Writing impressions revealed by scanners
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- Strach, S.J. Forensic Sci Med Pathol (2007) 3: 210. doi:10.1007/s12024-007-0015-4
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Heavily impressed writing impressions in paper and impressions in glossy brochures are often difficult or impossible to develop using the electrostatic detection apparatus (ESDA, manufactured by Foster and Freeman). Oblique and specular reflectance lighting can be used but they are time-consuming techniques and not always very successful. This paper briefly describes a scanning and image-processing method for producing excellent images of such writing impressions.
KeywordsForensic document examinationWriting impressionsScannerImage processing
Latent writing impressions in paper, caused by the act of writing on overlying documents, can provide important evidence concerning the provenance of questioned documents and about other writings that may not have been preserved on paper documents. Since approximately 1978, the electrostatic detection apparatus (ESDA)1  has been the preferred method of revealing such impressions. There are certain situations when ESDA can produce unsatisfactory results. One of these situations is when the writing impressions are the results of writing with very heavy pressure, which is the main focus of this paper. Other situations are when the writing impressions are on glossy paper or card, on paper which is later excessively handled, on paper that has frequently moved around within a file, on paper that has been chemically treated with fingerprint reagents or other solvents, and on low-quality paper such as newsprint or telephone directory pages.
The contents of documents are often scanned and saved in computer files during the course of forensic document examination. In one particular case, the author wished to record some heavily impressed writing impressions on some relatively unimportant documents for the case file without necessarily using the ESDA. To the author’s surprise, he was able to produce within a short time frame excellent images of the impressions using an office scanner and some simple image adjustments. This paper explores the use of scanning techniques to record heavy writing impressions and the use of Adobe Photoshop to enhance them.
Materials and methods
Several sets of heavy writing impressions were created on various sheets of paper, including a heavyweight letterhead paper, a glossy brochure, a sheet of high-gloss photographic quality ink jet paper, and a postcard photograph. Paper and card documents from actual cases were also tested.
An Epson scanner (1640SU) was used for most of the test scans reported in this paper.
Scans were made at an image resolution of 200 ppi (pixels per inch), with no sharpening at the scan stage. The original and manipulated files were stored in TIF (recommended) format but reproduced as high-quality JPG files for the purposes of this paper.
Basic images were enhanced using Adobe Photoshop 7.0 by manually adjusting the levels (under “Image/Adjustments”) to obtain the best images of the impressions, sometimes with an “auto contrast” adjustment and finally an “unsharp mask” filter sharpen function. There are other enhancement methods and other software that could be employed but the purpose of this paper was to report on the simple and effective results obtainable with commonly used software and its simpler functions. Scans made with the document rotated 90° (see the Results and Discussion section) were shifted back to normal viewing using the “rotate canvas” function. Scans made of the backs of documents were mirror inverted to make the images compatible with those of the fronts using the “flip canvas” function.
Images of multiple scans were combined by creating a second (or further) layer over the first image layer, cutting and pasting the second image into this layer and reducing the opacity of the second layer typically to between 40% and 50% to optimize the visibility of impressions in both layers. Images of impressions and/or embossments in the two (or more) layers were adjusted to optimize the degree of coincidence and/or visibility of the writing impressions using the “move” tool. The “free transform” function (under “Edit”) was used for rotational fine adjustments of the upper layer relative to the first layer.
In order to check the effect of the scanner model, some test scans were also made using a Canon multifunction laser printer, copier and scanner (model IRC2620p), a Canon USB-powered portable scanner (model CanoScan LiDE 20) and an Epson multifunction ink-jet printer, scanner and copier (model Stylus CX5300). There are other commercially available methods to produce side lighting on a document to view writing impressions. However, it was not the aim of this paper to evaluate all possible lighting methods but simply to discuss the effective use of widely used and inexpensive scanners.
Results and discussion
The Epson 1640SU and Stylus CX5300 scans gave outstanding results after image processing. The images shown in this paper are all from scans using the Epson 1640SU.
The CanoScan LiDE 20 gave usable results in terms of enhancing the writing impressions but not as good as the results obtained with the Epson machines.
The Canon IRC2620p did not produce any visible images of the writing impressions.
Initial attempts at producing images of heavy writing impressions in a colour postcard photograph using the scanning method were unsuccessful. This is ascribed to the complexity of the information with respect to the photograph and hence the difficulty in separating the information relating to the writing impressions from this complex background image.
The key to obtaining writing impression images using a scanner is the closeness of the light source, typically light-emitting diodes (LEDs), and the detector to the paper being scanned. The result is that, when the light source is close to the paper (and the detector necessarily slightly displaced from it), any irregularities in the paper surface, including writing impressions and embossments, will cause a shadow to be cast across the paper. In the Canon IRC2620p, the light source is much further away from the document (located as in conventional photocopying machines) and therefore will not cast a shadow of any irregularities on the paper.
This has implications for forensic document examiners with regard to the type of scanner to use in order to record paper surfaces. This applies not only to the research described in this paper but also to the nature of document images obtained by scanning in general. It is the author’s preference to record irregularities in the paper surface (including pressure effects of handwriting) rather than a generally flat view of the document.
Because the (moving) scanner light source is toward the top of the document, casting a beam of light down the paper, highlights appear at the lower part of the writing impression and shadows at the upper part; lower and upper here refer to position on the paper rather than depth. However, vertically oriented writing impressions will be poorly revealed because the light shines straight down the valley of the impression, casting little if any shadow. For this reason it was thought useful to obtain a scanned image with the document rotated (in this case anticlockwise) by 90°, bringing vertically oriented impressions into horizontal alignment with respect to the scanner.
