Transcript
Application Note
Hyperspectral Imaging as a Method of Colored Inkjet Printer Ink/Toner Comparisons Introduction Due to the increasing availability of inkjet and toner-based printing systems to households and businesses, forensic document examiners (FDE) are likely to face an increase in requests to analyze samples of this nature. Several reasons for performing analysis on inkjet or toner-based documents include: • To determine if two or more documents were printed with similar ink or toner. • To determine if a copied document is a fraudulent alteration of an original document.
Figure 1: Digital images with the approximate imaging areas outlined.
• To determine the possible make and model of a printing system that was used to produce a particular document. ChemImage instrumentation offers an advanced method for examining printed and/or copied documents, providing a method for FDEs to examine inkjet and toner evidence without any sample preparation or destruction. Hyperspectral imaging (HSI) examination measures the reflectance spectra of the individual ink/toner droplets. This enables the comparison of the three primary colors (magenta, yellow and cyan), avoiding any mixing of colors that could interfere with comparisons. Since the individual droplets are the basis for measurement, even very small fragments of documents or pictures can be analyzed.
Figure 2: HSI extract at 490 nm. The white dots seen in the image are cyan ink droplets which reflect the 490 nm light. The circles indicate the ink droplets selected for spectral comparisons, as shown in Figure 3.
HSI data is collected in the form of digital images as a function of wavelength, resulting in each pixel within the image having an individual, fully resolved spectrum associated with it. Since each individual pixel has its own spectrum, the pixels containing information for particular droplets can be selected. Therefore the spectrum from one droplet can be compared to that of another allowing intuitive spectral comparison to be used to support or exclude the presence of chemical similarities. Further differentiation can be made through the use of powerful multivariate statistical analyses. Chemometric functions such as Euclidean Distance Analysis (EDA) and Cosine Correlation Analysis (CCA) can be applied to hyperspectral datasets in order to provide a more quantitative method of spectral differentiation.
Experimental Information Figure 1 shows the digital images of two inkjet printer samples with boxes outlining the approximate area on the samples where HSI data was collected. Figure 2 shows
Figure 3: Spectra representing the selected ink droplets, as shown in Figure 2. EDA was used to determine the differences in spectral features around 670 nm, indicating that the two samples did not come from the same model of inkjet printer.
the HSI images of the two inkjet sample areas at 490 nm. The dots in the images that appear white are the cyan ink droplets which are reflecting the 490 nm light. The circles around some of the ink droplets indicate that these droplets were selected to show representative cyan spectra of each printer. Figure 3 shows the spectra of the cyan ink droplets which were selected from figure 2. Dissimilarities can be seen around 670 nm when the spectra from each printer are overlaid. Figure 4 shows the digital images of three inkjet printer samples with boxes outlining the approximate area on the samples where HSI data was collected. Figure 5 shows the HSI images of the three inkjet sample areas at 490 nm. The dots in the images that appear white are the cyan ink droplets which are reflecting the 490 nm light. The circles around some of the ink droplets indicate that these droplets were selected to show representative cyan spectra of each printer. Figure 6 shows the spectra of the cyan ink droplets which were selected from figure 5. The spectra do not show any dissimilarity in peak intensity or placement throughout the wavelengths collected which indicates that these three samples were printed from the same printer, or from printers of similar brands and models.
Figure 4: Digital images with the approximate imaging areas outlined.
(3A)
Figure 5: HSI extract at 490 nm.The white dots seen in the images are cyan ink droplets, with the circled dots indicating the areas chosen for spectral comparison between the samples, as shown in Figure 6.
Results and Discussion HSI provides a means of comparing both inkjet and toner samples from different models and brands of printing systems. The spectra representing the magenta, yellow and cyan ink droplets can be compared between samples to support a determination of similarity or to exclude a model as a possible source of the document. Both image and spectral comparisons are valuable to the forensic scientist in that they are intuitive in both the interpretation and explanation to a jury. Data acquisition is quick, and no sample preparation is required before analysis. Once an area of a particular sample is imaged, it can be compared to multiple samples without re-examining the evidence, which helps to maintain sample integrity.
Conclusions • HSI provides a method of comparing multiple inkjet printer samples to determine similarities or dissimilarities. • HSI is a nondestructive means of examination, and does not require any sample preparation.
Figure 6: Spectra representing the selected ink droplets, as shown in Figure 5. EDA was used to determine the spectra, indicating the samples originated from the same model of inkjet printer.
• HSI can successfully examine small pieces of printed samples which may not have been suitable for other methods of examination. • HSI provides spectral information for comparison, allowing for further sample characterization.
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ANREV002 06/2011
