Study Of Fingerprint Collection Method Based On Color-change Performance Of Materials

Fingerprinting played an increasingly prominent role in criminal investigation, making it the most direct, quickest and most convenient way to find and investigate criminals for law enforcement around the world. Hence the establishment of complete fingerprint databases is urgently needed in many countries. Currently, fingerprint scanners have three forms: vivo optical, capacitive and pressure sensitive types. New method of fingerprint collection by using color-change performance of materials has not been reported. Common color-change materials are usually divided into electrochromic, thermochromic, photochromic and piezochromic materials. Because of many color-change materials have good cycle reversibility. Moreover the synthesis technology of color-change film was simple and synthesis condition was mild, which inspired us to study their application on fingerprint collection. The color-change performance of the materials would influence the quality of the fingerprint images, so it’s important to choose appropriate color-change materials. In this paper, we considered from two aspects, which are preparing films with good color-change perfprmance and designing collection process. Choosing typical electrochromic and thermochromic materials, the new methods of fingerprint collection by using color-change performance were established. Based on the second-level details, third-level details of fingerprint images and ImageJ2 x software, the effect of the different color-change materials on the fingerprint images was studied preliminarily. The main results of the paper were given as the following three parts:Firstly, fingerprint collection using the electrochromismic method was presented with electrochemically deposited polyaniline film on an ITO plate. The film was characterized by optical microscopy, scanning electron microscopy and electrochemical method. Exploring the effects of different polyaniline films with different thickness on fingerprint acquisition, with the thickness of about 200 nm, the color of polyaniline film was darker enough and the film has good color-change performance. It was suitable for fingerprint collection. In the collection experiment, it is necessary to apply appropriate voltage to ensure that the color-change reaction could occur but the voltage should be better lower than the human safety voltage. When the applied potential was controlled in the range of 25-35 V, clear fingerprints can be obtained by rolling the bare finger on the polyaniline film. Analyzing the second-level details and third-level details of the fingerprint images, and based on the ImageJ2 x software, fingerprints collected in this experiment can be clearly observed by naked eyes.Secondly, based on the experiments above, the new method of fingerprint collection with electrochromic Prussian blue film was explored. We considered from three aspects, which are the preparation, property characterization, and application of the Prussian blue film. When the film thickness was about 500 nm, with the applied potentials were controlled in the range of 25-40 V and the humidity of the environment was in the range of 65-80%, clear fingerprints can be obtained. The fingerprint images obtained by this method were even much clearer. Moreover the Prussian blue film obtained by electrochemical method has better adhesion with the ITO electrode and better cycle reversibility. The color change was from blue to colorless, with higher contrast ratio. Therefore it was more suitable for fingerprint acquisition.Thirdly, to improve the selectivity of kinds of the color-change materials, fingerprint collection with a thermochromic approach was presented in this paper. The thermochromic microcapsules were synthesized by means of in-situ polymerization and characterized by thermogravimetric analyzer, differential scanning calorimeter, ultra-deep three-dimensional microscope, scan electron microscopy, laser particle size analyzer and UV reflectance spectra. The microcapsules were utilized for the preparation of the thermochromic film with the help of sodium carboxymethyl cellulose as the binding agent. The film obtained was compact and uniform, and its thermochromism was explored for fingerprint collection based on the fact that the temperatures of human fingers are almost constant and are substantially lower than the color-change temperature of the thermochromic microcapsule(49°C). When the film temperature was controlled in the range from 51°C to 53°C, the film surface temperature dropped below its color-change temperature quickly and fingerprints can be obtained by rolling the bare finger on the film.