This had the desired effect, as shown in Figs. 2–4. These images were enhanced as described in the Materials and Methods section. Those parts of the writing impressions that were poorly revealed with a scan made with the original orientation of the document (shown in Fig. 3a) are quite clearly revealed when a second scan is made with the document rotated by 90° (Fig. 3b). Note that the initial downstroke of the first letter “w”, the first letter “i” dot and the stems of the letters “g” as well as other generally vertically oriented impression lines are weakly revealed (or not at all for the “i” dot) in the image shown in Fig. 3a, but are strongly revealed in the image shown in Fig. 3b.
Combining these two images (as described in the Materials and Methods section) gives a good rendition of the writing impressions (Fig. 4). In some cases, such as this one, a second unsharp mask operation (shown in Fig. 4c) improves the clarity of the image of the writing impressions.
Further research was undertaken on test sheets of specialty paper with a weight of 100 gsm. Grids of vertical, horizontal, and oblique writing impressions were created in the paper by writing heavily on another sheet of similar paper located over the test sheet. A pad of multipage writing paper was used as backing for the test sheet when the writing impressions were created. Figures 5a and b, respectively, show the unadjusted and enhanced scanned images of one of the test sets of impressions in such paper. A procedure similar to that described in relation to Figs. 2–4 was used to create the combined image shown in Fig. 5b from scans with illumination in the vertical and horizontal directions with respect to the sheet of paper. It should be noted that only the lower part (approximately 75%, below the dark line) of Fig. 5b is the result of a combination of scans in two directions.
With heavy impressions on paper, the back will almost always bear embossments (or reverse impressions) created by the same act of writing. These can also be detected by observation under oblique lighting illumination. Figure 6a shows an enhanced image of the test impressions and Fig. 6b shows the enhanced image of the back of the same sheet of paper, in both cases with light coming only from the top of the sheet of paper. The image of the writing embossments is an approximate mirror image of the writing impressions shown in Fig. 6a. Therefore in Fig. 6b the image has been mirror reversed to obtain correspondence with Fig. 6a. For simplicity the demonstrations shown here are the results of a single direction of illumination (from the top) for both the impressions and embossments. This does not allow the vertically oriented impressions/embossments to develop well.
Since information about writing impressions is available from scans of the front and back of the document, it was thought of interest to try to enhance the image of the impressions by combining the scanned images of the front and back. Figure 7a shows the exact combination. Here there is some canceling of the images of writing impressions. However, by moving one image relative to the other some interesting and useful effects occur. By slightly shifting one image in the vertical direction and thereby slightly distorting the combination, images can be created that enhance the highlights (by extending them in the combination, as shown in Fig. 7b) or the shadows (by extending these in combination, as shown in Fig. 7c). This is useful in situations where the background paper is dark or light, respectively. This can be done because the image of the embossments is not an exact mirror image of the impressions image due to the different effects of the single orientation of the lighting. A horizontal line is illuminated on its lower portion (which therefore appears bright) as a writing impression on the front of the test paper but it is illuminated on its upper portion as an embossment on the back, by the lighting from the top. Conversely, the upper portion of a horizontal line is in shadow when in the form of a writing impression on the front whereas the lower portion is in shadow when it is in the form of an embossment on the back. Therefore combining the images of the front and back in exact registration with one another tends to cancel out the images of the impressions. In practice, because the back is not exactly a reversed image of the front, writing impressions are still visible in the combined images of the front and back in exact alignment (Fig. 7a). However, a shift upwards of the front will combine and accentuate the highlights (Fig. 7b), useful when the writing impressions are on dark paper, whereas a shift downwards of the front combines and accentuates the shadow (Fig. 7c), useful when impressions are on light coloured or white paper. These images of impressions/embossments, optimized for maximum visibility, are nevertheless slightly distorted combined images in that the front and back of the sheet of paper are no longer in exact registration with respect to the paper edges or print on the paper.
More complex enhancements can be undertaken by combining four images: front and back, as just described, plus images of the front and back with the paper reoriented by 90°. Overall image quality is also enhanced by scanning in full colour, but the disadvantage with this type of shadow enhancement is that spurious coloured lines and general false colouration sometimes appear as a result of the enhancement process. For this reason the author prefers to use scans in black-and-white photo mode (full gray scale). An example of an enhanced image from the combined four image scans of the lower part of the same document, for which impressions were shown in Figs. 5–7, is shown in Fig. 8. Unlike the images in Figs. 5–7, this combination also reveals the vertically oriented impressions because of the inclusion of images with lighting from the side.
This research has demonstrated the ease with which good images of writing impressions can be obtained using certain types of office scanners and some simple image-enhancement techniques. The methods described should form part of the range of standard forensic document examination techniques. The technique will be especially useful for those various situations where ESDA is sometimes not effective or in cases where sets of heavily written impressions need to be recorded simply and rapidly. Additional benefits occur in situations where heavy writing impressions are accompanied by corresponding embossments on the back of the same sheet. In such circumstances, combinations of scanned images can easily be obtained by slightly shifting one image of the front with respect to another of the back to produce maximum enhancements of the impressions that contrast with the background colour of the paper.
Heavily impressed writing impressions in paper can be recorded very simply using an inexpensive office scanner.
Simple image-enhancement techniques can be used to optimize the images of writing impressions.
Further improvements to the writing impression images can be obtained by combining images from two directions and by combining images from the front and back; shifts of the images can optimize the contrast with the background colour of the paper.
Electrostatic Detection Apparatus (ESDA) manufactured by Foster and Freeman Ltd, 25 Swan Lane, Evesham, Worcestershire, WR11 4PE, UK.
Comments and assistance on this work and manuscript by staff of Forensic Document Services are gratefully acknowledged.