispersive deposition of ink. The effectiveness of the method is determined by the selected signal (secondary or backscattered), by the accelerating potential of the incident electron beam, and by the specimen-to-electron detector geometry. Knowing these limitations is crucial to successful applications of deflection modulation, since this method is sensitive to the flaws contained in the original signal. The method qualifies for re-examination of the apparently uncharacteristic firing pin impressions of shotguns. The x-ray dot mapping method is well-suited for elemental matching comparisons of multilayered paint chips. Despite the time involved, this method and the backscattered electron imaging method are suitable for examination of fingerprints of magnetic inks.


Line Crossing Incident Electron Beam Gunshot Residue Deflection Modulation Paint Chip 
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Copyright information

© Plenum Press, New York 1986

Authors and Affiliations

  • Samarendra Basu
    • 1
  1. 1.New York State Police Crime LaboratoryAlbanyUSA

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